Changes in plasma atrial and ventricular natriuretic peptide concentrations after transfer of eels from freshwater to seawater or vice versa.

Changes in plasma atrial and ventricular natriuretic peptide (ANP and VNP) concentrations were examined in conscious eels, Anguilla japonica, after transfer from freshwater (FW) to seawater (SW) or vice versa. These changes were compared in ventral and dorsal aortic blood since clearance receptors are present in the gills. Plasma Na concentration and osmolality in the ventral aortic blood rose linearly after transfer of FW eels to SW. Six hours after transfer, plasma ANP levels increased from 28.1 +/- 2.9 to 44.8 +/- 5.4 fmol/ml (n = 6) (P < 0.05) and plasma VNP levels also increased from 63.6 +/- 3.8 to 91.2 +/- 6.2 fmol/ml (n = 6) (P < 0.05). However, the levels gradually returned to the FW levels despite plasma Na concentration and osmolality remaining elevated. Dorsal aotic plasma ANP and VNP levels were unchanged after transfer of FW eels to SW, possibly due to extraction of secreted peptides by the branchial clearance receptors. After transfer of SW-adapted eels to FW, plasma ANP and VNP levels were unchanged, while both plasma Na concentration and osmolality decreased. These results indicate that plasma ANP and VNP levels are increased by an increase in plasma Na concentration or osmolality after transfer of FW eels to SW. However, the sustained increase in plasma Na concentration or osmolality no longer stimulated ANP and VNP secretion, and plasma ANP and VNP levels gradually returned to the FW levels gradually on adaptation to SW.