M&R Flashcards
What is the only phospholipid not based on glycerol?
Sphingomyelin
List 5 general functions of biological membranes
- Continuous highly selective permeability barrier
- Allows control of the enclosed chemical environment
- Recognition - signalling molecules, adhesion proteins, immune surveillance
- Communication - control the flow of information between cells and their environment
- Signal generation in response to stimuli (chemical, electrical)
What significance does CIs double bonds in fatty acid chains on phospholipids have?
Introduce a kink in the chain, reducing phospholipid packing
What are glycolipids?
Sugar containing lipids
Given2 types of glycolipids
Cerebrosides - head group sugar monomers
Gangliosides - head group oligsosaccharides (sugar multimers)
What % respectively is a membrane constituted of lipid, protein and carbohydrate?
40% lipid
60% protein
1-10% carbohydrate
(20% of total weight is H2O)
What % of the total membrane lipid is cholesterol?
45%
Distribution of other lipids is related to function and tissue specific
List the 4 permitted modes of mobility for lipid molecules in the lipid bilayer
- Intra-chain motion - kink formation in the fatty acyl chains
- Axial rotation (fast)
- Lateral diffusion wishing plane of bilayer (fast)
- Flip-flop - movement of lipid molecules from one half of the bilayer to the other half on a one for one exchange basis.
How can peripheral membrane proteins be removed?
Changes to pH or ionic strength
How are integral membrane proteins removed?
Use of agents (detergents, solvents) that compete for non-polar interactions in the bilayer
List the modes of motion permitted for proteins in bilayers
- Conformational change
- Rotational
- Lateral
No flip flop!
List 4 mechanisms of restriction of membrane protein mobility
- Aggregates
- Tethering
- Interactions with other cells
- Lipid mediated effects - proteins tend to separate out into the fluid phase or cholesterol poor regions.
What are hydropathy plots used for?
For determining how many transmembrane regions a protein has
What proteins is the erythrocyte cytoskeleton composed of?
Spectrin and actin
(actin-spectrin network, attached to the membrane by adapted proteins ankyrin and glycophorin, maintaining shape of RBCs)
What causes symptoms in hereditary Spherocytosis?
Spectrin is depleted by 40-50%
Erythrocytes ‘round up’, so increased cell lysis, decreased RBC lifespan
What causes symptoms in hereditary Elliptocytosis?
Spectrum molecules are unable to form heterotramers (fragile elliptoid cells)
Give 6 roles of transport processes
- Maintenance of ionic composition
- Maintenance of intracellular pH
- Concentration of metabolic fuels and building blocks
- Regulation of cell volume
- The extrusion of waste products of metabolism and toxic substances
- The generation of ion gradients necessary for the electrical excitability of nerve and muscle
Give 3 mechanisms for regulating protein transporters
Ligand gated - respond to ligand binding to receptor site
Voltage gated - respond gets in potential difference across membrane
Gap junction - close when cellular Ca2+ rises above 10uM, or cell becomes acidic
Name 2 things which influence is movement is active or passive
Concentration gradient
Membrane potential
What is the free ion distribution across the cell membrane for Na+?
145mM Extracellular
12mM Intracellular
What is the free ion distribution across the cell membrane for K+?
Extracellular 4mM
Intracellular 155mM
What is the free ion distribution across the cell membrane for Cl-?
Extracellular 123mM
Intracellular 4.2mM
What is the free ion distribution across the cell membrane for Ca2+?
Extracellular 1.5mM
Intracellular 10^-7 M (0.0001mM)
What does a uniport transport?
Transports a single molecule in a single direction
What does a symport transport?
Transports 2 molecules in the SAME direction
What does and anti port transport?
Transports 1 molecule in and 1 molecule out in a single cycle
Name 4 Ca2+ transporters
NCX (Na+ / Ca2+ exchanger)
SERCA (sarco(endo)plasmic reticulum Ca2+ - ATPase)
PMCA (plasma membrane Ca2+ - ATPase)
Ca2+ uniporters (mitochondria)
Name 2 calcium transporters with high affinity, low capacity
PMCA
SERCA
Name a calcium transporter which works via secondary active transport
NCX
What does sodium calcium exchanger (NCX) exchange?
3Na+ in for 1 Ca2+ out
Therefore is electrogenic, membrane potential dependent, can reverse in depolarised cells
Name 2 acid extruders
NHE (Na+/H+ exchanger)
NBC (sodium bicarbonate cotransporter. Na+ dependent Cl-/HCO3- exchanger)
Name a base extruder
AE - acidifies cell
Cl-/HCO3- exchanger (anion exchanger)
NBC - alkalinises cell
Na+-bicarbonate-chloride cotransporter
How do you measure a membrane potential?
Use a microelectrode (fine glass pipette, tip diameter
Are membrane potentials expressed inside to outside, or outside to inside?
