Abstract
In eels, plasma osmolality rather than blood volume is a major regulator of atrial and ventricular natriuretic peptide (ANP and VNP) secretion. The present study examined the effects of changes in blood volume on ANP and VNP secretion stimulated by an increase in plasma osmolality in freshwater-adapted eels. Basal plasma ANP and VNP levels were decreased by 2 ml of blood withdrawal (28% of total blood volume), but not changed by blood volume expansion with 2 ml of 0.9% NaCl solution containing 2% dextran. The blood loss suppressed the increased plasma ANP level caused by an injection of 2.5 ml/kg of 1.7MNaCl solution at 60 min (120.5±31.0 fmol/ml,n=5) compared with controls without blood volume manipulation (586.6±43.6 fmol/ml,n=5), but the plasma ANP level transiently increased in bled fish immediately after osmotic stimulus, probably due to the release of ANP stored in the cardiac tissues after the blood loss. Changes in plasma VNP were not so evident as those of ANP. In contrast, blood volume expansion augmented the increase in plasma ANP and VNP levels within 60 min after osmotic stimulus compared with controls. The recovery of plasma VNP level was quicker than that of plasma ANP. Increases in plasma Na, Cl concentrations, and osmolality were not different among hypovolemic, normovolemic, and hypervolemic eels after osmotic stimulation. It is concluded that volume itself is a minor regulator for ANP and VNP secretion compared with osmotic stimulus, but it plays a modulatory role in osmotically induced ANP and VNP secretion in eels.
Published Version
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