Semester 1 Pharmacology Flashcards
What is the definition of a receptor?
It is a macromolecular component of a cell with which a drug interacts to produce its characteristic biological effect
What concentrations are receptors present in?
Receptors are present in low concentrations and show saturable binding
Receptor binding
Displays saturable binding
Saturable binding means that as you increase the amount of drug inside of the tissue, there will be an increase in the amount of receptors that have been bound to, until all receptors have been bound
Has a maximum binding, due to a finite number of binding sites
What affinity do drugs have for receptors?
Many drugs have high affinities for their receptors, meaning a drug will bind to its receptors at low concentrations
Affinity is measured using the equilibrium dissociation constant Kd
Do receptors show selectivity?
Receptors show strong selectivity
Sometimes referred to as a pharmacological profile
Each receptor has an order of affinity for each drug that reacts with the receptor
Are drug-receptor interactions reversible?
Drug-receptor interactions are usually fully reversible, neither the drug nor the receptor are permanently changed
Drug + Receptor <–> DR
A few exceptions:
- Toxins (bind so tightly that its irreversible)
- Phenoxybenzamine (Used to treat tumour of Adrenal Medulla)
What size are drugs?
Drugs are usually very small molecules
Their molecular weights are typically 200 (compared to typical -250,000 for receptor)
What is special about a receptors binding site?
A receptors binding site has a complementary structure to the drug
Drugs are only held by week bonds so a close fit is required
How do agonists effect receptors?
Agonists induce conformational change in their receptors
Receptors are not rigid
This ‘induced fit’ has been confirmed by structural biology
ANTAGONISTS do NOT cause conformational change
How can we quantify drug-receptor interactions?
To allow comparison between drugs their effects must be quantified:
- ASSUME LAW OF MASS ACTION
- D + R = DR
- Rate of forward reaction = k[D][R]
- Rate of reverse reaction = k-1[DR] - ASSUME ONLY NEGLIGIBLE AMOUNT OF DRUG IS BOUND
- Free drug = total drug - ASSUME REACTION IS AT EQUILIBRIUM
- k1[D][R] = k-1[DR]
What is the equation that allows you to derive the equilibrium dissociation constant of a drug for its receptor?
The langmuir equation!
Will result in a rectangular hyperbola
What is special about Kd and half fractional occupancy
The concentration of the drug at which you are receiving half maximal receptor occupancy is equal to Kd
What is special about nicotinic receptors?
How does occupancy relate to biological effect?
2 theories:
Occupation theory:
- Response [E] is directly proportional to the number of receptors occupied
Rate theory:
- Response [E] is directly proportional to the rate of receptor occupation
Occupation theory equation and explanation
More receptors binded the larger the response
Maximal response at full binding
Graphs of fractional response against drug concentration will have same shape as fractional occupancy against drug concentration
Therefore at the half maximal response, the [D] will equal the Kd
What is pD2?
pD2 is the negative log of the agonist concentration that gives a half maximal response
It is a very useful pharmacological parameter as it quantifies the affinity of an agonist for its receptor
Drugs with high values of pD2 act at low concentrations
pD2 is always positive and there are no units
How do we measure pD2?
From a log[concentration] response curve:
Take the negative log of the concentration of agonist that gives a half maximal response (EC50)
-log(EC50)
What is a competitive antagonist?
The agonist and antagonist bind to the same site
The block can be overcome by increasing the concentration of the agonist
What is a non-competitive antagonist?
The antagonist binds to a different site on the receptor, or acts irreversibly
The block CANNOT be overcome by increasing the concentration of agonist
Competitive antagonism graph differences
- Dose response curve shifts to the right in a parallel fashion
- The apparent pD2 decreases in the presence of the competitive antagonis
- There is no change in Emax (maximal response)
Non competitive antagonism graph differenes
- The pD2 is not changes
- Emax decrease (maximum response)
- Dose-response curves are not parallel
What does the ability to block a response depend on for a competitive antagonist?