Inside the cell relative to extracellular solution
What is the unit of measurement for membrane potentials?
Millivolts mV
What dominates the membrane ionic potential in most cells?
K+ channels
In the Nernst equation, what does z stand for?
X stands for the valency (charge) of the ion moving
What is depolarization?
Decrease in negativity
Cell interior becomes less negative
E.g. -70mV to -50mV
What is hyperpolarization?
Increase in negativity
Cell interior becomes more negative
E.g. Change from -70mV to -90mV
Given3 types of gating for channels
- Ligand gating - respond to binding of chemical ligand
- Voltage gating - respond to changes in membrane potential
- Mechanical gating - respond to membrane deformation e.g. In mechanoreceptors
What makes fast synaptic transmission?
Receptor is also a ligand-gated ion channel (binding & channel event are linked directly)
What do inhibitory synapses cause?
Hyperpolarization
Further from resting membrane potential/threshold
What makes slow synaptic transmission?
The receptor and channel are separate proteins
Name the 2 basic patterns for the separate proteins of slow synaptic transmission mechanisms
- Direct G-protein gating
2. Gating via an intracellular messenger
Name 2 factors other than ion channels which can influence membrane potential
- Changes in ion concentration
2. Electrogenic pumps (I.e. Na+/K+ - ATPase)
What is an action potential?
Change in voltage across membrane
Depends on ionic gradients & relative permeability of the membrane
Only occurs if a threshold level is reached - all or nothing
Propagated without loss of amplitude
By how much is the intracellular Na+ conc. Increases by during each action potential?
(Only) 40uM.
~0.4% increase
Name 3 ways of investigating the mechanism of action potential generation
- Voltage-clamping (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 currents flowing through individual ion channels to be measured)
Depolarization to threshold initiates an action potential where in the neurone?
Axon hillock
What is the ARP?
Absolute Refractory Period:
Nearly all Na+ channels are in the inactivated state
What is the RRP?
Relative Refractory Period:
Na+ channels are recovering from inactivation, the excitability returns towards normal as the number of channels in the inactivated state decreases
What is accommondation?
The longer the time taken for stimulus to reach threshold value, the larger the threshold value necessary to initiate an AP, and less positive (smaller) the peak. (More Na+ channels switched to inactive state)
Describe the structure of a Na+ channel
1 polypeptide chain, consists of 4 repeats.
Inactivating particle between repeat 3&4, which can plug pore.
Each repeat consists of 6 transmembrane domains.
No. 4 contains many +ve AAs, so causes conformational change on depolarization making it more likely inactivating particle will block pore (putting channel in the inactive state). This is reversed on hyperpolarization.
Describe the structure of a K+ channel
Consist of 4 alpha subunits
Each subunit has 6 transmembrane domains
No. 4 contains many +ve AAs, so causes conformational change on depolarization
How do local anaesthetics, e.g. Procaine, work?
Act by blocking Na+ channels (acting like inactivating particles)
In what order do local anaesthetics block conduction in nerve fibres?
Small myelinated axons
Non-myelinated axons
Large myelinated axons
How is the conduction velocity of an axon measured?
Distance between stimulating electrode and recording electrode, as well as time gap between stimulus as AP being registered are measured. S=D/T
List 3 properties that lead to high conduction velocity in an axon
- High membrane resistance
- Low membrane capacitance
- Large axon diameter (therefore low cytoplasmic resistance)
What is capacitance?
The ability to store charge - a property of the lipid bilayer
What cells myelinated peripheral axons?
Schwann cells
What cells myelinated axons in the CNS?
Oligodendrocytes
What is the rate of peripheral nerve regeneration?
1-5mm/day
Faster in bigger neurones
Describe the sequence of events that results in release of neurotransmitter at a synapse
Ca2+ entry via voltage-gated Ca2+ channels Ca2+ binds to synaptotagmin Vessel brought close to membrane Snare complex makes a fusion pore Transmitter released via this pore
What does synaptotagmin do?
Brings vesicle towards synaptic membrane
What does the snare complex do?
Enables formation of a fusion pore
How many ACh molecules bind to a nicotinic receptor? And what does it cause?
2
Opens ligand gated channel, enables K+ efflux and Na+ influx
Describe the structure of a nicotinic receptor
5 subunits
2 alpha subunits to which ACh binds, causing confirmational change which causes receptor pore to open
How does curare work?
Blocks ACh binding site (competitive)
Name the 2 types of blockers for nicotinic ACh receptors and describe their mechanism of action.
Competitive - block ACh binding
Depolarizing - agonists at ACh receptor, cause depolarization, Na+ channels open and therefore become inactivated. Prolonged activation as drug not broken down by AChE so not reset, Na+ channels remain inactivated.
What are nACh receptors?
