an equilibrium state;

at equilibrium a specific concentration of a drug (ligand) will occupy a fixed percentage of receptors at any moment in time;

- from moment to moment the particular receptor that is occupied might change, but the overall percentage of occupied receptors will not change drugs that sit in a receptor for longer periods of time will occupy greater percentage of receptors (at a specified concentration);

the concentration of drug that occupies 50% of the receptors is equal to the affinity;

- affinity= Kd

- affinity= K2/K1 (the rate of dissociation over the rate of association)

- affinity is NOT the EC50 (this = potency)

Y = fraction of receptors occupied by a drug

[D] = concentration of drug in molar units

KD = affinity of drug in molar units 

bind to receptors (affinity) -- weak, irreversible chemical bonds;

produce effects (efficacy) -- effect =

hard-wired (signal transduction); related to dose (dose-response fucntion) 

- the reponse is NOT linearly proportional to the # of receptors occupied;

- the max effect can be produced by an agonist when occupying only a small proportion of receptors

- different drugs can have varying capacities to initiate a respnose & consequently occupy different proportions of the receptors when producing equal responses 

- bind to receptors (affinity) -- weak irreversible chemical bonds;

- do NOT produce effects (no efficacy)

- block action of agonist

- effect is related to dose

occupy the agonist binding site w/o producing an effect;

classic competitive antagonism results in rightward parallel shifts of the agonist concentration-effect curve

interfere w/ agonist binding through a "distal binding site";

the non-competitive antagonsit changes the receptor conformation so that the agonist binding site is no longer available