- The relative affinity of the agonist (Kd) and the relative affinity of the antagonist (Ka) for the receptor
- The relative concentrations of the agonist [D] and the antagonist [A]
Fractional occupancy in the presence of agonist and antagonist equation
What is a dose ratio?
It is the ratio of the agonist concentrations that elicit the same response either in the absence [Do] or the presence of [Da] the antagonist
What is an assumption we can make about dose ratio?
If the response in the presence of and absence of the antagonist is the same, then it is reasonable to assume that the occupancy by the agonist is the same
This concept is used to derive the affinity of antagonists from dose-response curves
How can you use a series of dose response curves in the presence and absence of an antagonist to determine the antagonist affinity?
- Choose any response (usually 50%)
- Determine the agonist concentrations that give this response in the absence [Do] and presence [D1, D2…] of antagonist concentrations A1, A2…
- Calculate the dose ratio D1/D0, D2, D0… at each antagonist concentration
- Use the Gaddum-Schild Equation
What is the Gaddum-Schild Equation?
Analysis is based off of the law of mass action and assumes simple competitive antagonism
No assumptions are made about the relationship between response and the number of receptors occupied
It is independent of the agonist used - so long as it competes with the antagonist for the same receptor
Watch first 10 minutes of LHD RECEPTORS IV OCT 16TH
What do different pA2 values mean?
Higher the pA2 value, the stronger the affinity the antagonist has on the receptor
In the example shown, more atropine will be required in histamine receptors than acetylcholine receptors as the affinity for the histamine receptors is lower…
How is pA2 measured?
pA2 = -log(Ka)
What are the assumptions of the occupancy theory?
- There are specific receptors for specific agonists
- All agonists for a given receptor can produce the same maximum response
- The drug-receptor interaction is rapidly reversible
- All receptors are equally accessible to the drug
- The receptors do not interact with each other
- The maximum response occurs when all receptors are occupied
What did Stephenson (1956) discover?
He found that some agonist drugs act on receptors and only produce a weak response (partial agonist)
He described the action of n-alkyltrimethylammonium compounds on the guinea pig ileum
n = 4-6 full agonist
n = 7-9 partial agonist
Partial agonists act as competitive antagonists of the full agonist
Do all drugs produce the same effect when bound to the same receptor?
No
Drugs may differ in their ability to induce a conformational change in the receptor, once they have bound
D + R <–> DR <–> DR*
DR = Affinity
DR* = Ability to produce an effect (intrinsic activity - alpha)
What are “spare receptors” in pharmacology?
Spare receptors are the subset of receptors that remain unoccupied, yet the system still elicits a maximal response
They amplify tissue or system sensitivity: a significant response can be achieved even with low ligand concentrations
Serve as a protective mechanism, offering a buffer against scenarios of receptor loss or downregulation
Emphasise the dissociation between receptor occupancy and physiological response, showing a non-linear relationship
Why are spare receptors crucial in pharmacology and drug design?
Maximal effect (Emax) can be achieved even if not all receptors are occupied
Relevant primarily for agonists; antagonists often need to occupy more receptors to counteract endogenous ligands
Drugs acting on systems with spare receptors might be effective at low doses
What is a “spare receptors knockout test” in pharmacology?
A test designed to determine the presence and extent of spare receptors in a system
Method involves reducing the number of functional receptors, often through genetic modification or by using antagonists
If a maximal response can still be achieved after reducing the receptor number, it indicates the presence of spare receptors
Provides insights into the relationship between receptor occupancy and physiological response, as well as the system’s sensitivity to agonists
Will an agonist bind to a receptors active or rested state?
An agonist with high efficacy will preferentially bind to 2
and stabilise the active conformation of the receptor
An agonist with low efficacy may bind to both 1 and 2
An antagonist will only bind to 1
What are inverse agonists and where did the concept originate?
