Characterization of an endothelin ETB receptor in the gill of the dogfish shark Squalus acanthias

Endothelins (ETs) are potent vasoconstrictive peptides that are secreted by the vascular endothelium and other tissues in vertebrates. Previous studie s have demonstrated that ETs are expressed in a variety of fish tissues and contract various blood vessels. In order to determine if receptors for ET are expressed in fish gill tissue, we examined the binding kinetics of I-12 5-labeled, human ET-1 to membrane fragments isolated from the gill of the d ogfish shark, Squalus acanthias, I-125-ET-1 bound at a single site, with a dissociation constant (K-d) and binding site number (B-max) very similar to those described in a variety of mammalian blood vessels. ET-1 and ET-3 com peted equally with I-125-ET-1, suggesting that the receptor was ETB, which has been shown in mammalian systems to bind to both ligands equally. The ET B-specific agonists sarafotoxin S6c, IRL-1620, and BQ-3020 also competed ag ainst I-125-ET-1 at a single site, supporting this hypothesis, We conclude that the shark gill expresses an ETB receptor with substantial homology to the mammalian receptor and that ET may play an important role in modulating such vital gill functions as gas exchange, ion regulation, acid-base balan ce, and excretion of nitrogen.