Membranes And Receptors Flashcards

Question

What are the main movements of membrane proteins?

Answer 1

Conformational change- vibration Rotational Lateral NO FLIP FLOP

Answer 2

Mouse and human hybrid cells

Answer 3

Due to the thermodynamic constraints of moving large hydrophilic groups through the hydrophobic core

Answer 4

Proteins tend to separate into cholesterol rich fluid phases and cholesterol poor crystalline structure phases - LIPID MEDIATED EFFECTS - AGGREGATES Associations with membrane proteins on other cells- CLUMPING Associations with extra membranous proteins - TETHERING - fixes proteins in fixed positions to basement membrane, peripheral proteins or cytoskeleton

Answer 5

Peripheral | Integral - intrinsic (completely span the membrane) and extrinsic (don't completely span the membrane)

Answer 6

Bounds the surface of membranes Bonds- electrostatic and H bonding interactions Removed by changes in pH or in ionic strength

Answer 7

Interact extensively with hydrophobic domains of lipid bilayer (intrinsic and extrinsic) Cannot be removed by manipulation of pH and ionic strength Removed by agents that compete for non polar interactions - detergents and organic solvents Alpha helical arrangement across hydrophobic region of membrane R groups in hydrophobic region are largely hydrophobic (on outside of helix)

Answer 8

For function- receptors | E.g. Insulin receptor must have recognition site directed to extracellular space

Answer 9

Peripheral proteins in cytoplasmic surface- Spectrin Cross linked spectrin rods- actin, band 4.1 and adducin Adapter proteins- ankyrin and band 4.1 ``` Integral proteins- Band 3 (Anion exchanger) and band 7 (glycoporphorin A) ``` Example of tethering

Answer 10

They are susceptible to proteolysis only when the cytoplasmic face of the membrane is accessible

Answer 11

Attachment of integral membrane proteins to the cytoskeleton restricts the LATERAL mobility of the membrane protein

Answer 12

When spectrin is depleted by 40-50% Erythrocytes round up and become less resistant to lysis Erythrocytes are cleared by the spleen SPHERICAL RBCs =tiredness, low O2 delivery, treated with transfusion in crisis

Answer 13

Defect in spectrin molecule Unable to form heterotetramers Fragile elliptoid cells ELLIPTICAL RBCs

Answer 14

Where a protein has multiple trans membrane domains (e.g G coupled protein receptors- 7) It is likely that the folding of the nascent protein against the constraint of the first trans membrane segment is the driving force for the insertion of the other domains

Answer 15

Hydropathy graph / plot

Answer 16

``` Hydrophobic molecules (oxygen, Carbon dioxide, nitrogen, benzene) Small uncharged polar molecules (water, urea, glycerol) ```

Answer 17

Large uncharged polar molecules (glucose and sucrose) | Ions (H+, Na+, K+, Ca2+, Mg2+, Cl-, HCO3-)

Answer 18

Maintenance of ionic composition Maintenance of IC pH Regulation of cell volume Concentration of metabolic fuels and building blocks Extrusion of waste products of metabolism and toxic substances Generation of ion gradients necessary for the electrical excitability of nerve and muscle

Answer 19

Dependent on permeability and concentration gradient Rate of passive transport increases linearly with increasing conc gradient PASSIVE (does not require energy), uses CHANNELS

Answer 20

``` Permeability of the membrane for a substance is increased by the incorporation of a specific protein in the bilayer: Carriers molecules (ping pong proteins) Ion protein channels (ligand gated or voltage gated) ``` Saturable process - each carrier can interact with a few ions or molecules at any given moment CARRIERS AND CHANNELS, PASSIVE (no energy)

Answer 21

Allows the transport of ions or molecules against an unfavourable concentration and or electrical gradient requiring energy (directly or indirectly) the hydrolysis of ATP (30-50% cells ATP used on at) Whether or not energy is required depends on the free energy change of the transported species and by the electrical potential across the membrane bilayer when the transported species is charged

Answer 22

Uni port channel Gated ion channel Ligand gated ion channel (nicotinic acetlycholine receptor, ATP sensitive K+ channel) Voltage gated ion channel (Na+ channel) Passive diffusion and facilitated diffusion

Answer 23

Ping pong transporters Co transporters ( more than one type of ion or molecule may be transported on a membrane transporter per cycle) = symport (two molecules, same direction) (Na+ glucose) = anti port (two molecules, different direction) (Na+ Ca2+ exchange OR Na+ H+ exchange)

Answer 24

- A plasma membrane associated pump - Uses ATP to pump ions (active transport) - 25% of BMR is used for pump - Called a P-type ATPase (ATP phosphorylates Aspartate, producing phosphoenzyme intermediate) - a-Subunit – Binding sites for K+, Na+, ATP, ouabain - b-Subunit – Glycoprotein directs pump to the surface - The binding of ouabain to the a-Subunit inhibits Na+/K+-ATPase - Uses energy from ATP hydrolysis to move 2K+ into the cell and 3Na+ out of the cell. (Antiport) Forms Na+ and K+ gradients - Necessary for electrical excitability Drives Secondary Active Transport - Control of pH - Regulation of cell volume - Regulation of Ca2+ concentration - Absorption of Na+ in epithelia - Nutrient uptake, e.g. glucose from the small intestine

Answer 25

Plasma Membrane Ca2+-ATPase (PMCA) - Expels Ca2+ bound to calmodulin from the cell in exchange for H+ - Uses ATP - Antiport - High affinity, low capacity - Removes residual Ca2+

Answer 26

Accumulates Ca2+ bound to calmodulin into the SR/ER in exchange for H+ - Uses ATP - Antiport - High affinity, low capacity - Removes residual Ca2+

Answer 27

Na+/Ca+-exchanger (NCX) - Secondary Active Transport - Expels 1xCa2+ from the cell in exchange for 3xNa+ - Uses the Na+ concentration gradient set up by Na+/K+-ATPase - Antiport - Low affinity, high capacity - Removes most Ca2+ - Electrogenic – Current flows in the direction of the Na+ gradient - Expels intracellular Ca2+ during cell recovery - Activity is membrane potential dependent

Answer 28

Na+/H+ Exchanger (NHE) - Exchanges extracellular Na+ for intracellular H+ - Electroneutral 1:1 exchange - Uses the Na+ concentration gradient set up by Na+/K+-ATPase - Raises intracellular pH - Regulates cell volume - Activated by growth factors - Inhibited by amiloride

Answer 29

``` Sodium Bicarbonate Co-Transporter (NBC) Na+ Dependent Cl-/HC03- Exchanger - Acid out - Base in - Uses the Na+ concentration gradient set up by Na+/K+-ATPase - Raises intracellular pH - Both involved in regulating cell volume - acid extruder - Na+ and HCO3- in and H+ and Cl- out ``` ``` Anion Exchanger (AE) Cl-/HCO3- exchanger - Removes base from cell - Acidifies cell - Involved in cell volume regulation - Base extruder ```

Answer 30

``` Sodium potassium pump (sets up the gradient for NCX) PMCA SERCA NCX Ca2+ uni porters into mitochondria ```

Answer 31

Sodium potassium pump (all cells) NHE (most cells) - acidic extrusion NBC (some cells) - acid extrusion and alkali influx Na+-3HCO3- co transport (some cells) - alkali influx AE (most cells) - alkali extrusion

Answer 32

Mechanisms to resist cell swelling- Efflux of osmotically active ions ( sodium, potassium, chloride) or solute molecules Water follows Mechanisms to resist cell shrinking- Influx of osmotically active ions (sodium, potassium, chloride) Water follows

Answer 33

Na/K pump again drives other channels, by keeping intracellular Na+ concentration low, so NHE can pump H+ ions into the proximal tubule lumen. H+ goes into the lumen to “pick up” bicarbonate and bring it back into the cell. Under normal corcumstances the kidney reabsorbs all the bicarbonate filtered into the proximal tubule The main reason is to retain base for the pH buffer E.g. Renal control of circulating sodium conc is often a first line treatment for mild hypertension Water tablet- diuretic

Answer 34

The goal of Renal anti-hypertensive therapy is to reduce the reuptake of Na+ and other molecules, so less water is reabsorbed via osmosis. With less water being reabsorbed, blood volume and therefore blood pressure falls Aquaporin allows water to more readily cross the membrane. Its inclusion in the mmebrane is stimulated by anti-diuretic hormone (ADH) Loop diuretics block Na+ reuptake in the thick ascending limb Amiloride acts both in the Distal convoluted tubule (ENaC) and the proximal tubule (Na/H) to prevent Na+ reuptake Aldosterone up-regulates these transporters. Spironolactone (Glucocorticoid receptor antagonist) is used to treat (if Aldosterone is high)

Answer 35

Transport of Na+ out of cell by Na/K pump allows for the symport of 2Cl- out of the cell with Na+ and K+. Faulty CFTR protein leads to accumulation of Cl- in the cell. Water moves into the cell via osmosis, giving thick, viscous mucous in the lumen.

Answer 36

CFTR is overly active once phosphorylated by Protein Kinase A. Cl- is excessively transported into the lumen. Water follows, giving the symptoms of Diarrhoea.

