DRUG ANTAGONISM One drug or inhibits action of

DRUG ANTAGONISM One drug or inhibits action of another drug Types of Antagonism a) Physical antagonism b) Chemical antagonism c) Physiological/functional antagonism d) Pharmacological antagonism

A) Physical Antagonism v Based on physical property of drugs, e. g. charcoal (adsorb alkaloid) in alkaloidal poisoning B) Chemical Antagonism v v Chemical reaction between two drugs e. g. , Na. HCO 3 HCl

C) Physiological/Functional Antagonism Opposite effects of two drugs on same function v Two drugs act on two diff. types of receptors & antagonize action of each other, e. g. histamine & adrenaline (adrenaline for treatment of anaphylactic shock); Ad & insulin on blood sugar level

D) Pharmacological Antagonism v v Opposite effect of two drugs binding to same receptors Receptor antagonism is specific, e. g. atropine spam of intestine by acetylcholine not by Hist. or 5 -HT Types Competitive Equilibrium (Reversible) Non-competitive Non-equilibrium (Irreversible)

i) Competitive Antagonism (equilibrium or reversible) Competition between agonist & antagonist for specific site or receptor Action of agonist is blocked if conc. of antagonist is v Antagonism can be overcome (surmount) by conc. of agonist Cont. v

v v v Agonist can produce max. response in higher conc. Competitive antagonist shifts log Dose-response curve of agonist to right EC 50 of agonist in presence of antagonist, e. g. , Ach & atropine; Ad & Prop. ; Morphine & naloxone

COMPETITIVE ANTAGONISM

II) Non-Equilibrium(irreversible) Antagonist Ø Ø Ø Antagonist binds to receptor with covalent bond Irreversible blocking Antagonist shifts DRC to right+ max. response, e. g. adrenaline & phenoxybenzamine

iii) Non-competitive (nonsurmountable Antagonist ü ü ü Antagonist binds to another site of receptor DRC is flattened + max. response is e. g. , verapamil (noradrenaline)

Non-competitive Antagonist