Heterogeneity of muscarinic receptors in the guinea pig esophageal muscularis mucosae and ileal longitudinal muscle.

Pharmacologic characteristics of muscarinic receptors in the muscularis mucosae of the guinea pig esophagus were examined in vitro and compared with those of the longitudinal muscle of the guinea pig ileum. All cholinomimetics tested produced a sustained contraction of the muscularis mucosae, in a concentration-dependent manner, which was associated with a small biphasic change in membrane potential, initially a hyperpolarization followed by a depolarization. The contractile response was hardly modified by verapamil, but was depressed by calcium removal from the bathing medium. Both atropine and pirenzepine antagonized the contractile response in a competitive manner, with higher pA2 values than those in the ileum. These results indicate that the muscarinic receptors of the muscularis mucosae of the guinea pig esophagus mainly link with calcium ion channels which are independent of changes in membrane potential and that their subtype populations are probably pharmacologically distinct from those in the ileal longitudinal muscle.