Answer 37

Electrical potential (voltage) difference across the plasma membrane of a cell

Answer 38

Provides the basis of signalling in the nervous system, heart and other tissues

Answer 39

Always expressed as the potential inside the cell relative to the extracellular solution

Answer 40

Millivolts

Answer 41

Using a very fine micropippette- microelectrode that will penetrate the cell membrane Tip diameter < 1 micrometer Microelectrode is filled with a conducting solution (KCl)

Answer 42

Membranes have different selective permeabilities for different ions due to the presence of channel proteins (which are selective for specific ions)

Answer 43

Selectivity- the channel lets through only one (or a few) ion species (e.g. Specific to Na+ Ca2+ K+ Cl- and with non selective cation permeability) Gating- channel can be open or closed by a conformational change in the protein molecule A high rate of ion flow- always down the concentration gradient (passive)

Answer 44

Ligand gated Voltage gated Mechanical gated

Answer 45

Chemical ligands bind causing a conformational change and causing the gates to open or close E.g. Channels at synapses that respond to EC transmitters Channels that respond to IC messengers

Answer 46

Change in membrane potential causing the channel to open or close E.g. channels involved in action potentials

Answer 47

Membrane deformation causing the channels to open or close E.g. Channels in mechanoreceptors carotid sinus stretch receptors: hair cells

Answer 48

Provides the outward ionic gradient for K+ necessary for maintenance of the membrane potential Although electrogenic, (3Na+ out 2K+ in) the enzyme contributes little to the membrane potential (-5mV) So indirectly Na+ K+ ATPase pump (active transport) is responsible for entire membrane potential because it sets up and maintains ionic gradients K+

Answer 49

Voltage insensitive K+ channels (facilitated diffusion), which remain open despite the changes in potential across the membrane, are responsible for K+ movement that establishes the resting membrane potential ICK+ 160mmol/l ECK+ 4.5mmol/l K+ moves outwards down the concentration gradient Since large anions can't follow as they can't penetrate the membrane, a negative potential develops on the intracellular face of the plasma membrane Growing potential difference across the membrane then opposes the further efflux of K+ An equilibrium is reached when the chemical and electrical forces are balanced and there is no net movement of K+

Answer 50

At equilibrium, the electrical and chemical gradients for K+ balance so that there is no net driving force on K+ across the membrane Nernst equation allows us to calculate the membrane potential. At which K+ will be in equilibrium given the extracellular and intracellular K+ concentrations Looks at K+ alone ~-95mV

Answer 51

``` Ek= 61/z log(10) [K+]o/[K+]i Z= valency ```

Answer 52

Corresponds to an estimate of the resting membrane potential (assuming there are no other channels in the membrane- obviously not true and so actual resting membrane potential is always slightly different)

Answer 53

Plasma membrane is not totally impermeable to other ions and the passage of these ions through selective ion channels contributes to overall membrane potential Depends on number of ions, concentration of ions, type of ion, selectivity of channels etc.

Answer 54

A decrease in the size of the membrane potential from its normal value Cell interior becomes less negative Opening Na+ and Ca2+ channels (influx of Na+ and Ca2+)

Answer 55

An increase in the size of the membrane potential from its normal value Cell interior becomes more negative Opening K+ or Cl- channels (out flux of K+ and influx of Cl-)

Answer 56

Signalling Action potentials in nerve and muscle cells Triggering and control of muscle contraction Control of secretion of hormones and neurotransmitters Transduction of sensory info into electrical activity by receptors Post synaptic actions of fast synaptic transmitters

Answer 57

Have an intrinsic ion channel Opened by binding of ACh Channels let Na+ K+ through, but not anions Moves membrane potential towards 0mV = intermediate between ENa+ and EK+ Sodium predominates movement across membrane

Answer 58

At the synapse a chemical transmitter is released from the presynaptic cell and binds to receptors on the post synaptic membrane Fast synaptic transmission Slow synaptic transmission

Answer 59

Receptor protein is also an ion channel Transmitter binding causes channel to open Excitatory synapse- open ligand gated channels, depolarisation, Ca2+ and Na+, change in membrane potential = excitatory post synaptic potential (EPSP) (Longer than ap, graded with amount of transmitter, ACh, Glutamate (transmitters)) Inhibitory synapse- open ligand gated channels, hyper polarisation, K+, Cl-, change in membrane potential = inhibitory post synaptic potential (IPSP) (Glycine, gamma amino butyric acid GABA)

Answer 60

Receptor protein and channel protein are separate Direct G protein gating- localised and quite rapid (1 signal = 1 event) Gating via an intracellular messenger- throughout cell, amplification (1 signal= many events)

Answer 61

High concentration of potassium in the blood Less negative EK resting membrane potential So lesser change in ion conductance is required to depolarise the heart and so excite cardiac membranes - ventricular arrhythmia (life threatening) Na+ channels become inactivated more rapidly Less negative membrane potential also prevents the repriming of inactivated Na+ channels resulting in an electrically silent membrane - also contributes to arrhythmia

Answer 62

Change in voltage across the membrane

Answer 63

Ionic gradients | Relative permeability of the membrane

Answer 64

Voltage sensitive sodium channels

Answer 65

The membrane potential will move closer to the equilibrium potential for that ion

Answer 66

It is dependent on the number of channels for that ion that are open

Answer 67

Enables the membrane current to be measured at a set membrane potential By voltage clamping which controls the membrane potential so that the ionic currents can be measured Using different ionic concentrations the contribution of various ions can be assessed Patch clamping enables current flowing through individual ion channels to be measured

Answer 68

In an unclamped cell membrane potential can change freely | Voltage clamp prevents change in membrane voltage in response to membrane current

Answer 69

The initiation of an action potential depends on the membrane potential rising above a threshold value

Answer 70

An increased open probability for Voltage gated sodium and calcium channels which lead to membrane depolarisation to exceed the hyperpolarising effects of the resting potassium efflux The closure of potassium channels which reduces the hyperpolarising effect on membrane allowing the membrane potential to rise in the leak of sodium and calcium

Answer 71

``` Depolarisation to threshold Sodium channels open Sodium enters the cell Membrane depolarises Causes more sodium channels to open Positives feedback is the basis of all or nothing characteristic of the action potential ```

Answer 72

Depolarisation causes Potassium channels to open causing a potassium efflux and repolarisation Depolarisation inactivates sodium channels causing the influx of sodium to be stopped and thus repolarisation

Answer 73

Absolute refractory period | Relative refractory period

Answer 74

During the peak When excitability is Zero When nearly all the sodium channels are in the inactivated state Another AP cannot be generated here

Answer 75

After the Peak When excitability is increasing When sodium channels are recovering from inactivation (ie. returning to closed state so that they are available for another) excitability returns towards normal as the number of channels in the inactivated state decreases Another AP can be generated here

Answer 76

Yes however it is more difficult to reach the threshold

Answer 77

The longer the stimulus is, the larger the depolarisation necessary to initiate an action potential With a persistent stimulus, sodium channels become inactivated and the threshold becomes more positive (as less sodium channels are available to be opened) (If a stimulus lasts a long time the body becomes used to it and so a greater depolarisation is needed to isolate an action potential)

Answer 78

``` One peptide 4 homologous repeats Six transmembrane domains One domain is voltage sensitive S4 Function requires one subunit ```

Answer 79

``` Are similar in structure but each repeat is in fact a separate sub unit 4 peptides Six transmembrane domains One domain is voltage sensitive S4 Function requires 4 subunit ```

Answer 80

Procaine | It stops action potential generation

Answer 81

Small myelinated axons (sensory) Non myelinated axons Large myelinated axons (motor)

Answer 82

Local anaesthetics are weak bases and cross the membrane in their unionised form- they block sodium channels when the channel is open and also have a higher affinity to the inactivated state of the sodium channel Stop action potential generation Via hydrophobic or hydrophilic pathway

Answer 83

Occurs under a cathode (negative) Excitability will be reduced under an anode (positive) Electrodes can be used to raise the membrane potential to threshold to generate an action potential By recording changes in potential between the STIMULATING (cathode -ve) and RECORDING (anode +ve) electrodes along an axon, conduction velocity can be calculated Extracellular recording of action potentials can give info about the conduction velocities under various conditions

Answer 84

Conduction velocity= distance/ time Measuring the distance between the stimulating electrode and the recording electrode and the time gap between the stimulus and action potential being registered by the recording potential

Answer 85

The depolarisation of a small region of membrane produces transmembrane currents in neighbouring regions As sodium channels are voltage gated this opens more channels causing the propagation of an action potential The further the local current spreads the faster the conduction velocity of the axon

Answer 86

A high membrane resistance A low membrane capacitance A large axon diameter

Answer 87

Membrane resistance depends on the number of ion channels open So the higher the membrane resistance, the more sodium channels open, the easier to reach threshold

Answer 88

Capacitance is the ability to store charge So membrane with low capacitance will take less time to charge as its ability to store charge will be less, increasing conduction velocity

Answer 89

Results in a low cytoplasmic resistance Current is thus larger Action potential will travel further Increased conduction velocity

Answer 90

Ability to store charge

Answer 91

Myelinated axon- cv dp to diameter Unmyelinated axon - cv dp to square root (diameter) Diameter is limiting for the myelinated axon when small

Answer 92

Oligodendrocytes

Answer 93

Schwann cells

Answer 94

Conduction velocity is increased considerably by myelination of axons

Answer 95

It reduces membrane capacitance | Increasing conduction velocity

Answer 96

It increases the membrane resistance | Increasing conduction velocity

Answer 97

In myelinated axons | Where action potential jumps between nodes of Ranvier

Answer 98

Myelin acts as a good insulator, causing local circuit currents to depolarise the next node above threshold and generate an action potential

Answer 99

Nodes are a high density of sodium voltage gated channels | Evenly distributed in unmyelinated neurones

Answer 100

Demyelination Autoimmune disease where myelin is destroyed in certain areas of the CNS decreased conduction velocity

Answer 101

Na+ K+ ATPase pump

Answer 102

Na+ Ca2+ exchange NCX

Answer 103

NKCC2 furosemide

Answer 104

NCC on distal convoluted tubule | Hydrochlorothiazide

Answer 105

Na+ K+ Ca2+

Answer 106

Action potential arrives at nerve terminal Depolarisation causes the opening of voltage gated calcium channels Influx of calcium Rise in intracellular concentration of calcium causes release of neurotransmitter

Answer 107

Calcium binds to synaptotagmin Vesicle is brought close to the membrane and binds to the snare complex Snare complex makes a fusion pore with the membrane Transmitter (acetyl choline) is released through this pore

