The presence of atrial natriuretic peptide (ANP) and the nature of its binding sites were studied in freshwater (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.