Nicotinic Acetylecholine receptors
Which ACh receptor produces a faster response and why?
nAChR - fast as it is a ligand gated ion channel
mAChR - relatively slow as they are coupled to G-proteins which trigger a cascade of events in the cell.
Give 2 advantages of having a large inward gradient of Ca2+
- Changes in [Ca2+]i occur rapidly with movement of little Ca2+
- Little Ca2+ has to be removed to re-establish resting conditions
Give 2 disadvantages of having a large inward Ca2+ gradient
- Energy expensive
2. Inability to deal with Ca2+ easily leads to Ca2+ overload, loss of regulation and cell death.
How is the Ca2+ gradient maintained?
Relative Impermeability of the plasma membrane
Ability to expel Ca2+ across the plasma membrane via Ca2+-ATPase / Na+-Ca2+ exchanger
Ca2+ buffers
Intracellular Ca2+ stores
What is the feedback mechanism controlling intracellular Ca2+ levels?
[Ca2+]i levels increase
Ca2+ binds to calmodulin
Ca2+-calmodulin complex binds to Ca2+-ATPase
Ca2+-ATPase removes Ca2+
Give two types of channels which enable Ca2+ influx across the plasma membrane
Voltage-operated Ca2+ channels (VOCC) Ionotropic receptors (ligand gated)
What are the mechanisms for altering intracellular Ca2+ levels?
Ca2+ influx across plasma membrane
Ca2+ release from rapidly releasable intracellular stores (sarcoendoplasmic reticulum)
Non-rapidly releasable stores (mitochondria)
How is Ca2+ release from the Sarcoplasmic reticulum mediated?
G-protein coupled receptors (GPCRs)
What type of ACh receptors utilise G-proteins?
Muscarinic
How many parts is a heterotrimeric G-protein comprised of, and what are they called?
3
Alpha, beta, gamma
What is the active part of a heterotrimeric G-protein once it has been split?
Alpha
Beta and gamma remain stuck together
What does alpha q do?
Activates phosopholipase C, which converts PIP2 (a plasma protein) to
IP3 (free activator molecule)
+
diacylglycerol (remains in plasma membrane)
What does IP3 do?
IP3 binds to IP3 receptor on sarcoendoplasmic reticulum, enabling Ca2+ efflux from it.
In calcium induced calcium release, where does the Ca2+ come from?
VOCCs
Inotropic receptors
Intracellular stores
What activates the ryanodine receptor and what does it enable?
Ca2+ in cytoplasm (or possibly cyclic ADP-ribose)
Enables more Ca2+ to leave Sarcoplasmic reticulum
How are IP3 receptors effected by CICR?
Easier for Ca2+ to leave sarcoendoplasmic reticulum at low intracellular Ca2+ levels, and vice versa
What percentage Ca2+ comes from intracellular stores and extracellular in the cardiac myocyte?
85% intracellular stores
15% extracellular
What happens to NCX during influx of Ca2+?
Initial depolorisation reverses NXC to cause Ca2+ influx. Once Ca2+ levels are raised enough it reverts back to normal direction (Na+ influx, Ca2+ efflux)
Describe the Ca2+ channels on mitochondria. What are they important for?
Uniporter (Low affinity, high capacity) Important for: Ca2+ buffering Stimulation of mitochondrial metabolism Roll in cell death (apoptosis)
Why must Ca2+ levels be restored to basal state?
Too much Ca2+ for too long is toxic
Repetitive signalling requires restoration of basal state
How are Ca2+ levels restored to basal state?
Recycling of released (cytosolic) Ca2+
VOCC &/or capacitative Ca2+ entry
What is a ligand?
Any molecule that binds specifically to a receptor site
Define receptor
A molecule that recognises specifically a second molecule (ligand) or family of molecules & which in response to ligand binding brings about regulation of a cellular process. Silent at rest.
If ligand binding produces activation of a receptor, what is it termed?
Agonist
If ligand binding does not cause activation of a receptor (opposing the activation), what is it termed?
Antagonist
What is KD?
The contraction of substrate at which half of the receptor sites are filled of a particular receptor
What is KM?
The contraction of substrate at which half of the receptor sites are filled of a particular enzyme
Binding generally has higher affinity at receptor sites or enzyme active sites?
Higher affinity at receptor sites
How are receptors classified?
According to their signalling molecule (agonist) recognised, subclassified according to their affinity to a series of antagonists.
What’s are receptors and acceptors?
Receptor: Silent at rest, agonist binding stimulates a biological response.
Acceptor: Operate in absence of ligand. Ligand binding alone produces no response (modulated only).
Name 4 mechanisms of signal transduction
- Membrane-bound receptors with integral ion channels
- Membrane-bound receptors with integral enzyme activity
- Membrane-bound receptors which couple to effectors through transduction proteins
- Intracellular receptors
What is phagocytosis?
Internalisation of particulate matter
What is pinocytosis?