Answer 108

1 x 10 ^-7 M

Answer 109

1 x 10 ^-3 M

Answer 110

Exocytosis

Answer 111

Very similar structurally to sodium vg channels BUT calcium has some structural diversity in that a blocker that blocks one calcium channel will not necessarily block another Different calcium channels have different primary locations so selectively blocking one type of channel can have a localised effect

Answer 112

DHP Dihydropyridine (e.g. nifedipine)

Answer 113

All muscles, neurones, lungs

Answer 114

w-CTx-GVIA

Answer 115

R- neurones/heart? | T- neurones/ heart (SAN)

Answer 116

Nicotinic ACh receptor

Answer 117

Ligand gated ion channels

Answer 118

End plate potential

Answer 119

This depolarisation will raise the muscle above threshold so that an action potential is produced as it activates adjacent sodium channels and potassium channels due to the local spread of charge

Answer 120

ACh esterase

Answer 121

Fast synaptic transmission

Answer 122

Competitive blockers bind to the molecular recognition site for ACh Tubocurarine

Answer 123

Depolarising blockers cause a maintained depolarisation at the post junctional membrane. Adjacent sodium channels will not be activated due to accommodation Succinylcholine (used in operations to induce paralysis)

Answer 124

Autoimmune disease causing the destruction of nicotinic ACh receptors Caused by antibodies directed against nAChRs on the post synaptic membrane of skeletal muscle End plate potentials are reduced in amplitude, leading to muscle weakness and fatigue

Answer 125

Drooping eyelids Profound weakness, which increases with exercise Fatigue

Answer 126

Using AChesterase inhibitors, increasing the amount of time ACh remains in the synaptic cleft and hence increasing the likelihood of ACh binding to any remaining available receptors

Answer 127

Slow synaptic transmission | mACh receptors are coupled to G proteins which trigger a cascade of events in the cell

Answer 128

Many cellular processes are calcium sensitive, for example fertilisation, secretion, neurotransmission, metabolism, contraction, learning and memory, apoptosis and necrosis. As Ca2+ cannot be metabolised, the cell has to regulate intracellular Ca2+ concentration based largely on moving Ca2+ into and out of the cytoplasm.

Answer 129

Changes in intracellular [Ca2+] occur rapidly with little movement

Answer 130

Ca2+ overload leads to loss of regulation and cell death

Answer 131

1. The relative impermeability of the plasma membrane 2. The ability to expel Ca2+ across the plasma membrane using: - Ca2+ ATPase - Na+-Ca2+ Exchanger 3. Ca2+ Buffers 4. Intracellular Ca2+ Stores: - Rapidly releasable - Non-Rapidly releasable

Answer 132

The permeability of the membrane is regulated by the open/closed state of ion channels.

Answer 133

Ca2+ ATPase PMCA & SERCA Considered to be high affinity, low capacity. - Intracellular [Ca2+] Increases - Ca2+ binds to calmodulin – a binding trigger protein - Calmodulin-Ca2+ binds to Ca2+ ATPase - Ca2+ is removed from cell Na+/Ca2+ Exchanger Considered to be low affinity, high capacity - Na+ Gradient used as driving force (Na+/K+-ATPase) (M&R LO 2.3) - Transports 3Na+ into the cell per 1Ca2+ out - Antiporter is electrogenic - Works best at resting membrane potential

Answer 134

Ca2+ buffers limit diffusion, through ATP and Ca2+ binding proteins such as parvalbumin, calreticulin, calbindin and calsequestin. Diffusion depends on the concentration of binding molecules and their level of saturation. Many other proteins also bind Ca2+, altering their function, e.g. calmodulin.

Answer 135

LARGE INWARD GRADIENT

Answer 136

Stores are either rapidly releasable or non rapidly releasable An alternative way of raising cellular calcium concentration

Answer 137

1. Ca2+ influx across the plasma membrane (Altered permeability) - Voltage Operated/Gated Ca2+ channels (VOCC/VGCC) - Receptor operated ion channels (Ionotropic receptors) 2. Ca2+ release from ‘rapidly-releasable’ stores - G-protein coupled receptors (GPCRs) - Ca2+ induced Ca2+ release (CICR) 3. Ca2+ release from ‘non-rapidly releasable’ stores

Answer 138

Voltage-Gated Calcium Channels (VGCC) Channels that open to allow the influx of calcium down its concentration gradient, triggered by membrane depolarisation. Receptor Operated Ca2+ Channels A ligand/agonist binds to the channel, opening it and allowing Ca2+ to enter down its concentration gradient.

Answer 139

Stores of Ca2+ are set up inside the Sarco/Endoplasmic Reticulum by the SERCA protein (Sarco/Endoplasmic Reticulum Ca2+ ATPase). Ca2+ is moved in using the energy from ATP hydrolysis and binds to proteins such as calsequestrin. G-Protein Coupled Receptors (GPCRs) A ligand binds to the GPCR on the cell membrane, activating its Gaq subunit. This subunit then binds to the membrane phospholipid PIP2, releasing IP3, which in turn binds to its receptor on the sarcoendoplasmic reticulum, triggering the release of calcium down its concentration gradient into the cell. Ca2+ induced Ca2+ release (CICR) Ca2+ binds to the Ryanodine receptor on the side of the Sarco/endoplasmic reticulum, triggering the release of calcium down its concentration gradient into the cell.

Answer 140

G-Protein Coupled Receptors (GPCRs) A ligand binds to the GPCR on the cell membrane, activating its Gaq subunit. This subunit then binds to the membrane phospholipid PIP2, releasing IP3, which in turn binds to its receptor on the sarcoendoplasmic reticulum, triggering the release of calcium down its concentration gradient into the cell.

Answer 141

Ca2+ induced Ca2+ release (CICR) Ca2+ binds to the Ryanodine receptor on the side of the Sarco/endoplasmic reticulum, triggering the release of calcium down its concentration gradient into the cell

Answer 142

An example of an important physiological role for CICR is in the cardiac myocyte. Here, Ca2+ entry through VOCCs following depolarisation of the membrane binds to the ryanodine receptors, causing an explosive release of large amounts of Ca2+ from intracellular stores.

Answer 143

Ca2+ entry through VOCCs following depolarisation of the membrane binds to the ryanodine receptors, causing an explosive release of large amounts of Ca2+ from intracellular stores (CICR) During the very early part of the action potential (at the height of depolarisation) conditions will favour the reversal of the sodium calcium exchanger (NCX) which will result in a small amount of Ca2+ entry As the intracellular concentration of calcium increases and membrane repolarisation starts, the sodium calcium exchanger (NCX) will revert to Ca2+ extrusion to lower the intracellular concentration of calcium Calcium will also be pumped back into the SR are by SERCA in preparation for another release event

Answer 144

Ca2+ is taken up into mitochondria when [Ca2+]I is high as a protective mechanism, but mitochondria also participate in normal Ca2+ signalling due to microdomains (areas of cytoplasm with a higher concentration of Ca2+ due to their proximity to a channel). Mitochondria take up Ca2+ to aid in buffering, regulating signalling, and stimulation of ATP production. They do this via a Ca2+ uniporter that is driven using respiration.

Answer 145

Repetitive signalling requires a return to the basal state. Further to this, too much Ca2+ for too long is toxic to cells. A return to basal levels requires: - Termination of signal - Ca2+ Removal- Ca2+ ATPase and NCX - Ca2+ store refilling- Ca2+ stores are refilled both by the recycling of cytosolic Ca2+ (SERCA) (e.g. cardiac myocyte) and by using Calcium stored in mitochondria. Mitochondrial Ca2+ is used to replenish SR stores via the store-operated Ca2+ channel (SOC).

Answer 146

``` PMCA SERCA NCX Mitochondrial calcium uptake Calcium binding protein ```

Answer 147

``` Inositol 1,4,5 triphosphate receptors (IP3) (GPCR) Ryanide receptors (CICR) Voltage sensitive calcium channels (VOCC/VGCC) Sodium calcium exchanger (NCX) Ligand gated calcium channels ```

Answer 148

Voltage sensitive calcium channels mediate excitation contraction coupling in skeletal muscle Physical coupling between voltage sensitive calcium channels and ryanodine sensitive calcium channels in sarcoplasmic particular is required to release the vesicular stores of calcium required for contraction

Answer 149

Calcium entry through the voltage sensitive calcium channels (VOCC/ VGCC) is required for excitation contraction Release of calcium from vesicular stores required for contraction is mediated by calcium induced calcium release channels (CICR) Entry of extracellular calcium may be mediated in part by the sodium calcium exchanger (NCX) working in reverse mode in the depolarised sarcolemma In cardiac muscle calcium antagonist exert their anti-dysrhythmic action by blocking voltage sensitive calcium channels (VOCC/ VGCC)

Answer 150

Calcium entry through voltage gated calcium channels (VOCC/VGCC) may lead to smooth muscle contraction Release of calcium from vesicular stores in response to activation of phospholipase C beta and production of inositol 1,4,5 triphosphate (IP3) may lead to smooth muscle contraction M3 muscarinic receptor activation may stimulate the release of intracellular stores of calcium and smooth muscle contraction Alpha-1 adrenoreceptor activation may stimulate the release of intracellular stores of calcium and smooth muscle contraction

Answer 151

Hormones - signalling between cells in different tissues via circulation

Answer 152

Neurotransmitters- signalling at specified cell junctions in the nervous system at synapses

Answer 153

Local chemical mediators- signalling between adjacent cells in the same tissue

Answer 154

A molecule that recognises specifically a second molecule (ligand), or family of molecules and in response to binding brings about the regulation of a cellular process.

