Presence of a natriuretic factor in urine of normal men undergoing water immersion

Abstract

Immersion of normal subjects in water to the neck (NI) results in a redistribution of blood volume, relative central hypervolemia, and a significant natriuresis which is equal in magnitude to that produced by 10% expansion of the extracellular fluid volume by isotonic saline. The natriuresis of NI is independent of changes in GFR and is not prevented by the administration of DOCA. The present study was undertaken to determine whether the NI natriuresis is associated with increased activity of the natriuretic factor (NF) that has been observed in volume-expanded dogs and uremic dogs and patients. Twelve normal subjects were studied following overnight water restriction for 10 hr. Each subject was studied under control conditions, during NI in the seated position, and at the same time of day. U(Na)V, while remaining constant during control studies, increased significantly during NI. Bioassays for natriuretic activity were performed in rats by using urine fractions from NI and control studies. The NI fractions in the test animals resulted in significant increments in both U(Na)V (1.27 ± 0.28 μEq/min) and FE (Na) (1.39 ± 0.21%) ( P < 0.001 for both). With the control fractions, there was no significant change in U(Na)V, and there was a small increment in FE (Na) (0.70 ± 0.25%; P < 0.05). The natriuresis of NI, thus, is associated with increased activity of natriuretic factor. The demonstration of increased NF in normal subjects during a period of increased sodium excretion strengthens the possibility that the natriuretic factor may participate in the regulation of sodium excretion and of sodium homeostasis.