Cholinergic regulation of Na absorption by turtle colon: Role of basolateral K conductance

The mechanism underlying the muscarinic inhibition of colonic Na absorption is unknown. In this study the effects of carbachol on active Na transport and basolateral K conductance were compared in the isolated turtle colon. Carbachol produced a biphasic response in both Na transport and basolateral K conductance. The response consisted of a transient activation followed by a sustained inhibition and was blocked by atropine. Submucosal cholinergic neurons were implicated in the regulation of colonic transport by employing depolarizing agents to release endogenous acetylcholine. Depolarizing agents produced a carbachol-like response that was atropine-sensitive. Finally, experiments with the Ca ionophores, A23187 and ionomycin, suggested that the muscarinic response may be mediated, at least in part, by changes in cellular Ca. These experiments provide evidence that 1) cholinergic neurons are present in the turtle colon submucosa, 2) muscarinic agonists cause a change in basolateral K conductance that may be an important event in the regulation of colonic Na absorption, and 3) a Ca second messenger system may be involved in mediating the response.