Inhibitory effect of acetylcholine on muscular tonus of the small intestine of a lizard.

The present report concerns the effects of cholinergic agonists on the isometric tension of "in vitro" preparations of the esophagus and distal part of the small intestine of the lizard Liolaemus gravenhorsti. Acetylcholine (Ach) and carbachol elicited a dose-related increase of isometric tension in the esophagus, whereas the small intestine was slowly relaxed by these drugs. Eserine induced a synergistic action on the cholinergic responses of both organs, and atropine completely antagonized the respective effects. The esophagus and the intestine showed different thresholds for the cholinergic evoked responses, the former being about 30 times more sensitive than the intestine. Assessment of cholinesterase activity revealed that Ach is hydrolyzed at a significant lesser speed in the intestine than in the esophagus. The results do not provide information about the nature of the chemical mediator causing the inhibitory effect observed in the intestine. The eventual role of an adrenergic mechanism mediated by muscarinic receptors is under study in our laboratory.