Atrial natriuretic peptide in the eel, Anguilla anguilla L.: its cardiac distribution, receptors and actions on isolated branchial cells

Abstract

The presence of atrial natriuretic peptide (ANP) and the nature of its binding sites were studied in fresh-water (FW)- and seawater (SW)-adapted eels using a heterologous analogue, that of the rat (rANP). Rat ANP-like immunoreactivity was demonstrated in the cardiac atria and ventricles of both FW and SW eels, and electron-dense ANP-like granules were observed. The atria and ventricles of FW eels contained significantly more granules than those of SW animals and, in both types, the atria were more granular than the ventricles. Specific binding sites for rANP were demonstrated by displacement and uptake experiments using labelled rANP in dispersed eel branchial cell preparations, enriched in chloride cells. The concentration of rANP required to produce a 50% inhibition of binding in FW cells was significantly lower than that in SW cells. Scatchard analyses revealed the presence of two classes of binding site in SW eel branchial cells but only a single class of receptor in FW cells. The affinity of the FW receptor was not significantly different from that of the SW high affinity site. Rat ANP stimulated the production of cyclic GMP (cGMP) in a dose-dependent manner, and both basal and stimulated levels of cGMP were significantly greater in SW branchial cells. These studies suggest that ANP is involved in the adaptation of the euryhaline eel to differing environmental salinities; the levels of the peptide in the heart alter with changing salinity, and the nature of the receptors in the sodium chloride-transporting epithelium of the gill changes in response to the need either to eliminate or to absorb sodium chloride.

Characterisation of natriuretic peptide receptors in eel gill

“…Since eel gills contain large amounts of NPR-C and/or NPR-D (Sakaguchi et al 1993, it is possible that the Scatchard analysis could not detect a small quantity of the CNP-specific NPR-Bs, which are present in the chloride cells of FW-adapted eels. By contrast, Broadhead et al (1992) demonstrated two classes of receptor in chloride cell-enriched gill cells of SW-adapted Anguilla anguilla using rat 125 I-ANP, but only one class was detected in those of FW-adapted eels. The reason for the difference between A. anguilla and A. japonica is not known at present.…”

“…The possible roles of ANP in osmoregulation have been described in the eel by Takei & Balment (1993). ANP receptors in eel gill cells have previously been examined using rat 125 I-ANP (Broadhead et al 1992).…”

Regulation of interrenal function in freshwater and sea water adapted tilapia (Oreochromis mossambicus)

“…A N P than atria, con firms results from morphometric and biosynthetic studies on other fish species (Baranowski and Westenfelder 1989). Because we used a heterolo gous R IA , we cannot compare our data directly with other studies on fish, in which a variety of homologous and heterologous antisera was used (Baranowski and Westenfelder 1989;Evans et al 1989;Evans 1990;Broadhead et al 1992;Powell and Miller 1992;Takei et al 1992). Most likely, our antiserum recognized both A N P and VNP (Takei et al 1992).…”

“…Clearly, additional work is warranted using multiple fish species comparing the actions of (Katafuchi et al 1994). Furthermore, receptor profiles for atrial natriuretic peptide in the gills, another target tissue for A N P, varies between FW-and SW-adapted eels (Broadhead et al 1992). …”