Endothelin A receptors mediate relaxation of guinea pig internal anal sphincter through cGMP pathway

Abstract

Endothelin (ET) modulates motility of the internal anal sphincter through unclear receptor subtypes. We measured relaxation of guinea pig internal anal sphincter strips caused by ET-related peptides and binding of (125)I-ET-1 to cell membranes prepared from the internal anal sphincter muscle. Visualization of (125)I-ET-1 binding sites in tissue was performed by autoradiography. In the guinea pig internal anal sphincter, ET-1 caused a marked relaxation insensitive to tetrodotoxin, atropine, or omega-conotoxin GVIA. ET-2 was as potent as ET-1. ET-3 caused a mild relaxation. The relative potencies for ETs to cause relaxation were ET-1 = ET-2 > ET-3. The ET-1-induced relaxation was inhibited by BQ-123, an ET(A) antagonist, but not by BQ-788, an ET(B) antagonist. These indicate that ET(A) receptors mediate the relaxation. The relaxant response of ET-1 was attenuated by LY 83583, KT 5823, Rp-8CPT-cGMPS, tetraethyl ammonium, 4-aminopyridine and N(omega)-nitro-L-arginine, but not significantly affected by N(G)-nitro-L-arginine methyl ester, N(G)-methyl-L-arginine, charybdotoxin, apamin, KT 5720, and Rp-cAMPS. These suggest the involvement of cyclic guanosine 3',5'-cyclic monophosphate (cGMP), and potassium channels. Autoradiography localized (125)I-ET-1 binding to the internal anal sphincter. Binding of (125)I-ET-1 to the cell membranes prepared from the internal anal sphincter revealed the presence of two subtypes of ET receptors, ET(A) and ET(B) receptors. Taken together, these results demonstrate that ET(A) receptors mediate relaxation of guinea pig internal anal sphincter through the cGMP pathway.