Suppression of plasma renin and plasma aldosterone during water immersion in normal man

Previous studies from this laboratory have demonstrated that the redistribution of blood volume and concomitant relative central hypervolemia induced by water immersion to the neck (NI) results in a significant natriuresis which is quantitative identical to that induced by the acute administration of 2 liters of saline. Since the central hypervolemia induced by NI occurs without concomitant alterations in serum sodium and potassium concentration, the NI model was utilized to assess the role of volume in the regulation of both plasma renin (PRA) and plasma aldosterone (PA) in man. Nine normal subjects were studied on two occasions while in balance on a 10 meq Na, 100 meq K diet; Control and NI. The conditions of seated posture and time of day were identical. Blood for PRA and PA was obtained at 30-min intervals for 6 h. NI produced a profound suppression of PRA as early as 30 min with maximal suppression (62%) by 180 min (P less than 0.001). Recovery from NI was associated with a prompt return to pre-study levels. The changes in PA paralleled those of PRA with regard to both the rapidity and magnitude of the suppression (r = 0.993: P less than 0.001). These data emphasize the importance of central volume per se as a primary determinant of PRA and PA regulation in normal man. Furthermore, the current studies confirm the importance of the renin-angiotensin axis in the control of volume-related changes in PA in normal man. The ability of NI to induce a prompt and parallel suppression of PRA and PA without concomitant alterations in plasma composition, suggests that NI may be a preferred investigative tool for assessing the effects of volume expansion on renin-aldosterone.