Atrial and ventricular natriuretic peptide concentrations in plasma of freshwater- and seawater-adapted eels

Plasma levels of atrial and ventricular natriuretic peptide (ANP and VNP) were measured in freshwater (FW)- and seawater (SW)-adapted eels using homologous radioimmunoassays. Anesthesia with tricaine (MS222) and exposure of the pericardium for blood sampling from the ventral aorta affected plasma ANP and VNP levels. ANP and VNP levels in the ventral aorta were higher than those in the dorsal aorta when blood was collected simultaneously from both aortae of cannulated conscious FW eels, perhaps reflecting branchial extraction of the peptides. This phenomenon was not observed in SW eels. Based on these results, blood was sampled from conscious eels through a catheter implanted in the ventral aorta because it appears to reflect most accurately ANP secretion. Plasma ANP and VNP levels measured in more than 100 conscious FW and SW eels displayed large variations at each month despite acclimation to laboratory conditions at 180 for more than 1 week in FW eels or for more than 2 weeks in SW eels. However, concentrations of ANP and VNP averaged from all data were not different between FW and SW eels: 68.1 +/- 2.9 (FW: n = 155) vs 60.7 +/- 3.6 (SW: n = 104) for ANP, 51.6 +/- 3.0 (FW: n = 69) vs 55.0 +/- 3.7 (SW: n = 54) for VNP (fmol/ml in each case). Plasma Na concentration and osmolality were significantly higher in SW eels than in FW eels. Plasma ANP and VNP levels thus do not seem to be affected by chronic changes in plasma Na concentration and osmolality.