Answer 155

Stimulates a biological response

Answer 156

Molecules that operate in the absence of a ligand | Ligand binding alone produces no response

Answer 157

Dihydrofolate reductase - methotrexate | Sodium channel- local anaesthetic agent, neurotoxins

Answer 158

Receptors are classified primarily by their specificity to a physiological signalling molecule (agonist) They are then often divided further on the basis of their affinity to a series of antagonists. Receptor type: acetylcholine muscarinic receptors Agonist: muscarine Receptor subtype: M1 M2 and M3 Antagonist: M1 (pirenzipine) M2 (gallamine) M3 (hexahydrosiladiphenol)

Answer 159

The affinity of ligand binding at receptor sites is generally much higher than binding of substrates to enzyme sites. This is because ligands may only be present in very small concentrations.

Answer 160

Binding is specific Specificity determined by shape of binding cleft Binding is reversible Binding induces a conformational change and change in the activity of molecule - induced fit No chemical modification of ligand

Answer 161

Any molecule that binds specifically to a receptor site Ligand binding may produce ACTIVATION of a receptor- agonist Ligand binding may NOT CAUSE ACTIVATION but PREVENT BINDING OF AN AGONIST = antagonist

Answer 162

It must produce specific receptor proteins which recognise and produce a response to the signalling molecule

Answer 163

Extracellular surface

Answer 164

Intracellular surface

Answer 165

Membrane bound receptors with integral ion channels Membrane bound receptors with integral enzyme activity Membrane bound receptors which couple to effectors through transducing proteins Intracellular proteins

Answer 166

Agonist binding to ligand-gated ion channels results in a conformational change and the opening of a gated channel. The channel then permits the flow of ions down an electrochemical gradient. Several of these receptors belong to the classical ligand-gated ion channel family, sharing similar pentameric subunit structures with four transmembrane domains E.g. Nicotinic Ach Receptors (nAchR) (gated Na+, K+, Ca2+ channels) [other examples: gamma amino butyric acid GABA (gated Cl- channels), Glycine receptor (gated Cl- channels)] In addition to these non-classical ligand-gated ion channels can also be present. E.g. Ryanodine receptor (gated Ca2+ channels), P2x purine receptors (gated Na+/Ca2+ channels)

Answer 167

Agonist binding to the extracellular domain of these receptors causes a conformational change, which activates an intrinsic enzyme activity, contained within the protein structure of the receptor for example tyrosine kinase linked receptors. E.g. Platelet Derived Growth Factor (PDGF) linked directly to Tyrosine Kinase Tyrosine Kinase Linked Receptors autophosphorylate upon ligand binding. Phosphorylated receptor tyrosine residues are recognised either by transducing proteins, e.g. insulin receptor substrate-1 (IRS-1) or directly by enzymes containing phosphotyrosine recognition sites, Src-homology-2 (SH2) domains. On association with receptor or transducing protein, effector enzymes become activated allosterically/by tyrosine phosphorylation by the receptor kinase. This transduces the message into an intracellular chemical event. E.g. Insulin receptor

Answer 168

Seven transmembrane domain receptors (7TMDR) couple to effector molecules via a transducing molecule, a GTP-binding regulatory protein (G-Protein). This family of receptors are therefore known as G-Protein Coupled Receptors. Effectors may be enzymes (e.g. adenylyl cyclase) or ion channels (e.g. Ca2+/K+). Many extracellular signalling molecules have a structure including seven transmembrane domains. Examples include muscarinic 2 Ach receptors (m2AchR) (K+ channel), dopamine receptors, B adrenergic adrenoreceptors (adenylyl cyclase) 5-HT receptors, light, smell and taste receptors. Often a number of different types of G-protein receptors exist for a particular agonist, e.g. M1-5 mAchRs. Often, separate G-protein coupled receptors will act simultaneously to both stimulate/inhibit the effector. This is Integrated Signalling, and the two inputs combine to produce a measured effect.

Answer 169

Hydrophobic ligands, such as the steroid hormones cortisol, oestrogen and testosterone and the thyroid hormones T3 and T4 can pass through the plasma membrane. They therefore bind to receptors inside the cell. These intracellular receptors in their resting state are bound to heat shock or chaperone proteins. The activated receptor dissociates from the stabilising protein and translocates to the nucleus, where it binds to control regions in DNA, regulating gene expression. Compared to extracellular receptors, which act through ion channels or enzymes (see above), the action of intracellular receptors is relatively slow, as they are dependant on transcription and translation.

Answer 170

The concentration of many extracellular signalling molecules is very low. With all receptor mechanisms there is a possibility of molecular amplification. For example, by stimulating the activity of an enzyme, the binding of a chemical signal molecule to a single receptor can cause the modification of hundreds or thousands of substrate molecules. An enzymatic cascade can produce further amplification.

Answer 171

E.g. Cardiac Pacemaker Cells: Noradrenaline à b1-Adrenoceptors à Increased Heart Rate Acetylcholine à M2-Muscarinic Receptors à Slowing of Heart Rate E.g. Hepatocytes: Insulin à Stimulates Glycogen Synthesis from glucose Glucagon à Stimulates Glycogen Breakdown to glucose

Answer 172

Pinocytosis is the invagination of the plasma membrane to form a lipid vesicle. This permits the uptake of impermeable extracellular solutes and retrieval of plasma membrane. Pinocytosis can be sub-divided into two forms, fluid-phase and receptor mediated endocytosis.

Answer 173

In mammals, phagocytosis is found only in specialised cells, i.e. macrophages and neutrophils. In response to the binding of a particle to receptors in the plasma membrane, the cell extends pseudopods that permit further receptor interactions and membrane invagination/particle internalisation via a ‘membrane zippering’ mechanism. Internalised phagosomes fuse with lysosomes to form phagolysosomes in which the particulate material is degraded. This process permits the clearance of damaged cellular materials and invading organisms for destruction.

Answer 174

Selective internalisation of molecules into the cell by binding to specific cell receptors

Answer 175

Specific binding of molecules to cell surface receptors permits the selective uptake of substances into the cell.

Answer 176

Low Density Lipoproteins (LDL) originate in the liver and consist of a core of cholesterol molecules esterified to fatty acid, surrounded by a lipid monolayer containing phospholipids, cholesterol and a single protein species, Apoprotein B. Animal cells that require cholesterol synthesise cell surface LDL-Receptors that recognise specifically Apoprotein B. These receptors are localised in clusters over Clathrin Coated Pits that cover approximately 2% of the cell’s surface. These pits form spontaneously, just as Clathrin spontaneously forms cages. When LDL binds to the receptors, the pit invaginates to form coated vesicles. The vesicles are uncoated in a process that requires ATP (As they formed spontaneously) and fuse with larger, smooth vesicles called endosomes. The pH of the endosome is lower than that of the cytoplasm (5.5-6.0), maintained by an ATP-dependent proton pump. At this pH, the LDL receptor has a low affinity for the LDL particle and the two dissociate. Because of this, the endosome is also known as the Compartment for the Uncoupling of Receptor and Ligand (CURL). The receptors are sequestered to a domain within the endosome membrane, which buds off as a vesicle and recycles the LDL-receptor to the plasma membrane (maybe via the Golgi apparatus). The endosomes containing the LDL fuse with lysosomes, and the cholesterol is hydrolysed from the esters and released into the cell.

Answer 177

Non-Functioning Receptor If there is a mutation to the LDL binding site of the LDL-Receptor, it will prevent the binding and uptake of LDL. Receptor Binding Normal If the receptor binding is normal, a mutation can still lead to Hypercholesterolaemia. If there is a deletion of the C-terminal cytoplasmic domain, which prevents the interaction between the receptor and the Clathrin coat, the LDL-Receptors will be distributed over the entire cell surface instead of being concentrated in 2%. Receptor Deficiency A deficiency caused by a mutation that prevents expression of the LDL-Receptor.

Answer 178

Two Fe3+ ions bind to Apoptransferrin to form Transferrin in the circulation. Transferrin, but not Apoptransferrin, binds to the Transferrin Receptor at neutral pH and is internalised in a similar way to LDL as described above. Upon reaching the acidic endosome, the Fe3+ ions are released from the transferrin, but at this pH the Apoptransferrin remains associated with the transferrin receptor. The complex is sorted in the CURL for recycling back to the plasma membrane, where at pH 7.4 the Apoptransferrin dissociates from the receptor again.

Answer 179

Insulin receptors only congregate over Clathrin coated pits when their agonist is bound (unlike with LDLs and iron). Insulin binding induces a conformational change in the receptor that allows it to be recognised by the pit. In the endosome Insulin remains bound to the receptor and the complex is targeted to the lysosomes for degradation. This mechanism allows for the reduction in the number of insulin receptors on the membrane surface, desensitising the cell to a continued presence of high circulating insulin concentration.

Answer 180

Some ligands that remain bound to their receptors may be transported across the cell. This is Transcytosis. Examples include maternal immunoglobulins to the foetus via the placenta and the transfer of immunoglobulin A (IgA) from the circulation to bile in the liver. During transport of IgA the receptor is cleaved, resulting in the release of immunoglobulin with a bound ‘secretory component’ derived from the receptor.

