Regulation of vasoactive intestinal peptide release and metabolism by sodium

To determine whether vasoactive intestinal peptide (VIP) might act as a humoral mediator for a proposed portal or hepatic sodium monitor, we measured plasma VIP levels and urinary sodium excretion after portal and intravenous sodium loading in rabbits equilibrated on normal and low sodium diets. Sodium excretion was significantly less after intravenous (2 h: p less than 0.005; 4 h; p less than 0.005; 8 h; p less than 0.025) than after portal sodium administration in rabbit on a low-sodium diet. Plasma VIP levels fell after intravenous (p less than 0.005) but not after intraportal sodium in this group. To determine whether this fall in VIP levels reflected increased metabolism or decreased secretion, metabolic clearance studies were performed in rabbits on a low sodium diet. The metabolic clearance rate of VIP and its theoretical secretion rate increased after intravenous sodium loading in rabbits on a low salt diet (MCR, p less than 0.025; SR, p less than 0.05). We conclude, therefore, that in rabbits on a low-salt diet, intravenous sodium increases VIP metabolism, causing a decrease in plasma levels that may explain the difference in sodium excretion.