RESETTING OF 24-H SODIUM AND WATER-BALANCE DURING 4 DAYS OF SERVO-CONTROLLED REDUCTION OF RENAL PERFUSION-PRESSURE

This study examines whether an increase in renal perfusion pressure (R PP) is necessary to escape endogenously stimulated Na- and water-retai ning mechanisms. In seven dogs stimulation was accomplished by a servo -controlled reduction of RPP (rRPP) below the threshold for pressure-d ependent renin release for 4 days. Oral intake was standardized. Plasm a renin activity (PRA) rose from 2.5 in controls to similar to 5 ng AN G I.ml(-1) h(-1) during rRPP days. Plasma aldosterone concentration (P AC) increased by similar to 50% only on day 1 of rRPP but fell at or b elow control levels thereafter. The PAC-to-PRA ratio decreased during rRPP days. Atrial natriuretic factor (ANF) rose to values three times higher than in controls. Mean systemic blood pressure (MABP) rose from 111 +/- 12 in controls to 142 +/- 14 mmHg on day 4 of rRPP. On day 1 of rRPP 60% of the Na and 24% of the water intake were retained. Howev er, after 2-3 days the input-output balance was restored but on a high er level of total body Na and total body water (new ''set point''). Be cause elevated systemic MABP could not exert direct pressure effects o n the kidneys due to servo control of rRPP, there must be other factor s, e.g., fall in PAC, increase in ANF, and changes in intrarenal hemod ynamics and physical factors that may have contributed to the resettin g of input-output balances during rRPP.