Mechanisms and sites of action of endothelins 1 and 2 on the opossum internal anal sphincter smooth muscle tone in vitro.

Endothelins, localized in the enteric nervous system, may play important roles in the morphogenesis of the gastrointestinal (GI) tract and in the regulation of GI motility. However, the role of endothelins in the GI sphincters, including the internal anal sphincter (IAS) have not been examined. We examined the actions of endothelins on the basal tone of the opossum IAS circular smooth muscle strips before and after different neurohumoral antagonists or inhibitors. Endothelins 1 and 2 produced a concentration-dependent biphasic effect on the basal tone of the IAS, an initial brief fall followed by a sustained rise. The fall in the IAS smooth muscle tone was not modified by atropine, guanethidine, or tetrodotoxin but was significantly attenuated by the nitric oxide synthase inhibitor L-NNA, the specific neuronal nitric oxide synthase inhibitor, 1-(2-trifluoromethylphenyl)imidazole, the N-type neuronal Ca++-channel blocker omega-conotoxin GVIA, and by the calmodulin antagonist W-13. Endothelin-induced contraction of the IAS, on the other hand, was not affected by any of the neurohumoral antagonists but was significantly inhibited by the selective protein kinase C inhibitor H-7 or the calmodulin inhibitor W-13. The combination of H-7 and W-13 had no additive effect in attenuating the contractile action of endothelin 1. There was clear evidence of a cross-tachyphylaxis to the actions of endothelin 1 and endothelin 2. We conclude that the endothelins exert important neuromodulatory effects on the basal tone of the IAS. The contractile action occurs directly at the smooth muscle and the relaxant action by the activation of neuronal nitric oxide synthase at the nerve terminals. The contraction and relaxation of the smooth muscle caused by endothelins 1 and 2 may involve distinct receptors that are similar for both endothelins. The excitatory actions of endothelin 1 involve both the protein kinase C and the Ca++-calmodulin pathways that may lie in series.