Invagination of the plasma membrane to form a vesicle. Permits uptake of extracellular solutes.
What is endocytosis?
The selective internalisation of molecules into the cell by binding to specific cell surface receptors
Give an example of receptor-mediated endocytosis
Cholesterol uptake
Describe the structure of LDLs
Core of sterilised cholesterol esters, covered by a phospholipid and cholesterol monolayer, containing apoprotein B.
How are clathrin coats assembled?
Triskelions spontaneously form clathrin coats
How is a clathrin coat disassembled?
Uncoated by an ATP-dependent uncoating protein
Describe the structure of triskelions
Coat structures made up of hexagons and pentagons
Comprise 3 clathrin heavy chains, and 3 light chains.
What is the CURL?
Compartment for the Uncoupling of Receptor and Ligand, also known as the endosome.
How is the low pH of the endosome maintained?
ATP-dependent proton pump
Describe the process of the receptor mediated endocytosis of LDLs
Receptors, located in coated pits, binds to LDL
Coated pit invaginate and pinch off from the plasma membrane to form coated vesicle
Coated vesicles are quickly uncoated
Uncoated vesicles then fuse with larger smooth vesicle CURL
At lower pH here LDL-receptor has low affinity for LDL particle, so dissassosiate
Transmembranous receptors are sequestered to a domain within CURL - buds off as a vesicle & recycled back to plasma membrane
CURL containing LDL fuse with lysosomes such that cholesterol can be hydrolysed from the esters and released into the cell
Name 3 possible mutations in LDL receptors that can lead to hypercholesterolaemia
- Receptor deficiency
- Non-functional receptor
- Receptor binding normal, but no internalisation due to deletion in the c-terminal of receptor that makes the interaction with the coated pits.
How is the clathrin coat attached to the plasma membrane?
By a number of integral membrane adapted proteins which form associations with both the clathrin and receptors, locating receptors over the coated pit.
Describe the process of endocytosis of Fe3+ (ferric ions) by transferrin
Ferrotransferrin (containing Fe3+) binds to transferrin receptor at coated pit
Coated vesicle buds off and is uncoated
Uncoated vesicle reaches CURL (acidic), Fe3+ ions are released
Apotransferrin remains bound to receptor at this pH
Complex sorted in the CURL for recycling back to plasma membrane
At neutral pH apotransferrin dissociates from transferrin receptor again
Describe the 2 unique parts of endocytosis of insulin
Insulin receptors only congregate over the covered pits when agonist is bound (binding causes conformational change in receptor enabling it to be recognised by coated pit)
In CURL, insulin remains bound to receptor. Complex is targeted to lysosome for degradation (enables reduction in no. insulin receptors (down regulation) desensitising bell to continued presence of high circulation insulin concs.)
Describe unique parts of endocytosis of immunoglobulin
Remains bound to receptor in CURL, together targeted for transfer vesicle, in turn for bile
Once in bile, immunoglobulin is released from receptor by proteolytic cleavage
What is transcytosis? Give an example
When ligand remains bound to its receptor in CURL
E.g. Immunoglobulin - secreted with a bound ‘secretory component’ derived from the receptor
Are coated pits unique for each type of receptor?
No - receptors for different ligands enter cell via same coated pit, pathway from such to endosome is common for all proteins, targeted to discreet regions of CURL once there.
How do viruses take advantage of RME?
Enron cells via clathrin-coated pore.
Once transported to favourable in pH within CURL are activated, so undergo replication ect.
What are the 3 ‘superfamilies’ of cell surface receptor?
- Ligand-gated (receptor operated) ion channels
- Receptors with intrinsic enzyme activity (receptor tyrosine kinases)
- G protein-coupled (7TM) receptors
What does an agonist do?
Binds to receptor and activates it
What does an antagonist do?
Binds to receptor without activating it (blocks effects of agonist)
Describe the shared basic structure of all GPCRs
Single polypeptide chain (300-1200 amino acids)
7-transmembrane (7TM) spanning regions
Extracellular N-terminal
Intracellular C-terminal
Describe the 2 regions of GPCRs that can be responsible for ligand binding
Ligand binding site formed by:
2-3 of the transmembrane domains
N-terminal region
What does ‘G protein’ stand for?
Guanine-nucleotide binding protein
How is G-protein signalling terminated?
Alpha or beta/gamma interaction with the effectors lasts only until the alpha subunits GTPase activity hydrolases GTP back to GDP. Alpha-GDP and beta/gamma subunits then reform an inactive heterotrimeric complex.
How are G-proteins activated?
GPCR -GProtein interaction activates the G protein by causing GTP to exchange for GDP on the G-Protein alpha subunit. The alpha-beta/gamma complex immediately dissociates, and each part can then interact with effector proteins
What does an agonist do?
Binds to receptor and activates it
What does an antagonist do?
