ANTAGONISTS Antagonists are substances which block the action of agonists There are a number of types COMPETITIVE NON-COMPETITIVE PARTIAL AGONISTS PHYSIOLOGICAL.

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ANTAGONISTS Antagonists are substances which block the action of agonists There are a number of types COMPETITIVE NON-COMPETITIVE PARTIAL AGONISTS PHYSIOLOGICAL CHEMICAL PHARMACOKINETIC

Typical curve for an agonist Guinea pig ileum - carbachol log concentration response 150 100 Response (mm) 50 -2 -1 1 2 3 4 Log conc carbachol (µM)

Typical curve for agonist plus antagonist Effect of atropine on carbachol log concentration response CARB + ATR 10-7 M 150 100 Response (mm) 50 -2 -1 1 2 3 4 Log conc carbachol (µM)

Typical curve for agonist plus antagonist II Effect of atropine on carbachol log concentration response + ATR 10-7 M CARB 150 + ATR 10-6 M 100 Response (mm) 50 -2 -1 1 2 3 4 Log conc carb (µM)

Effect of atropine on carbachol log conc response In the presence of atropine Curves are shifted to the right - higher concentration of carbachol required Curves are parallel Still get the same maximum response Carbachol is able to overcome the presence of the atropine

Antagonist This type is COMPETITIVE antagonism antagonist competes with agonist for same receptor site can be surmountable or insurmountable if surmountable then increasing conc of agonist will overcome the block sometimes this is known as reversible competitive antagonism eg ATROPINE competes with CARBACHOL at muscarinic receptors

Surmountable antagonism Agonist and antagonist molecules occupy the same receptor sites Dynamic process Drug molecules of both types rapidly associating and disassociating If agonist conc is increased - more chance of an agonist molecule occupying the receptor Therefore get a response

Insurmountable Antagonism Again a dynamic situation BUT rate of dissociation of antagonist may be MUCH SLOWER Agonist molecule does not get the chance to bind Unable to overcome the block This may happen to varying extents sometimes this is known as irreversible competitive antagonism

Insurmountable Antagonism Curve AGONIST 100 75 AGONIST + ANTAGONIST % Maximum response 50 25 0.001 0.01 0.1 1 10 100 log conc (µM)

Insurmountable antagonist cont.. if insurmountable, increasing conc of agonist unable to overcome the block probably because dissociation of the antagonist is very slow eg -BUNGAROTOXIN on ACh receptors at the neuromuscular junction

Competitive Antagonism Recommended Reading Competitive Antagonism Medical Pharmacology at a glance, 5th Ed: Pages 10 & 11 Pharmacology, 5th Ed: Pages 11-14 and 18 & 19 Basic and Clinical Pharmacology Online Book: Section I, Chapter 2, Relation Between Drug Concentration and Response, Competitive and Irreversible Antagonists