To discover the action of a drug "X" in isolated intestinal smooth muscle preparations a study was carried out. It was seen that, drug X binds to cholinergic receptors of intestine and relaxes the preparation. While in the absence of drug X, Acetylcholine acts on same receptors and contracts the preparation. It was also seen that, at a low conc. of drug X, the EC50 of acetylcholine remains unchanged, but the Emax was decreased. At a high conc. of drug X, high conc. of acetylcholine had no effect. Which of the following statement is true about the drug X?
High-Yield Explanation
Options including chemical/ physiologic antagonism are wrong because drug X acting at same receptor as acetylcholine. Drug X acts opposite of acetylcholine when given alone. So it cannot be an agonist. As the drug has opposite effect on same receptors, it is an antagonist. Now, we need to understand about EC50 and Emax. EC50 represents Km in enzymes and Emax represents Vmax in enzymes. As Emax decreases and EC50 does not change, so it is a case of non-competitive or irreversible antagonism.