Binds to receptor without activating it (blocks effects of agonist)
Describe the shared basic structure of all GPCRs
Single polypeptide chain (300-1200 amino acids)
7-transmembrane (7TM) spanning regions
Extracellular N-terminal
Intracellular C-terminal
Describe the 2 regions of GPCRs that can be responsible for ligand binding
Ligand binding site formed by:
2-3 of the transmembrane domains
N-terminal region
What does ‘G protein’ stand for?
Guanine-nucleotide binding protein
How is G-protein signalling terminated?
Alpha or beta/gamma interaction with the effectors lasts only until the alpha subunits GTPase activity hydrolases GTP back to GDP. Alpha-GDP and beta/gamma subunits then reform an inactive heterotrimeric complex.
How are G-proteins activated?
GPCR -GProtein interaction activates the G protein by causing GTP to exchange for GDP on the G-Protein alpha subunit
How are G-proteins activated?
GPCR -GProtein interaction activates the G protein by causing GTP to exchange for GDP on the G-Protein alpha subunit
Give 2 examples of possible effectors
Ion channels e.g. VOCCs
Enzymes e.g. Adenyl cyclase, phospholipase C, phosphoinostidide 3-kinase (PI3K), cGMP phosphodiesterase
What type of membrane protein is Adenylyl Cyclase?
Integral
How does activated adenylyl cyclase exert its effect with the cell?
Hydrolases cellular ATP to generate cyclic AMP, which interacts with a specific protein kinase (PKA), which in turn can phosphorylate a variety of other proteins within cell to effect their activities.
What does activated adenylyl cyclase do?
Increases glycogenolysis/gluconeogenesis in the liver
Increases lipolysis in adipose tissue
Relaxation of a variety of types of smooth muscle
Positive inotropic and chronotropic effects in the heart
What is PIP2 cleaved into?
Two second messengers, IP3 and DAG (diacylglycerol)
What does Phosopholipase C do?
Catalyses the cleavage of the membrane phosopholipid PIP2 (into IP3 and DAG)
What parts of the body does activated Phospholipase C effect?
Smooth muscle contraction of GI tract, airways/vasculature
Mast cell degranulation
Platelet aggregation
Give the 3 key features of signal transduction in biological membranes
- Diversity
- Specificity
- Amplification
What governs receptor activation?
Intrinsic efficacy
What is the efficacy of a drug governed by?
Intrinsic efficacy PLUS other things that influence the response (cell/tissue dependent factors)
What is intrinsic efficacy?
The ligands ability to produce n active receptor
What is Kd?
Dissociation constant
A measure of drug-receptor affinity. The concentration of ligand required to occupy 50% of the available receptors.
(Lower value = Higher affinity)
What is the EC50?
The Effective Concentration giving 50% of the maximal response
What is efficacy a measure of?
Potency
Affinity + intrinsic efficacy + cell/tissue dependent events
What is the IC50?
Inhibitory Concentration giving 50% of maximum inhibition
How may selectivity in drugs be achieved?
Drugs may have selective affinity or selective efficacy
What are ‘spare receptors’?
When a cell has more receptors than those which would need to be occupied to produce the maximal response. Spare receptors being activated will not amplify signal any more. Often in amplification/transduction systems. Increases sensitivity.
Why is having spare receptors advantageous?
Increases sensitivity
Name the 3 types of antagonist
- Reversible competitive antagonism
- Irreversible competitive antagonism
- Non-competitive antagonism (generally allosteric or even post-receptor)
What is Kb?
Kd, except determined pharmacologically
How could first pass metabolism be avoided?
Using a parenteral, sublingual or rectal route
What are 4 questions you should always ask before prescribing drugs?
- Is drug getting into patient? (pharmaceutical process)
- Is drug getting to site of action? (pharmacokinetic process)
- Is drug producing desired effect? (Pharmacodynamic process)
- Is this translated to a therapeutic effect? (Therapeutic process)
What are pharmacokinetics?
‘What the body does to a drug’
What are pharmacodynamics?
‘What the drug does to a body’
Name enteral sites of drug administration
Sublingual, oral, rectal
Name parenteral sites of drug administration
Subcutaneous, intramuscular, intravenous, inhalation, transdermal
Define the bioavailability of a drug
The proportion of drug given orally/any route other than IV, that reaches circulation unchanged.
Can be expressed as amount or rate
What is the therapeutic ratio?
Maximum tolerated dose LD50
___________________________ _________
Minimum effective dose ED50
What is the volume of distribution?
The theoretical volume into which drug is distributed, if this occurred instantaneously. Obtained by extrapolation of plasma levels to zero time.
It’s the free drug which exerts an effect, not total level. When is this particularly important?
If drug is highly bound to albumin (>90%)
Has a small volume of distribution
Has a low therapeutic ratio
What is a class 1 (object) drug?
