Effects of trout endothelin on the motility of gastrointestinal smooth muscle from the trout and rat

Trout endothelin (ET), previously isolated from the kidney of the rainbow trout Oncorhynchus mykiss, contains four amino acid substitutions at residues 4-7 compared with rat ET-1. Trout ET produced sustained and concentration-dependent contractions of strips of longitudinal smooth muscle from trout stomach (pD₂ = 7.52 ± 0.06) and proximal small intestine (pD₂ = 7.80 ± 0.10) and from rat fundus (pD₂ = 7.78 ± 0.14). Rat ET-1 was equipotent with trout ET for contraction of rat fundus and 2- to 3-fold more potent for contraction of trout gastrointestinal tissues. In contrast, rat ET-1 was 10- to 20-fold more potent than trout ET in constricting isolated rings of vascular tissue from trout efferent branchial artery and cardinal vein and from rat aorta (Y. Wang et al., 1999, Am. J. Physiol. 277, R1605-R1611). It is known that the contractile effects of ET-1 on rat fundus are mediated through the ET_B receptor and effects on the rat aorta are mediated through the ET_A receptor. We propose, therefore, that trout gastrointestinal tissues express an ET_B-type receptor that differentiates poorly between trout ET and rat ET-1, whereas trout vascular tissues express an ET_A-type receptor that is preferentially activated by rat ET-1. The rat ET-1-induced contractions of the trout gastrointestinal tissues are in part indirect, involving a serotoninergic neuronal pathway in the intestine and a noncholinergic, nonserotoninergic pathway in the stomach.