Answer 181

Receptors for different ligands enter the cell via the same Clathrin coated pits and the pathway from coated pits to the endosome is common for all proteins that undergo endocytosis. Different modes of this process can be defined on the basis of the destination of the internalised receptor and ligand. Receptors targeted to different cellular destinations, by short amino acid motifs, are sorted within the CURL to discrete regions of membrane, which bud off into transport vesicles

Answer 182

``` Mode 1 Fate of Receptor- recycled Fate of Ligand- degraded Examples- LDL Function- Metabolite uptake ```

Answer 183

``` Mode 2 Fate of Receptor- recycled Fate of Ligand- recycled Examples- transferrin Function- metabolite uptake ```

Answer 184

Mode 3 Fate of Receptor- degraded Fate of Ligand- degraded Examples- insulin, epidermal growth factor Function- immune complexes, receptor down regulation, removal from circulation of foreign antigen

Answer 185

``` Mode 4 Fate of Receptor- transported Fate of Ligand- transported Examples-maternal IgG; IgA Function- Transfer of large molecules across cell; E.g. Maternal immunity to foetus via placenta; E.g. Circulation to Bile ```

Answer 186

***** Long lasting hyperglycaemia can result in type 2 diabetes because the tissue becomes desensitised and had a reduced response to insulin as the excess production of insulin means that the enzymes are down graded, increasing the hepatic glucose production, thus contributing more to hyperglycaemia

Answer 187

Membrane-enveloped viruses and some toxins exploit endocytic pathways to enter cells after adventitious binding to receptors in the plasma membrane. Once in the endosome, the acidic pH allows the viral membrane to fuse with the endosomal membrane, releasing the viral RNA into the cell where it can be translated and replicated by the host cell’s machinery to form new viral particles. E.g. Cholera Toxin and Diphtheria toxin, both of which bind GM1 Ganglioside.

Answer 188

Appropriate receptors

Answer 189

Thyroid hormone | Steroid hormones

Answer 190

Transferrin Insulin ACTH

Answer 191

Ligand gated ion channels Receptors with intrinsic enzymatic ion channels G protein coupled (7TM) receptors

Answer 192

Changes in light (e.g. Rhodopsin) odours and tastes

Answer 193

Ions (H+, Ca2+) Neurotransmitters (ACh, Glutamate) Peptide and non peptide hormones (glucagon, adrenaline) Large glycoproteins (TSH)

Answer 194

Single polypeptide chain (300-1200 aa's) 7TM spanning regions Extracellular N terminal Intracellular C terminal

Answer 195

Ligand binding/ activation of GPCR Activation of G Protein Signalling Termination

Answer 196

Two regions of GPCRs can be responsible for ligand binding: N terminal region and other extracellular domains Within the transmembrane domain

Answer 197

An activated GPCR must interact with another protein- guanine nucleotide binding protein (= G protein) The GPCR- G protein interaction activates the G protein by causing GTP to exchange for GDP on the G protein alpha sub unit

Answer 198

The alpha(GTP)- betagamma complex immediately dissociates into alpha(GTP) and betagamma and each can then interact with effector proteins (second messengers) generating ion channels or enzymes

Answer 199

``` The alpha(GTP) and/or betagamma interaction with effectors lasts until the alpha subunit GTPase activity hydrolyses GTP back into GDP Alpha(GDP) and betagamma then reform an inactive heterotetrameric complex ```

Answer 200

GTPase is integral to alpha subunit

Answer 201

Activated GPCRs preferentially interact with specific types of G protein. The G (alpha) subunit is a primary determinant. In turn G(alpha) subunits and G(beta gamma) subunits interact with specific effector proteins In this way an extracellular signalling molecule working by a specific GPCR activate a single, or a small sub population of G proteins and effectors in the cell to bring about a specific cellular response.

Answer 202

Adrenaline, noradrenaline G(alpha)s, G(betagamma) Stimulates adenylyl cyclase glycogenolysis, lipolysis

Answer 203

Adrenaline, noradrenaline G(alpha)i, G(betagamma) Inhibits adenylyl cyclase

Answer 204

Adrenaline, noradrenaline G(alpha)q, G(betagamma) Stimulates Phospholipase C Stimulates smooth muscle contraction

Answer 205

Light G(alpha)t, transducin Stimulates cyclic GMP phosphodiesterase Visual excitation

Answer 206

Acetylcholine G(alpha)i, G(betagamma) Inhibits adenylyl cyclase, stimulates K+ channels Slowing of cardiac pacemaker

Answer 207

Acetylcholine G(alpha)q, G (betagamma) Stimulates Phospholipase C

Answer 208

Adenylyl cyclase ATP--> cyclic AMP Phospholipase C PIP2 --> IP3 + DAG Phosphoinositide-3-kinase (PI3K) PIP2--> PIP3 cGMP Phosphodiesterase cyclic GMP --> 5'GMP

Answer 209

Voltage gated Ca2+ channels | G protein regulated inwardly rectifying K+ channels (GIRKs)

Answer 210

(Noradrenaline, dopamine) Agonist binds to GPCR (beta-adrenoreceptors, D1 dopamine receptors, H2 histamine receptors) stimulating Gs protein G protein splits into alpha-GTP and betagamma Alpha-GTP stimulates ADENYLYL CYCLASE Stimulates ATP to cAMP conversion. cAMP activates cAMP dependent protein kinase (PKA), Epacs- guanine nucleotide exchange factors, Cyclic nucleotide gated ion channel Causes increased glycogenolysis and gluconeogenesis in the liver, increased lipolysis in adipose tissue, relaxation of a variety of types of smooth muscle and positive inotropic and chronotropic effects in the heart.

Answer 211

(Noradrenaline, dopamine) Antagonist binds to GPCR (alpha2-adrenoreceptors, D2 dopamine receptors, mu-opoid receptors) stimulating Gi protein G protein splits into alpha-GTP and betagamma Alpha-GTP inhibits ADENYLYL CYCLASE Inhibits ATP to cAMP conversion. cAMP no longer activates cAMP dependent protein kinase (PKA), Epacs- guanine nucleotide exchange factors, Cyclic nucleotide gated ion channel

Answer 212

Through cAMP dependent protein kinase (PKA)

Answer 213

cAMP dependent kinase is made up of 2 regulatory subunits and 2 catalytic subunits The 2 R subunits rest in the C site preventing their action Binding of 2 cAMPs per R subunit causes the release of the C subunits which are thus free to act as protein kinases and add phosphates to aa residues (serine and threonine) on proteins Phosphorylation is a critical way for the cell to send out instructions

Answer 214

Agonist binds to the GPCR (alpha1- adrenoreceptors, M1 muscarinic receptors, H1histamine receptors) stimulating Gq protein G protein splits into alpha(GTP) and betagamma Alpha(GTP) activates Phospholipase C Phospholipase C catalyses the cleavage of membrane phospholipid (PIP2) into 2 second messengers- IP3(hydrophilic) and DAG(lipophilic) IP3 stimulates release of calcium from endoplasmic reticulum via IP3 proteins Increase in calcium in the cell and DAG activates Protein kinase C causing phosphorylation

Answer 215

Key feature of many cell signalling pathways E.g.- a few molecules of adrenaline binding to the cell surface B adrenoreceptors may cause a relatively massive cellular response. The B-adrenoreceptor--> Gs protein--> adenylyl cyclase part of the cascade causes relatively little amplification- never the less activation of adenylyl cyclase generates many molecules of cyclic AMP which then activate the enzyme PKA.

Answer 216

Cyclic GMP Phosphodiesterase is a specialised mechanism found in the photoreceptive cells of the retina. It regulates the breakdown of the second messenger cyclic GMP phosphodiesterase by Gt (Transducin). Light sensing protein rhodopsin activates Gt which activates cGMP phosphodiesterase enzyme to hydrolyse cyclic GMP to 5'GMP

Answer 217

1. Once a receptor has productively interacted with a G-Protein, the binding of the agonist is weakened and agonist-receptor dissociation is likely to occur. 2. Whilst activated, the receptor is susceptible to a variety of protein kinases that phosphorylate the receptor and prevent it activating further G-Proteins. This comprises an important part of the receptor desensitisation phenomenon observed for most G-Protein-Coupled Receptors. 3. The active lifetime of a-GTP may be limited by cellular factors that stimulate the intrinsic GTPase activity of the Ga subunit. 4. Enzymatic activities in the cell are such that the basal state is favoured. Therefore cells contain high activity enzymes that metabolise second messengers, rapidly returning their levels to the basal. 5. Enzymatic cascades activated downstream of second messenger/protein kinase activation act to oppose their effect.

Answer 218

The rate at which the sinoatrial node fires an action potential can be affected by Ach release by the parasympathetic nerves. The predominant receptor type is M2 muscarinic cholinoceptors and activation of these increases the open probability of K+ channels via Gi. Increase plasma membrane permeability to K+ causes hyperpolarisation, slowing the intrinsic firing rate, resulting in a negative chronotropic effect.

Answer 219

Sympathetic innervation of the cardiac ventricles (and/or circulating adrenaline) can influence the force of contraction (inotropy). Activation of B-Adrenoceptors (predominantly B1), increases the open probability of voltage operated calcium channels (VOCCs) via Gs. Gs both interact directly with the VOCCs, and indirectly via cyclic AMP --> PKA --> Phosphorylation and activation of VOCCs. The influx of Ca2+ brings about a positive inotropic effect.

Answer 220

Noradrenaline --> alpha1-adrenoceptors (vasculature smooth muscle cells)--> Gq --> Phospholipase C --> IP3 (and DAG)--> Release of Ca2+ from ER --> Rise in Ca2+ and DAG activates protein kinase C pathways --> vasoconstriction

Answer 221

(CNS and PNS) Pre-synaptic G-Protein-Coupled Receptors can influence Neurotransmitter release. For example, pre-synaptic mu-opiod receptors can be stimulated, either by endogenous opiods or by analgesics such as morphine to couple to GaI proteins. Morphine --> (mu-opoid) GPCR --> Gi protein --> alpha GTP and betagamma --> betagamma causes a decrease in VOCC activity and decrease in calcium influx and neurotransmitter release

Answer 222

ACh --> M3 muscarinic receptor (bronchiole smooth muscle cells)--> Gq --> Phospholipase C --> IP3 (and DAG)--> Release of Ca2+ from ER --> Rise in Ca2+ and DAG activates protein kinase C pathways --> bronchoconstriction

Answer 223

Diverse range of stimuli, receptors, G proteins and effectors

Answer 224

Specific ligand receptor interactions, specific G protein alpha subunits (betagamma) recruited which are coupled to particular effector pathways

Answer 225

For an extracellular stimulus (which may amount to only a few molecules of a hormone interacting with appropriate cell surface receptors) to generate an intracellular response, amplification of the signal is essential. Therefore an important role of the receptor-G protein-effector signalling system is to allow such amplification to occur. Amplification of the initial signal can be achieved at a number of levels: 1. Activated receptor can cause (sequential) GTP/GDP exchange on more than one G protein 2. An activated G alpha-GTP/free Gβγ can activate multiple effector molecules 3. Effector molecules act catalytically. Thus, activation of adenylyl cyclase by alpha s-GTP results in conversion of 100-1000s of molecules of ATP to cyclic AMP. Similarly, the opening of an ion channel by alpha-GTP allows 100-1000s of ions to move across the plasma membrane.