Used at lower dose than no. of albumin binding sites
What is a class 2 (precipitant) drug?
Used at doses greater than number of albumin binding sites, and thus displaces class 1 (object) drug. Free drug levels of object drug will rise transiently, as will its elimination rate.
What drugs may act as class 2/precipitant drugs to warfarin?
Sulfonamides, aspirin, phenytoin
What drugs may act as class 2/precipitant drugs to tolbutamide?
Sulfonamides, aspirin
What drugs may act as class 2/precipitant drugs to phenytoin?
Valproate
What happens is a patient is taking an object drug, and then adds in a precipitant drug as well?
Patient will temporarily have higher levels of free object drug - increasing risk of toxicity. Elimination rate of object drug in patient will also rise though, steady state is restored after a few days.
What does a drug with 1st order kinetics mean?
Rate of elimination is proportional to drug level. Constant fraction of drug eliminated per unit time, half life can be defined.
What does a drug with 0 order kinetics mean?
Rate of elimination is constant (rare)
When is a steady state of drug metabolism established?
Within 5 half lives of that drug
What is a loading dose?
A large initial dose of a drug used to quickly get drug within therapeutic range
What are the 2 phases of liver metabolism?
- Oxidation, reduction, hydrolysis, by mixed function oxidases.
- Conjugation (glucuronide, Acetyl, methyl, sulphate)
What is the enzyme used in 1st phase of liver metabolism of a drug?
Cytochrome P450 reductase
Has low substrate specificity, affinity for lipid soluble drugs. Inductively and inhibitable
Drug interactions are most likely to matter clinically if…
Involves drugs with a low therapeutic ratio
Drug is being used at minimum effective concentration e.g. Oral contraceptive
Drug metabolism follows 0 order kinetics
What is phenobarbitone a metabolic inducer of?
Warfarin, phenytoin
What is rifampicin a metabolic inducer of?
Oral contraceptive
What is cigarette smoke a metabolic inducer of?
Theophylline
What is cimetidine a metabolic inhibitor of?
Warfarin
What drugs do you have to be particularly careful of?
Drugs with a low therapeutic ratio
What is passive reabsorption of a drug dependent upon?
pH
Only the non-ionised moiety is lipid soluble so can cross membranes easily
For weak acid drugs, acidic urine has what effect upon reabsorption of drug?
Increases absorption
For weak acid drugs, alkaline urine has what effect upon reabsorption of drug?
Decreases reabsorption
For weak base drugs, acidic urine has what effect upon reabsorption of drug?
Decreases absorption
For weak base drugs, alkaline urine has what effect upon reabsorption of drug?
Increases reabsorption
What is the effect of kidney disease on the half life of drugs?
Half life is prolonged - Lower maintenance dose
What, with regards to drugs, can be altered by kidney disease?
Longer half life - therefore takes longer to reach stable state
Protein binding can be altered (due to loss of protein)
Give 2 ways cholesterol may stabilise the plasma membrane
Intercalation of the rigid planar conjugated ring structure reduces phospholipid packing, therefore increasing fluidity.
Rigid conjugated ring structure reduces phospholipid aliphatic tail mobility, so reducing fluidity.
Describe 2 ways a membrane protein may interact with the hydrophobic domain of a membrane bilayer
Transmembrane sequence of ~20-22 amino acids with hydrophobic R-groups (often in alpha helix structure)
Lipid-linked proteins through insert action of hydrophobic lipid moiety e.g. Post-translational modification with fatty acid e.g. G-proteins
Give an example of a passive transporter that transports K+ ions
ROMK (in kidney)
Name the transporter inhibited by loop diuretics and their location in the kidney tubule
Na+/K+/2Cl- cotransporter
Thick ascending limb
What is the approximate resting membrane potential of a nerve cell?
-70mV
How is the approximate resting membrane potential of -70mV within a nerve cell maintained?
Efflux of K+ via voltage-insensitive K+ channels
Aaaaand minor leak of Na+/Ca2+ ions
What property does the absolute refractory period confer on nerve cells?
Prevents re-entrant excitation, directional impulse propagation
What happens to conduction of a nerve impulse in a neurone immediately after the loss of the myelin sheath?
Conduction failure due to increased neuronal membrane capacitance and current leak preventing no dally distributed channels from being raised to threshold value
What happens to conduction of a nerve impulse in a neurone after a period of recovery from the loss of the myelin sheath?
Re-establishment of nerve impulses, with slower conduction velocity, due to redistribution of nodal ion channels in the nerve membrane
Describe signal transduction by insulin receptors on binding of insulin
Insulin India to receptor, resulting in conformational change, which activates integral tyrosine kinase within the cytoplasmic domain.
