NONPEPTIDE ENDOTHELIN RECEPTOR ANTAGONISTS .4. IDENTIFICATION OF RECEPTORS IN RABBIT COLONIC MUCOSA AND SMOOTH-MUSCLE AND CORRELATION WITH PHYSIOLOGICAL-EFFECTS

Endothelin (ET) has been previously demonstrated to be a potent intest inal secretory stimuli acid to elicit intestinal smooth muscle contrac tions. In rat distal colon, the EF ,receptor antagonist cyclo[D-Asp-L- Pro-D-Val-L-Leu-D-Trp] abolishes the changes in ion transport (as meas ured by short-circuit current) elicited by ET-1. In this study, effect s of ET-1, sarafotoxin 6c and ''big'' ET in the absence and presence o f the endothelin receptor antagonists cycle [D-Asp-L-Pro-o-Val-L-Leu-D -Trp] and nedioxy)phenyl]-5-(prop-1-yloxy)indan-2-carboxylic acid on i on transport in rabbit distal colon were assessed through measurement of short-circuit current changes in segments of muscle-stripped mucosa in Ussing chambers. In addition, changes in smooth muscle contraction in response to acetylcholine, ET-1, ET-3 and sarafotoxin 6c in the ab sence and presence of nedioxy)phenyl]-5-(prop-1-yloxy)indan-2-carboxyl ic acid and of cycle [D-Asp-L-Pro-D-Val-L-Leu-D-Trp] were measured in mucosa-free preparations oriented along their longitudinal axes. Recep tor binding studies with mucosal or smooth muscle homogenates were con ducted with [I-125]ET-1, with [I-125]ET-3 and with ]Tyr(13)SUC-[Glu(9) ,Ala(11,15)]-endothelin-1(8-21) to identify the subtypes of ET recepto rs present in these preparations. From studies with rabbit colonic muc osa, both binding studies and measurement of short-circuit current con firm that the ET(B) receptor subtype is the predominant subtype and is responsible for the changes observed in ion transport, Studies with r abbit colonic smooth muscle preparations demonstrate that endothelins produce similar to 30% of the response observed with acetylcholine. Bi nding and physiological studies demonstrate that the receptors respons ible for the smooth muscle contractions observed with endothelins are both ET(A) and ET(B).