Comparison of the Suppressive Effects of Water Immersion and Saline Administrationon Renin-Aldosterone in Normal Man*

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Abstract
Although previous studies have demonstrated that water immersion to the neck (NI) results in a relative central hypervolemia and a profound suppression of PRA and plasma aldosterone(PA), it is unclear whether the magnitude of suppression of the renin-aldosterone axis is comparable to that induced by extracellular fluid volume expansion associated with acute saline administration. The present study was undertaken, therefore, to compare the relative suppressive effectsof NI and saline administration on the renin-aldosterone axis. Nine normal subjects were studied on three occasions while in balance on a 10-meq Na, 100-meq K diet: control, NI, and saline. Blood for PRA and PA was obtained at 30-min intervals for 6 h. The conditions of seated posture and time of day were identical. NI produced a profound suppression of PRA (54 ° 10%) and a 59% suppression of PA which were similar both in magnitude and temporal profile to those observed during saline. The present study characterizes the dynamics of PRA and PA responsiveness during NI, demonstrating that in addition to similar central hemodynamic effects, the alterations of renin-aldosterone during NI and saline are similar. The delineation of the NI model commends its use as an alternative investigative tool for assessing renin-aldosterone responsiveness in a variety of edematous states.