Physiological actions of a family of natriuretic peptides in eels

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I.We cloned cDNA of eel atrial natriuretic peptide (ANP). The C-terminus of proANP predicted from the cDNA sequence has glycine residue which could not read by the amino acid sequencing. Detailed HPLC analysis revealed that glycine residue was used for amidation of the C-terminus of ANP.Southern blot analysis indicated that ANP and VNP genes are present in birds. Since BNP has already been identified in birds, VNP appears to be a new hormone distinctly different from BNP.II.In cooperation with Professor S.Hirose of Tokyo Institute of Technology, we cloned cDNAs of eel ANP-B and ANP-C receptors. By means of autoradiography and radioligand binding assay, we found dense population of ANP-C receptors in the gill cartilage as well as gill cells.III.Plasma ANP levels increased immediately after transfer of eels from fresh water to seawater. However, in eels adapted to seawater for two weeks, plasma ANP levels decreased significantly from the freshwater levels. The ANP level in the dorsal aorta was decreased to half compared with the level in the ventral aorta, which is explained by the extraction of ANP by ANP-C receptors in the gill.IV.ANP inhibited water intake and intestinal water and sodium absorption in the eel. ANP also caused antidiuresis and increased cortisol excretion. The former actions on the brain and intestine are advantageous to freshwater adaptation, but the latter two actions, antidiuretic and steroidogenic actions, are favorable to seawater adaptation. Thus it is not yet established whether ANP is a freshwater-adapting hormone or seawater adapting hormone in the eel.