ROLE OF THE RENIN-ANGIOTENSIN SYSTEM IN MEDIATING THE EFFECTS OF POSTURE ON RENAL-FUNCTION

This study was designed to quantitate the influence of the neurohumora l activation associated with orthostatic stress on renal hemodynamics and sodium excretion and, furthermore, to determine the importance of the renin-angiotensin system in mediating these changes in renal funct ion. Seven conscious dogs were studied while lying in the recumbent po sition and, subsequently, after standing in a supporting sling. Experi ments were conducted under control conditions and after plasma angiote nsin II (ANG II) concentration was fixed at control levels by chronic infusion of captopril (14 mu g . kg(-1). min(-1)) and ANG II (0.5 +/- 0.02 ng . kg(-1). min(-1)). During control experiments, 45 min of stan ding increased plasma renin activity twofold, whereas mean arterial pr essure, heart rate, and plasma norepinephrine concentration remained u nchanged. During standing, glomerular filtration rate (GFR) and renal plasma dow (RPF) fell to 88 +/- 2 and 77 +/- 3% of recumbent values, r espectively, whereas filtration fraction (FF) increased 16 +/- 1%. Add itionally, urinary (UNaV) and fractional sodium excretion (FE(Na)) dec reased to 27 +/- 6 and 30 +/- 7% of recumbent values, respectively. Wh en plasma ANG II concentration was fixed at control levels during stan ding, there were no significant changes in GFR, whereas increments in FF and reductions in RPF, UNaV, and FE(Na) were attenuated by 63, 40, 30, and 33%, respectively. These data suggest that, in conscious dogs, standing in a supporting sling causes reflex activation of the sympat hetic nervous and renin-angiotensin systems, eliciting reductions in G FR, RPF, and UNaV. Furthermore, ANG II contributes significantly to th e effects of passive standing on renal hemodynamics and UNaV.