Answer 226

Currently 40 % of all available prescription drugs exert their therapeutic effects directly (as agonists or antagonists) or indirectly at GPCRs

Answer 227

Binds to the receptor and activates it (leading to intracellular signal transduction events) - Antiasthma- B2 adrenoreceptor agonists= bronchodilation- SALBUTAMOL, SALMETEROL - Analgesia/ anaesthesia - mu-opiod receptor agonists= modulation betagamma, decrease in VOCC and decreased Ca2+ influxe- MORPHINE, FENTANYL

Answer 228

Bind to the receptor but do not activate it (block the effects of agonists at the receptor) - cardiovascular (e.g. Hypertension) - B adrenoreceptor antagonists- PROPRANOLOL, ATENOLOL - neuroleptics (e.g. antischizophrenic) - D2 dopamine receptor antagonists- HALOPERIDOL, SULPIRIDE

Answer 229

Nephrogenic diabetes insipidus can be caused by a loss of function mutation to the V2 vasopressin receptor Retinitis pigmentosa can be caused by a loss of function mutation to rhodopsin

Answer 230

Familial male precocious puberty is caused b a gain of function mutation to the Luteinising hormone (LH) receptor

Answer 231

``` Toxin complexes bind to the cell and an enzyme is injected into the cell Both toxins (cholera and pertussis) are important experimental tools used to study GPCR- G protein signalling Cholera and pertussis toxins are enzymes that ADP-ribosylate specific G proteins ```

Answer 232

CTx eliminates GTPase activity of G(alpha)s G(alpha)s becomes irreversibly activated ADP RIBOSYLATION of G(alpha)s by CTx prevents the de activation of Gs protein mediated signalling

Answer 233

PTx interferes with GDP/GTP exchange on G(alpha)i G(alpha)i becomes irreversibly inactivated ADP RIBOSYLATION of G(alpha)i by PTx prevents Gi protein activation by GPCRs

Answer 234

By binding to a target (most of the time reversibly) | Binding governed by association and dissociation rates

Answer 235

Proteins | There are some exceptions – some antimicrobial and antitumour drugs fbnd DNA

Answer 236

``` Enzymes GPCRs Ion channels Transporters Nuclear hormone receptors Other receptors Integrins Miscellaneous ```

Answer 237

The concentration of drug molecules around receptors is critical in determining drug action Drugs of equivalent molar concentrations have the same concentration of drug molecules Drugs of the equivalent concentrations by weight may not have the same concentration of drug molecules

Answer 238

6 x 10 ^23

Answer 239

1mole in 1litre

Answer 240

Molarity = grams/litre divided by molecular weight

Answer 241

The likelihood of a ligand binding to its target

Answer 242

The likelihood of activation of the receptor and a response (taking into account cell and tissue components e.g number of receptors)

Answer 243

The likelihood of activation of the receptor

Answer 244

Drugs that bind to receptors and cause a response – have affinity and efficacy

Answer 245

Drugs that bind to receptors but don't cause response – they have affinity only

Answer 246

Information is often obtained by binding of a radioligand (radio active version of the ligand) Affinity can be determined by means other than radioligand binding

Answer 247

Proportion of bound receptors versus concentration of drug in logarithmic terms

Answer 248

Rectangular hyperbole

Answer 249

The maximum binding capacity | Tells us about the total number of receptors

Answer 250

Measure of affinity; concentration of drug needed for 50% occupancy

Answer 251

High affinity (because a lower concentration of drug is required to occupy 50% of the receptors)

Answer 252

Low affinity (because a higher concentration of drug is required to occupy 50% of the receptors)

Answer 253

Rectangular hyperbole

Answer 254

A change in the signalling pathway | A change in cell /tissue behaviour (e.g.contraction)

Answer 255

Where we have a known concentration of drug at the site of action Using measuring a response and cells/tissues

Answer 256

Where the concentration at the site of action is unknown | Used when measuring response in a whole animal

Answer 257

The maximum response

Answer 258

Effective concentration giving 50% of the maximum response; a measure of potency (dependent on intrinsic efficacy and affinity and number of receptors and efficacy)

Answer 259

Efficacy is measured in relative terms, with no absolute scale. Agonists with different E max values have different efficacy. However agonists with the same E max values may not have identical efficacy The two drugs MAY DIFFER IN AFFINITY, meaning that the relationship between the occupancy and response will be different for the two agonists – one may be more able to convert binding into function Efficacy cannot be determined without knowing affinity

Answer 260

Reversible airflow obstruction and from here plasm Treatment goal is to activate B2 adrenoreceptors (GPCR) to relax airways But there are B adrenoreceptors elsewhere in the body (e.g. B1 in the heart) - increase the rate and force of contraction (angina) Salbutamol- Kd B1 20 micromolar B2 1 micromolar (low affinity (in comparison to salmeterol e.g.) but utility is enhanced by B2 selective efficacy and route of administration) Salmeterol- Kd B1 1900nM B2 0.55nM (high affinity; no selective efficacy- selectivity based on high affinity)

Answer 261

Affinity: does it bind? Efficacy: does it do what it supposed to do? Selectivity – off target effects? Drug metabolism/pharmacokinetics – how does the body deal with it? Physiochemical properties – solubility, pH, stability, crystallinity

Answer 262

In some cases less than 100% receptor occupancy will give 100% response EC50 < Kd (50% of the maximal response takes less than 50% of receptor occupancy) The relationship between receptor occupancy and response is non-linear and influenced by amplification in the signal transduction pathway and the fact that the response is limited by postreceptor event/properties of the tissue Some tissues have more receptors than required to produce a maximum response. They have spare receptors or a receptor reserve. Spare receptors increase sensitivity – allowing a response at low concentrations of agonist

Answer 263

Changing receptor number changes agonist potency and can affect the maximum response

Answer 264

When comparing the ability of different agonists to evoke responses in tissues, it is sometimes observed that some drugs cannot produce a maximal effect, even with full receptor occupancy. These drugs are referred to as partial agonists

Answer 265

EC50 (measure of potency) = Kd (measure of affinity) for a partial agonist The potency of a drug is dependent on both its affinity and efficacy; therefore partial agonists can be more or less potent than full agonists

Answer 266

Binding to target, preventing the binding of the full agonist to the target

Answer 267

Increasing receptor number can change a partial agonist into a full agonist The partial agonist still has low intrinsic efficacy at each receptor BUT there are sufficient receptors to contribute to a a full response (E.g. Partial agonists may each bind to LOTS of targets and activate them a little bit (low efficacy); all the activated receptors accumulate to give a FULL response)

Answer 268

Maximal response indicates intrinsic activity (on a conc response graph) Partial agonist have lower efficacies than full agonists but full agonists with identical intrinsic activities may have different efficacies

Answer 269

Opioids are used for pain relief and recreationally (heroin) – but can cause respiratory depression and DEATH They act through mu- opioid receptors (GPCRs) Morphine is a full agonist of the receptor- used for pain relief Buprenorphine is a partial agonist, with a higher affinity but lower efficacy than morphine - Can be advantageous if it provides adequate pain control, as there is less respiratory depression - Used in gradual withdrawal from overuse of opioids *example of a partial agonist acting as an antagonist of a full agonist

Answer 270

Block the effects of agonists Prevent receptor activation by agonists Affinity not efficacy

Answer 271

Reversible competitive antagonism Irreversible competitive antagonism Non-competitive antagonism

Answer 272

Commonest and most importantly therapeutics Competitive antagonists compete with agonists for binding Inhibition is surmountable- still get full biological response Adding more agonist makes it harder antagonist to compete IC50 = concentration of antagonist giving 50% of the inhibition response = index of antagonist potency * *(determined by strength of agonist) Kd is used to describe antagonist affinity

Answer 273

Causes a parallel shift to the right of the agonist concentration response curve

Answer 274

Naloxone- high affinity for mu opioid receptors | * Reversal of opioid mediated respiratory depression

Answer 275

Causes a parallel shift to the right of the agonist- concentration response curve at higher concentrations suppress the maximal response

Answer 276

Phenoxybenzamine- binds to alpha 1 adrenoreceptors * Hypertension episodes in pheochromocytoma - -> tumour of adrenal chromaffin cells - -> excessive adrenaline/ NA - /(Phenoxybenzamine)-> vasoconstriction (alpha 1 adrenoreceptors)

Answer 277

Antagonist binds to other allosteric sites Provides binding site for: agonists and molecules that enhance or reduce effects of agonists (no competition of the binding site but affects orthosteric ligand affinity or efficacy)

Answer 278

NMDA receptor Glutamate site and ketamine site (antagonist does not competed with glutamate site but with the ketamine site- it lowers the ionic flow Analgesia- reduced central excitation

Answer 279

Antagonist dissociates slowly or not at all With increased concentration of antagonist or increased time more receptors are blocked by the antagonist- non surmountable- can't get full biological response back

Answer 280

What the body does to a drug

Answer 281

Pharmaceutical process – is the drug getting into the patient? Pharmacokinetic process – is the drug getting into the site of action? Pharmacodynamic process – is the drug producing the desired pharmacological effect? Therapeutic effect – is the pharmacological effect translated into a therapeutic effect?

