Endothelin receptors in teleost fishes: cardiovascular effects and branchial distribution

Abstract

By observing gill blood flow using epi-illuminating microscopy, in parallel with cardiovascular recordings and immunohistochemistry, we have tried to identify the receptor mediating endothelin (ET) type 1 (ET1)-induced pillar cell contraction in the lamellae of the Atlantic cod ( Gadus morhua). Intra-arterial injection of the specific ETBreceptor agonist BQ-3020 induced dose-dependent increases in ventral aortic blood pressure, gill vascular resistance, and pillar cell area (indicating contraction). The specific ETAreceptor antagonist BQ-610 did not prevent either pillar cell contraction or increased gill vascular resistance induced by ET-1 injection. The cardiovascular responses were corroborated by the detection of ETBreceptor-like immunoreactivity (IR) associated with pillar cells in the lamellar region and in neuroendocrine cells. ETBreceptor-like IR was also found lining the muscle layer of lamellar arterioles and filament arteries. In contrast, strong ETAreceptor-like IR was found on branchial nerves throughout the filaments. In addition, ET-like IR was concentrated in neuroendocrine cells in the filament and lamellae. We also present data suggesting that ET-mediated pillar cell contraction is widespread among teleost fish, including Atlantic cod, rainbow trout ( Oncorhynchus mykiss), sculpin ( Myoxocephalus scorpius), and mackerel ( Scomber scombrus). Taken together, our results suggest that an ETB-like receptor mediates pillar cell contraction in fishes, whereas ETA-like receptors may serve another function in the gill, inasmuch as ETAreceptor-like IR is found on branchial nerves.