Effects of methacholine on the carotid chemoreceptors.

The present electrophysiological study shows that methacholine (MCh), generally regarded as a muscarinic agonist, stimulates the carotid chemoreceptors in pentobarbitone anaesthetized cats. The response consisted of a primary increase in discharge, attributable to nicotinic actions of MCh since it was unaffected by atropine but abolished by mecamylamine, and a delayed secondary increase in discharge, due mainly to bronchoconstriction evoked by MCh. Physostigmine caused similar potentiation of responses to acetylcholine and MCh which implies that acetylcholinesterase is located close to the site(s) at which the drugs act to stimulate chemoreceptor activity. The findings are in agreement with the general principle that chemosensory activity is increased by nicotinic agonists but not by muscarinic agonists.