Mechanism of decrease in plasma renin activity during volume expansion

The importance of filtered load of sodium and extracellular volume expansion (ECVE( per se on plasma renin activity (PRA) was studied in two groups of hydropenic dogs. Group I protocol consisted of bilateral ureteral obstruction (UO) followed by isotonic ECVE and finally UO release. During UO, PRA rose significantly above control (15.4 leads to 41.7 ng/ml per h). Superimposition of ECVE did not significantly alter PRA, but UO release returned PRA toward control (24.8 ng/ml per h). Fractional sodium excretion (FE(Na)) after UO release was higher than control (0.6 leads to 12.6%). In group II, ECVE preceded UO and caused a fall in PRA below control (16.8 leads to 4.8 ng/ml per h). FE(Na) was significantly increased over control (0.3 leads to 14.7%). Superimposition of UO reversed the fall in PRA to a value not significantly different from control. On release of UO PRA fell, but not significantly, whereas FE(Na) fell to 12.6%, a value not different from that during ECVE alone. The results indicate that filtration is required for ECVE to elicit a fall in PRA, under the present experimental condition.