Answer 282

Absorption Distribution Metabolism Elimination

Answer 283

Solid (tablet) or liquid – if it's solid, solubility and acid stability in stomach must be considered; rate of action depends on dissolution

Answer 284

Focal – eye, skin, inhalation Systemic – enteral – sublingual, oral, rectal; parenteral – subcutaneous, intramuscular, intravenous, inhalation, transdermal

Answer 285

Proportion of a drug given orally (or other routes except intravenously) that reaches the circulation unchanged Can be expressed as: amount – depends on GI absorption (food/disease) and first pass effect– Measured by area under curve of blood drug level versus time plot Can be expressed as: rate – depends on pharmaceutical factors and rate of gut absorption – measured by peak height and rate of rise of drug levels in the blood

Answer 286

100 x (AUC oral/ AUC injected)

Answer 287

Tells us about the safety of the drug | Allows us to maximise the time that the drug is in the therapeutic window

Answer 288

Maximum tolerated dose/minimum effective dose LD 50/ED 50 Lethal dose to 50% of people/ Effective dose to 50% of people

Answer 289

Desired therapeutic effect and unwanted adverse effect

Answer 290

Drugs administered intravenously enter directly into the systemic circulation and have direct access to the rest of the body FPM- Drugs administered orally are first exposed to the liver via the portal system and may be extensively metabolised before reaching the rest the body (systemic circulation)- lignocaine, opiates, propranolol, glyceryl trinitrate

Answer 291

Parenteral route- IV, SC, IM Rectal route- drainage to portal and systemic system Sublingual route- glyceryl trinitrate in angina (GTN spray)

Answer 292

Theoretical volume into which the drug has distributed assuming that this occurs instantaneously (calculated as amount given/plasma concentration at T =0) Obtained by extrapolation of plasma levels at zero time Tells us about solubility of drugs