Receptor autophosphorylation
Binding of transducing protein (insulin receptor substrate IRS) via specific domains which recognise phosphotyrosine residues on receptor
Tyrosine phosphorylation of IRS and consequent binding and activation of effector enzymes
Activation of intracellular enzymes by tyrosine phosphorylation
What is homologous desensitisation?
The process by which ONLY the signal from the stimulated receptor is reduced
What is heterologous desensitisation?
The process by which receptors for several agonists become less effective even when only one has been continuously stimulated
What receptor subtype activation results in dilatation of airways smooth muscle
Beta 2
What receptor subtype activation results in negative chronotrophy of the SAN?
M2
What receptor subtype activation results in positive ionotrophy of the heart ventricle?
Beta 1
What receptor subtype activation results in the stimulation of glandular secretions ?
M3 (and some M1)
What is the GTP-binding protein activated by alpha1-adrenoceptors, and results in release of Ca2+ from intracellular stores?
Gq
What is the effector activated by the G-protein Gq?
Phospholipase C
What does IP3 stand for?
Inositol-1,4,5-triphosphate
How is IP3 signal terminated?
IP3 phosphatase converts IP3—->IP2 (inactive)
Why is movement of protein in lipid bilayers more restricted than that of lipid constituents?
Proteins for membrane protein associations, may reduce rotation and lateral movement
Proteins form associations with peripheral membrane proteins e.g. Cytoskeleton
Lipid mediated effects - proteins separate out into fluid phase of bilayer in cholesterol poor areas
What proportion of fluid in the body is intracellular/extracellular?
~3/4 Intracellular (28l)
~1/4 Extracellular (9.4l)
Approx 4.6l in blood plasma
What does NCX stand for?
Sodium calcium exchanger
Apart from glucose, what other metabolites use the sodium gradient for their uptake into cells against the concentration gradient?
Amino Acids
What are the consequences for conduction of the nervous impulse of demyelination that is partial?
Slower conductance
What are the consequences for conduction of the nervous impulse of demyelination that is complete?
No conductance
What can desensitisation of GPCRs result from?
Modification of the receptor by phosphorylation
Reversible receptor internalisation
Down-regulation
Are parasympathetic preganglionic neurones long or short?
Long
Are parasympathetic postganglionic neurones long or short?
Short
Are sympathetic preganglionic neurones long or short?
Short
Are sympathetic preganglionic neurones long or short?
Long
Where do parasympathetic neurones originate?
Lateral horn of the medulla and sacral regions of the spinal cord
Where are parasympathetic ganglia found?
Located in tissues innervated by the postsynaptic fibres
Where do sympathetic nerves originate?
Lateral horn of the lumbar and thoracic spinal cord
Where are sympathetic ganglia located?
In the paravertebral chain close to the spinal cord
What receptors are activated upon release of ACh by a preganglionic nerve?
Nicotinic ACh (ligand gated ion channel)
What are the 2 major classes of adrenoceptor NA interacts with?
Alpha and beta
What subdivisions can alpha adrenoceptors be divided into?
Alpha 1 and alpha 2
What subdivisions can beta adrenoceptors be divided into?
Beta 1, beta 2, and beta 3
What type of receptor are all adrenoceptors?
G protein coupled receptors
What might some non-adrenergic, non-cholinergic transmitters release? (Or co-release along with NA/ACh)
ATP
NO
Neuropeptides
Serotonin
How might chromaffin cells be considered?
Post ganglionic sympathetic neurons that do not project to a target tissue
Where are chromaffin cells found?
Adrenal glands
What are the basic steps in neurotransmission?
- Uptake of precursors
- Synthesis of transmitter
- Vesicular storage of transmitter
- Degredation of transmitter
- Depolarisation by propagated action potential
- Depolarisation-dependent influx of Ca2+
- Exocytosis release of transmitter
- Diffusion to post-synaptic membrane
- Interaction with post-synaptic receptors
- Inactivation of transmitter
- Re-uptake of transmitter
- Interaction with pre-synaptic receptors
What enzyme catalyses ACh synthesis?
Choline acetyltransferase (CAT)
What enzyme catalyses acetylcholine degredation?
(Acetyl) Cholinesterase (AChE)
What is the equation for Acetylecholine synthesis?
Acetyl CoA + Choline —> Acetylcholine + Coenzyme A
What is the equation for acetylcholine degradation?
Acetylcholine —> Acetate + Choline
What might lack of selectivity of a drug result in?
Side effects
Describe the structure of most post-ganglionic sympathetic neurons
Highly branched axonal network with numerous varicosities, each of which is a specialised site for Ca2+ dependent noradrenaline release
Describe noradrenaline synthesis
Tyrosine—>DOPA—>Dopamine—>Noradrenaline
Describe the mechanisms of termination of noradrenaline transmission
Uptake 1: NA actions terminated by re-uptake into pre-synaptic terminal by a Na+ dependent, high affinity transporter
Uptake 2: NA not recaptured via uptake 1
What enzymes are used to metabolise noradrenaline in the pre-synaptic neurone?
