Two classes of endothelin receptors mediating contraction in esophageal muscularis mucosae

Endothelin (ET) causes contraction of the esophageal muscularis mucosae. To characterize the ET receptor subtypes involved in contraction, we measured contraction of isolated muscularis mucosae strips caused by ET-related peptides and binding of (125)I-ET-1 to cell membranes prepared from the guinea pig esophageal muscularis mucosae. Autoradiography demonstrated (125)I-ET-1 binding to the muscularis mucosae and muscularis propria. ET-1 caused tetrodotoxin and atropine-insensitive contraction of esophageal muscularis mucosae strips. The relative potencies for ET isopeptides to cause contraction were ET-1=ET-2>ET-3. FR-139317, an ET(A) receptor antagonist, or BQ-788, an ET(B) receptor antagonist, alone did not alter responses to ET-1. However, the combination of both antagonists almost abolished the ET-1-induced contraction, indicating synergistic inhibition. Desensitization to sarafotoxin S6c, an ET(B) receptor agonist, failed to abolish the response to ET-1, which was completely inhibited by FR-139317. These indicate the involvement of both ET(A) and ET(B) receptors in the contraction. Binding of (125)I-ET-1 to cell membranes of the muscularis mucosae was saturable and specific. Analysis of dose-inhibition curves demonstrated the presence of ET(A) and ET(B) receptors.This study demonstrates that the esophageal muscularis mucosae possesses both ET(A) and ET(B) receptors mediating muscle contraction. There is cooperation between these two subtypes of ET receptors in the esophagus mediating muscle contraction.