Answer 293

The drug is lipid soluble

Answer 294

The drug is water soluble

Answer 295

``` Amount of free drug determines the drug actions at receptors- not the total level Displacement of drugs from binding sites causes protein binding drug interactions (class II drugs free up class 1 drugs from binding sites allowing them to ACT ON RECEPTORS or BE ELIMINATED) Protein binding interactions are important when the OBJECT DRUG (class I) - is highly bound to albumin, has a small volume of distribution (water soluble) and has a low therapeutic ratio (not very safe) because administration of a PRECIPITANT DRUG (Class II) frees up the object drug to allow it to have its action or be eliminated Object drug (Class I) - is used at a dose which is much lower than the number of albumin binding sites Precipitant drugs (Class II) - is used at a dose which is much greater than the number of available albumin binding sites (and will displace class I drugs from binding sites) Binding interactions are TRANSIENT/ TEMPORARY - although free drug levels rise (greater effects at receptors)- elimination rate will also rise (since this depends on free drug levels); steady state is restored quickly in a few days ``` *bear in mind though that when a patient is taking the object drug, taking a supplementary precipitant drug will temporarily lead to higher free levels of the object drug and hence there is a higher risk of toxicity.

Answer 296

1- warfarin- sulphonamides, aspirin, phenytoin 2- tolbatamide- sulphonamides, aspirin 3- phenytoin- valproate

Answer 297

If drugs are metabolised by enzymes that obey the Michaelis Menten kinetics then: Rate of metabolism/ elimination = Vmax [C] / Km + [C] Can be zero or first order kinetics

Answer 298

Rate of elimination is directly proportional to the drug level Constant fraction of drug is eliminated in unit time Half life can be defined - as the rate of decline of plasma drug level is directly proportional to drug level [C] < Km --> rate of metabolism = Vmax[C]/Km Gives a predictable therapeutic response from dose increases (most drugs behave this way) Straight line- logs

Answer 299

Rate of elimination is a constant [C] > Km --> rate of metabolism = Vmax[C]/[C] = Vmax So enzyme is saturated Therapeutic response can suddenly escalate as elimination mechanisms saturate Straight line- not logs

Answer 300

Rate of drug metabolism vs dose of a drug At low doses drug metabolism is first order, that is proportional to drug dose At high dose, drug metabolism is zero order = constant and independent of drug dose

Answer 301

A steady state will be reached within 5 half lives, irrespective of dose or frequency of administration

Answer 302

If an immediate effect is necessary If a half life is long - often determined by volume of distribution

Answer 303

Metabolism by the liver | Excretion by the kidney

Answer 304

Phase I- reactive group exposed/ added to stable/ unreactive drug (prodrug); oxidation, reduction, hydrolysis; cytochrome P450 enzyme system; NADPH; enzymes are inducible or inhibitable Phase II- conjugation with a polar molecule = water soluble complex; glucuronic acid, glutathione, sulphate ions; specific enzymes & UDPGA

Answer 305

Phenobarbitone- warfarin, phenytoin Rifampcin- oral contraceptive Cigarettes- theophylline

Answer 306

Cimetidine- warfarin, diazepam

Answer 307

Drugs have a low therapeutic ratio Drug is being used at the minimum effective concentration Drug metabolism follows zero order kinetics

Answer 308

Free unbound drug enter the glomerular filtrate There is active secretion of other drugs in the proximal tubule In the distal tubule there is reabsorption of the lipid soluble unionised drug (passes membrane easily) to be recirculated in the blood- dependent on pH Ionised lipid insoluble drug is passed into the urine pK = pH at which half of the drug is ionised and half of it is unionised

Answer 309

Unionised/ uncharged lipid soluble drugs can pass the membrane more easily For weak acids (e.g. aspirin), making the urine alkaline will make the drug ionised, so there will be less tubular absorption because the charged drug stays in the tubule lumen. For weak bases (e.g. amphetamine), making the urine acidic will make the drug ionised, so there will be less tubular absorption because the charged drug stays in the tubule lumen.

Answer 310

In renal disease, 1. If the drug or active metabolite is excreted as its main route of elimination, T1⁄2 is prolonged. Therefore, lower the maintenance dose. 2. It takes 5 T1⁄2s to reach a new equilibrium every time you change the dose. 3. The loading dose is unchanged, unless volume of distribution changes (e.g. digoxin) 4. Protein binding of drugs is altered

Answer 311

Alcohol- inhibits metabolism Aspirin, sulphonamides, phenytoin- displacement from plasma proteins Broad spectrum antibiotics- reduced vitamin K synthesis by gut Aspirin- reduced platelet function

Answer 312

Barbiturates, rifampicin- Induces liver metabolizing enzymes

Answer 313

In liver disease, be careful with drugs with a low therapeutic ratio. Cellular dysfunction- warfarin, phenytoin, theophylline Portasystemic shunts- opiates, propranolol Reduced blood flow- opiates, propranolol, lignocaine Reduced albumin- affects drug binding to plasma protein

Answer 314

Amphipathic molecules form one of two structures in water, micelles and bilayers. Bilayer formation is spontaneous in water and is driven by the van der Walls forces between the hydrophobic tails. Co-operative structure is stabilised by non-covalent forces; electrostatic and hydrogen bonding between hydrophilic moieties and interactions between hydrophilic groups and water.

Answer 315

All or nothing- only occur if they reach a threshold Distinct signals- due to inactivation of Na+ channels and Hyperpolarisation in RRP Propagated without loss of amplitude- local current theory, length constant, previous region in hyperpolarised state

Answer 316

Length constant- distance its takes for the potential to fall to 37% of its original value The further the local current spreads the faster the conduction velocity of the axon

Answer 317

Controls involuntary functions Controls all efferent outputs of the body, except innervation of skeletal muscle - somatic efferent nervous system Regulated by afferent inputs SNS and PNS Tissues are not always innervated by both branches but where they are SNS and PNS may have opposing effects- notable exceptions= ANS control of salivary secretions

Answer 318

Stressful situations, fight or flight Thoraco-lumbar outflow (lateral horn of grey matter in the spinal cord) Short myelinated preganglionic neurone nAchR (ligand gated ion channel) - sympathetic chain Long unmyelinated postganglionic neurone NA alpha1,2 and beta 1,2 (GPCR)

Answer 319

Sweat glands- nAchR and mAchR | Adrenal glands- nAchR and chromaffin cells --> adrenaline into BLOOD

Answer 320

Atria/ ventricles SAN- tachycardia Ventricles- positive inotropy

Answer 321

Vasculature- arteriolar venous contraction Lungs/ GI/ GU tract- bronchiolar, intestinal, uterine relaxation GU tract- bladder spinchters contraction Eye- radial muscle contraction

Answer 322

Salivary- increased viscous secretion

Answer 323

Renin release

Answer 324

Basal activities, rest and digest, basal heart rate Cranial sacral outflow (lateral horn of grey matter in medulla and spinal cord) Long myelinated preganglionic neurone nAChR (ligand gated ion channel) Short unmyelinated post ganglionic neurone mAchR (GPCR)

Answer 325

SAN- bradycardia | AVN- reduced cardiac conduction velocity

Answer 326

Lungs- bronchiole, bronchiolar contraction GI tract- increased intestinal mobility/ secretion GU tract- bladder contraction/ relaxation and penile erection Eye- ciliary muscle and iris spinchter contraction

Answer 327

Increased sweat/ salivary/ lacrimal secretion

Answer 328

Catecholamines disorders- pheochromocytoma, baroreflex failure Central autonomic disorders- multiple system atrophy (MSA)- autonomic dominated Orthostatic intolerance syndrome Paroxysmal autonomic syncopes Peripheral autonomic disorders - Guillain Barré syndrome - familial dysautonomia

Answer 329

Hereditary sensory and autonomic neuropathy (HSAN) type III Autosomal recessive Defective gene encodes IkB kinase complex associated protein affecting development and survival of sensory sympathetic and some parasympathetic neurons Almost exclusively seen in children of Ashkenazi Jewish descent (1/3700 live births in Israel carry a mutant gene) FD infants normally present because of feeding/ swallowing difficulties FD symptoms include- dysautonomic crises, GI tract dyscoordination, cardiovascular and respiratory dysfunction, altered sensory perception, spinal curvature

Answer 330

Acetylcholine is synthesized by the enzyme choline acetyltransferase from choline (an essential dietary constituent) and the metabolic intermediate acetyl CoA in the cytoplasm of cholinergic terminals. Although some of the ACh is degraded by cytoplasmic cholinesterase, the majority is transported into synaptic vesicles by an indirect active transport mechanism similar to that described above for noradrenaline. Cholinergic terminals possess numerous vesicles containing high concentrations (>100 mM) of ACh that can be released by Ca2+-mediated exocytosis. Released ACh can interact with both pre- and post-synaptic cholinoceptors. However, the opportunity to interact with receptors is limited by ACh in the synaptic cleft being acted upon by cholinesterase, which rapidly degrades ACh to choline and acetate. The activity of this enzyme is higher at fast (nicotinic) cholinergic synapses limiting the synaptic cleft half-life of ACh to a few milliseconds. Most choline is recaptured by a choline transporter present in the synaptic terminal.

Answer 331

ay have a preferential ganglion (e.g. trimethaphan) or neuromuscular (e.g. tubocurarine, pancuronium) blocking action. The former class of agent is rarely used clinically, whilst the latter are used to cause muscle paralysis during anaesthesia.

Answer 332

Muscarinic cholinoceptor agonists vary in their muscarinic/nicotinic selectivity and resistance to degradation by cholinesterase. Note that no agent shows significant selectivity between muscarinic receptor subtypes (i.e. M1, M2, M3 selectivity). The major clinical use is in the treatment of glaucoma (raised intraocular pressure) where the agent (usually pilocarpine) can be applied in the form of eye drops. Minor uses include suppression of atrial tachycardia, increasing gastrointestinal activity after abdominal surgery, and stimulation of bladder emptying.

Answer 333

Muscarinic cholinoceptor antagonists show little selectivity for receptor subtypes, but vary in their peripheral versus central actions. Hyoscine (methylscopolamine) was used as anaesthetic premedication as it decreases bronchial and salivary secretions, prevents reflex bronchoconstriction, reduces any bradycardia induced by the anaesthetic and also has a sedative effect. Local application of a poorly absorbed muscarinic cholinoceptor antagonist (e.g. ipratropium bromide) can be used to treat bronchoconstriction in asthmatics where the constriction is caused by increased parasympathetic discharge. Pupillary dilatation and paralysis of accommodation can be caused by muscarinic cholinoceptor antagonists (e.g. homatropine, tropicamide) facilitating opthalmoscopic examination and having a beneficial effect in various (rare) eye conditions.

Answer 334

Cholinesterase inhibitors (e.g. edrophonium, physostigimine, dyflos) differ in their longevity of action and their peripheral versus central effects. They are used to acutely reverse the effects of non-depolarizing neuromuscular blocking agents used in anaesthesia, in the topical treatment of glaucoma and in the treatment of myasthenia gravis. Recently, cholinesterase inhibitors (e.g. tacrine, donepezil) have been introduced for the treatment of the early stages of Alzheimer’s disease.

Answer 335

Noradrenaline (and the other signalling molecules dopamine and adrenaline) is synthesized from tyrosine within the nerve terminal (see diagram below). The rate-limiting enzyme is tyrosine hydroxylase. The presence of phenylethanolamine N-methyltransferase in the chromaffin cells of the adrenal medulla allows adrenaline to be synthesized as the main product for release. The enzyme dopamine B-hydroxylase is located within synaptic vesicles and therefore newly synthesized dopamine is transported into the vesicle prior to conversion to noradrenaline. The vesicular transporter recognizes not only dopamine, but also noradrenaline allowing noradrenaline to be recycled following release and re-uptake (see below). Under most circumstances the cytoplasmic noradrenaline concentration is low, whilst the intravesicular concentration is very high (0.5-1.0 M), this is possible because the vesicular transporter exploits a H+- ATPase-generated cytoplasm:vesicle H+-gradient to move catecholamines against their concentration gradient. Cytoplasmic noradrenaline is susceptible to enzymic breakdown by monoamine oxidase (MAO) Noradrenaline release is triggered by depolarization of the nerve terminal membrane, Ca2+-entry and fusion of vesicles with the pre-synaptic plasma membrane (Ca2+-mediated exocytosis). Released noradrenaline can interact with both pre-and post-synaptic adrenoceptors. However, the opportunity to interact with receptors is limited by a high affinity reuptake system (called “Uptake 1”) which acts to rapidly remove noradrenaline from the synaptic cleft, rapidly decreasing the localized concentration increase following release and thus terminating its actions. Any noradrenaline escaping from the synaptic cleft is removed from the extracellular space by another, widespread, lower affinity re-uptake system (“Uptake 2”). Noradrenaline recaptured by the nerve terminal has two fates: it can be re-vesiculated and therefore undergo further release/ reuptake cycles or it can be metabolized (initially by MAO).

Answer 336

Alpha-Methyl-tyrosine competitively inhibits tyrosine hydroxylase and, therefore, blocks de novo synthesis of noradrenaline. Only clinical use is to inhibit noradrenaline synthesis in pheochromocytoma (noradrenaline-secreting tumour).

Answer 337

Alpha-Methyl-DOPA is taken up by adrenergic neurones and is converted to alpha-methyl- noradrenaline by the sequential action of DOPA decarboxylase and dopamine beta-hydroxylase. Unlike the true neurotransmitter, alpha-methyl- noradrenaline is poorly metabolized and therefore accumulates in the synaptic vesicles of noradrenergic terminals. It is released by Ca2+- mediated exocytosis, but differs from noradrenaline in that it preferentially activates pre-synaptic alpha2-adrenoceptors reducing transmitter release. The ability of α-methyl-DOPA to form the “false transmitter” alpha-methyl- noradrenaline has been exploited in the treatment of hypertension.

Answer 338

CarbiDOPA inhibits DOPA decarboxylase in the periphery, but not in the CNS (because it does not cross the blood-brain-barrier). It is used in combination with L- DOPA in the treatment of Parkinson’s disease (a dopamine deficiency in the basal ganglia).

Answer 339

Adrenergic blocking drugs (e.g. guanethidine, bretylium) are selectively concentrated in terminals by Uptake 1. They act via a variety of mechanisms, including a local anaesthetic action reducing impulse conduction and Ca2+-mediated exocytosis, a partial blocking action on reuptake of neurotransmitter, and depletion of noradrenaline from synaptic vesicles. They are now rarely used therapeutically, because of severe side-effects (postural hypotension).

Answer 340

Indirectly-acting sympathomimetic agents (IASAs)(e.g. tyramine, amphetamine, ephedrine) are structurally related to noradrenaline. Because they are only weak agonists at adrenoceptors they are thought to exert their actions by other/additional mechanisms. IASAs are recognised and transported into the adrenergic terminal by Uptake 1 and are taken up into synaptic vesicles where they cause noradrenaline to leak from the vesicle. The displaced noradrenaline can leak into the synaptic cleft by a mechanism unrelated to Ca2+-mediated exocytosis. The extent to which noradrenaline leaks into the synaptic cleft can be greatly enhanced by inhibition of the noradrenaline-degrading enzyme MAO.

Answer 341

Uptake 1 inhibitors comprise an important class of therapeutic agents - the tricyclic antidepressants (e.g. amitriptyline) - however, these agents exert their therapeutic actions centrally and their possible peripheral actions (enhancement of sympathetic actions to cause, e.g. tachycardia and cardiac dysrhythmias) are unwanted side-effects avoided by choice of drug and dose.

Answer 342

Adrenoceptor agonist pharmacology has produced highly receptor subtype-selective agents, therefore a knowledge of the pre- and post- synaptic adrenoceptor subtype disposition may allow the rational use of adrenoceptor agonists to achieve specific therapeutic ends. Important uses of adrenoceptor agonists are given below:  Selective beta 1-agonists (e.g. dobutamine) can cause positive inotropic and chronotropic effects which may be useful in treating circulatory shock - however, all beta 1-agonists are prone to causing cardiac dysrhythmias  Selective beta 2-agonists (e.g. salbutamol, terbutaline) are highly effective in reversing bronchconstriction in asthmatics  Selective alpha 1-agonists (e.g. phenylephrine, oxymetazoline) are used as nasal decongestants. Alpha 1-agonists (though more usually adrenaline) may be given in conjunction with a local anaesthetic injection to cause local vasoconstriction and so retard the dissipation of the anaesthetic.  Selective alpha 2-agonists (e.g. clonidine) can be used as anti- hypertensive agents. This action is brought about partly through stimulation of inhibitory pre-synaptic receptors which decrease noradrenaline release and partly through a centrally-mediated action

Answer 343

Adrenoceptor antagonists are also widely used therapeutically. Most useful drugs are alpha or beta- adrenoceptor-selective and increasingly drugs which distinguish alpha and beta- subtypes are being used to reduce the unwanted side-effect profiles associated with therapy. Important uses of adrenoceptor antagonists are given below:  alpha-adrenoceptor antagonists (e.g. phentolamine, and the irreversible blocker phenoxybenzamine) are used to cause peripheral vasodilatation (i.e. oppose sympathetically-mediated vasoconstriction) in the treatment of peripheral vascular disease. They are not used to treat hypertension because they cause postural hypotension and reflex tachycardia.  Selective alpha 1-adrenoceptor antagonists (e.g. prazosin) are used in the treatment of hypertension, although postural hypotension and impotence are still common unwanted side-effects.  beta-adrenoceptor antagonists (e.g. propranolol) or beta 1-adrenoceptor antagonists (e.g. atenolol) are used to treat hypertension, cardiac dysrhythmias, angina and myocardial infarction. Possible unwanted side-effects include bronchoconstriction (particularly using non-selective β-adrenoceptor antagonists in patients susceptible to parasympathetically-mediated brochospasm), bradycardia, cold extremities, insomnia and depression. Some trials report that the use of partial agonists (e.g. alprenolol, oxprenolol) cause fewer side-effects as they provide a low tonic stimulation of beta -adrenoceptors whilst still blocking receptor stimulation by noradrenaline.