Monoamine oxidase (MAO) Catechol-o-methyltransferase (COMT)
What are the mechanisms for modulating NA transmission?
- Presynaptic GPCR by inhibiting Ca2+ dependent exocytosis
- Indirectly acting sympathomimetic agents, taken into vesicles, displacing NA which then leaks out by mechanism other than Ca2+ mediated release
- Uptake 1 inhibitors. Work primarily in the CNS
- Adrenoceptor agonists and antagonists
What can be used to treat asthma/oppose bronchoconstriction?
Beta2-Adrenoceptor selective agonists (e.g. Salbutamol). Minimal side effects as are subtype-selective
How do beta2 agonists, e.g. Salbutamol, cause bronchiodilation?
Receptors are coupled to G-protein of Adenylyl cyclase, which produces second messenger cyclic AMP, which decreases Ca2+ concentrations within cells, and activates protein kinase A. Both of this inactivated myosin light chain kinase, and activate myosin light chain phosphatase.
Can also open Ca2+ activated K+ channels, thereby hyperpolorising airway smooth muscle cells
What is an endogenous substance?
Naturally occuring in the body
What is an exogenous substance?
An administered compound, I.e. A drug
What type of drug is it easy to predict response for?
Drugs with 1st order kinetics
If a drug is potent, what does this mean?
Has a greater response at a lower drug concentration
What is tachyphylaxis?
Reduced sensitivity to a drug as a result of excessive exposure to agonist
Via what mechanisms could altered sensitivity to a drug arise?
Change in receptor number
Change in receptor coupling to second messengers
Change in the availability of second messengers
Change in cell responsiveness
What receptors are activated in by opiates?
Gamma-opioid receptors
What is tamoxifen used for?
Oestrogen-receptive Breast cancer
Why is tamoxifen so beneficial?
Selective oestrogen modulator. Different action depending on tissue receptor - acts as an antagonist at the breast, but an agonist at bone and uterus.
What is phaeochromocytoma?
Tumour of the adrenal medulla
What does phaeochromocytoma cause?
Intermittent increased secretion of catecholamines (adrenaline/noradrenaline)
Therefore intermittent signs and symptoms of sympathetic nervous system over activity (sweating, tremor, anxiety, high blood pressure)
What is the risk of sudden removal of beta blockers in treatment of angina?
Suprasensitivity may have developed (beta receptor up-regulation). Symptoms would get significantly worse.
How is age associated with catecholamine sensitivity?
Decreasing sensitivity to endogenous catecholamines
Reduced heart rate responsiveness to exogenously administered catecholamines
Potential excess pharmacological efficacy of administered drugs
What does Gs bring about?
Adenylyl cyclase activation
What does Gt bring about?
GMP phosphodiesterase (in visual excitement)
What does Gi bring about?
Adenylyl cyclase inhibition (activation of K+ channels)
What does Gq bring about?
PIP2 activation (M3 smooth muscle contraction)
How does cholera toxin work?
ADP ribosylation by cholera toxin prevents GTP hydrolysis by alpha subunit
How does pertussis toxin work?
ADP ribosylation by pertussis toxin prevents receptor G-protein interaction
How is the actin-spectrin network of the erythrocyte cytoskeleton attached to the membrane?
By adapted proteins ankyrin and glycophorin
What specifically does PMCA transport?
2H+ in for 1Ca2+ out
Requires hydrolysis of ATP
Give an example of a depolarising nAChR blocker
Succinylcholine
What do trigger proteins do?
Bond to Ca2+ and alter function e.g. Synaptotagmin
What do Ca2+ buffers do?
Regulate free Ca2+
How are ionotropic receptors activated?
By a ligand/agonist binding
What are ‘classical’ ligand-gated channels like?
Pentameric (2xalpha, beta, gamma and delta subunits)
How do membrane bound receptors with integral enzyme activity exist?
In dimers
What are membrane bound receptors that signal through transducing proteins always?
7TMD
E.g. GTP binding proteins
Where is cyclic GMP phosphodiesterase found?
Found in photoreceptive cells of retina (rods and cones)
What does cyclic GMP phosphodiesterase do?
Regulates breakdown of 2nd messenger cGMP on activation by Gt, following excitation of rhodopsin by a photon of light.
On exposure to light, activation causes a decrease in cGMP, leading to channel closure and membrane hyperpolarisation, thus altering signal output to CNS
What is buprenorphine
Partial agonist of of morphine - higher affinity for receptors but lower efficacy
What drug is filtered in the kidney?
Only free unbound drug is filtered through the glomerular tuft?
What drug actually exerts an effect in the body?
Only free unbound drug (not the total level!)
How many mACh receptor subtypes are there?
5
What is sludge syndrome?
Over activity of the parasympathetic nervous system