The concept originated from studies on anxiolytics and anxiogenics (benzodiazepines) that work through the GABA-A receptor
Some receptors have some activity in the resting state – constitutive activity
This is reversed by inverse agonists, which decrease the basal activity
This is a wide-spread phenomenon also applies to GPCRs
What are the main types of agonists in pharmacology?
Full Agonist:
- Binds to and fully activates the receptor, eliciting a maximal physiological response
Partial Agonist:
- Binds to the receptor but only induces a sub-maximal response, even when occupying all receptors
Inverse Agonist:
- Binds to and stabilises the receptor in an inactive conformation, decreasing its basal activity
What is the autonomic nervous system?
ANATOMICALLY it is defined as all those neural pathways that leave the brain that do not innervate the voluntary muscles
PHYSIOLOGICAL ROLE: Homeostasis
Organization of the nervous system
Two main sections:
- CNS
- PNS
Under PNS also two main sections:
- Somatic
- Autonomic
Under Autonomic also two main sections:
- Sympathetic
- Parasympathetic
What is homeostasis and how is it controlled?
Homeostasis is defined as a self-regulating process by which a living organism can maintain internal stability while adjusting to changing external conditions
After sensory input homeostasis is controlled by 2 factors:
- Neuronal response (ANS) (faster response)
- Hormonal response (Hypothalamus) (more sustained response)
What are the differences between the two types of autonomic nervous system?
Sympathetic (Fight or Flight)
Parasympathetic (Rest and Digest)
These two cannot coexist, only ever sympathetic or parasympathetic at one time
What are the Physiological roles of Autonomic Nervous System?
Important in regulation of:
- Pupillary dilation
- Accommodation for near vision
- Dilation and constriction of blood vessels
- Force and rate of heart beat
- Movements of the gastrointestinal tract
- Secretions from most glands
- Energy metabolism, particularly in liver and skeletal muscle
What is a ganglion?
The point of contact between the first and second efferent neurone in the pathway occurs in a neural structure called a GANGLION
A GANGLION is a group of nerve cell bodies that lie outside the central nervous system
What are the differences in the preganglionic and postganglionic nerves in the para and sympathetic ns?
Sympathetic has:
- short preganglionic nerve
- long postganglionic nerve
Parasympathetic has:
- long preganglionic nerve
- short postganglionic nerve
Names of drugs that influence the ANS?
Mimic or block the effects of the two primary neurotransmitters:
- Acetylcholine
- Norepinephrine/Epinephrine
Drugs that mimic neurotransmitters are referred to as:
- Receptor agonists
- These drugs activate receptors
Drugs that block neurotransmitters are referred to as:
- Receptor antagonists
- These drugs block the endogenous neurotransmitters from activating receptors
Structure and function of sympathetic nervous system?
- Innervates many different tissues
- Preganglionic neurone located in midbrain, medulla or lateral horn of spinal cord
- Ganglia form the ‘sympathetic chain’
- Short preganglionic neurone
- Long postganglionic neurone
- Provides diffuse innervation of target tissues
Structure and function of parasympathetic nervous system?
- Innervates many different tissues
- Preganglionic neurone located in medulla or sacral segment of spinal cord
- Ganglia are located in the target tissue
- Long preganglionic neurone
- Short postganglionic neurone
- Provides discrete innervation of target tissues
What does it mean to have antagonistic inputs for sympathetic and parasympathetic ANS?
What is the structure of Noradrenaline?
Noradrenaline is a CATECHOLAMINE
What is the synthetic pathway of tyrosine to noradrenaline?
Tyrosine is taken from diet
Enzyme:
- Tyrosine hydroxylase
Affect:
- Adds an OH group to form a catechole group
Resultant formed:
- L-DOPA
Enzyme:
- Dopadecarboxylase
Affect:
- Removes the carboxyl group from L-DOPA
Resultant formed:
- Dopamine
Enzyme:
- Dopaminebetahydroxylase
Affect:
- Adds a hydroxyl group to the beta carbon of dopamine
Resultant formed:
- Noradrenaline
Enzyme:
- PNMT
Affect:
- Adds a methyl group to noradrenaline
Resultant formed:
- Adrenaline
What were the two uptake mechanisms that Iversen discovered for noradrenaline?
