What is the incorrect definition of a receptor?
Any target molecule with which a drug combines to elicit a specific effect; includes enzymes, carrier molecules, ion channels, and receptors.
What is the correct definition of a receptor?
A protein activated by a natural agonist to produce a physiological response, which can also interact with antagonists.
Why are enzymes, transporters, and ion channels not considered receptors?
Because they are not activated by agonists to produce physiological responses.
Give an example of a true receptor.
β-adrenoreceptor on heart muscle cells, activated by adrenaline to increase heart rate and contractility.
What determines the tendency of a drug to bind to a receptor?
Affinity.
What determines the tendency of a drug to activate a receptor?
Efficacy.
Do agonists have affinity and efficacy?
Yes, agonists have both affinity and efficacy.
Do antagonists have affinity and efficacy?
Antagonists have affinity but no efficacy.
How does potency relate to affinity?
Drugs with high potency generally have high affinity and occupy more receptors at lower concentrations.
How is drug affinity measured?
Using radiolabelled drugs and plotting binding versus concentration to find the drug concentration at 50% binding (KA).
What is KA?
The concentration of drug that produces 50% maximal binding; inversely related to affinity.
What is Bmax?
The maximum binding capacity representing receptor density when all sites are occupied.
What is the binding equation?
B = (Bmax × [A]) / ([A] + KA). When [A] = KA, half the receptors are occupied (B = Bmax/2).
How are KA and Bmax estimated?
Using nonlinear regression or Scatchard plots; slope = KA, x-intercept = Bmax.
What do K+1 and K-1 represent?
K+1 = rate of binding; K-1 = rate of dissociation.
How is affinity related to binding kinetics?
Affinity (KA) = K-1 / K+1; low KA indicates high affinity.
What does binding measure vs. what does effect measure?
Binding measures receptor occupancy; effect measures physiological response.
What does a concentration-response curve show?
The relationship between drug concentration and its physiological effect, indicating Emax and EC50.
What is Emax?
The maximal response a drug can produce.
What is EC50 or ED50?
The concentration or dose that produces 50% of the maximal response.
What is the difference between full and partial agonists?
Full agonists produce maximal responses; partial agonists produce submaximal responses even when all receptors are occupied.
What is competitive antagonism?
When an antagonist competes with an agonist for the same binding site on the receptor.
Give an example of competitive antagonism.
Isoprenaline (agonist) increases heart rate, while propranolol (antagonist) competes with it to block the response.
What is non-competitive antagonism?
The antagonist reduces the agonist’s response by blocking a step in the signaling pathway, not the receptor itself.
