EFFECT OF ACUTE CHANGES IN GLOMERULAR-FILTRATION RATE ON NA+/H+ EXCHANGE IN RAT RENAL-CORTEX

Studies were undertaken in Munich-Wistar rats to assess the influence of changes in filtered bicarbonate (FL(HCO3)), induced by changes in GFR, on Na+/H+ exchange activity in renal brush border membrane vesicles (BBMV). Whole-kidney and micropuncture measurements of GFR, FL(HCO3), and whole-kidney and proximal tubule HCO3 reabsorption (APR(HCO3)) were coupled with BBMV measurements of H+ gradient-driven Na+-22 uptake in each animal studied. Na+-22 uptake was measured at three Na+ concentration gradients to allow calculation of V(max) and K(m) for Na+/H+ exchange. GFR was varied by studying animals under conditions of hydropenia, plasma repletion, and acute plasma expansion. The increase in GFR, FL(HCO3), and APR(HCO3) induced by plasma administration correlated directly with an increase in the V(max) for Na+/H+ exchange in BBMV. The K(m) for sodium was unaffected. In the plasma-expanded rats, the V(max) for Na+/H+ exchange was 22% greater than in the hydropenic rats (P < 0.025) whereas APR(HCO3) was 86% greater (P < 0.001). These results indicate that increases in FL(HCO3), induced by acute increases in GFR, stimulate Na+/H+ exchange activity in proximal tubular epithelium. This stimulation is a mechanism which can, in part, account for the delivery dependence of proximal bicarbonate reabsorption.