Uptake 1:
- High affinity
- Low capacity
- Present in nerve terminal
- Requires Na+ gradient
- ATP dependant
Uptake 2:
- Low affinity
- High capacity
- Present in extra-neuronal tissue
- Co-transport with Sodium, no energy required
- Inhibited by cortisol
What is the MAO metabolic pathway of noradrenaline?
Noradrenaline + MAO –> Dopegal
Dopegal + AR –> DHPG
DHPG + COMT –> MHPG
MHPG + ADH –> Mopegal
Mopegal + AD –> VMA
What are some beta adrenoreceptor antagonists?
Propranolol
- Non-selective beta blocker
- Antihypertensive results
- Has local anaesthetic action
- Can cause bronchoconstriction
Atenolol
- Beta1 selective antagonist
- Cardioselective
Overview of noradrenaline adrenoreceptor pathways
What is the structure of acetylcholine?
Packaging of acetylcholine?
Acetylcholine is taken up into presynaptic vesicles by an active transport process (blocked by vesamicol)
Transporter is caleld VAChT
Requires ATP
Acetylcholine is released in response calcium entry into the presynaptic terminal
How is acetylcholine inactivated in the synaptic cleft?
The synaptic cleft is also rich in the enzyme Acetylcholinesterase which breaks ACh down into choline and acetic acid
Choline is taken back up into the nerve terminal by the choline transporter. This is blocked by hemicholinium.
Acetylcholinesterase mechanism
Has two important sites:
Anionic site:
- Drives electrostatic interaction with the nitrogen group on acetylcholine
Esteric site
- Hydrolyses acetylcholine into choline and acetic acid
What did Dale’s experiment show?
It showed that acetylcholine produces 2 different effects on heart rate and blood pressure based off of the concentration of acetylcholine used
The same compound triggered 2 different responses as there are 2 receptors present (muscarinic and nicotinic)
Muscarinic have high affinity meaning a only a low dose is required of ACh
Nicotinic receptors have low affinity and therefore requires high concentrations
What is the structure of muscarine?
What are some agonists of muscarinic receptors?
What are the 2 muscarinic receptor antagonists?
Atropine
Pirenzepine
Nicotinic receptors basic structure
Not G-Protein coupled
All are ligand-gated ion channels and are permeable to cations
When acetylcholine binds to the receptor:
- Sodium will be imported
- Potassium will be exported
- Exchange of ions, drives downstream response
The isoform at ganglia is different from at muscle
What are the subunits in the nicotinic receptor?
2 alpha
1 beta
1 delta
1 gamma
Binding site for acetylcholine is present in the alpha subunits (2 binding sites)
What is hexamethonium?
It is an antagonist of nicotinic receptors
It blocks ganglionic nicotinic receptors (N2 receptors)
What is polymethylene bismethonium series?
It is a family of compounds
It contains hexamethonium, the antagonist of nicotinic receptors
The image displays a generalised formula, with a variable number of carbons
Members with lower number of carbons (4-8):
- Drive ganglionic block
Members with higher numbers of carbons(7-13):
- Drive neuromuscular block
By changing number of carbons, can select different nicotinic receptor types
How do drugs pass the cell membrane?
MEMBRANE PORES – For drugs with low molecular weights (<200Da) or small ions (Li+)
DIFFUSION THROUGH MEMBRANE – For lipid soluble molecules
CARRIER MEDIATED – Drug must resemble natural ligand or substrate
What is the Henderson-Hasselbach equation
Used to determine the extent of ionisation of a drug
pH – pKa = log [conjugate base] / log [conjugate acid]
