Effect of acute increases in filtered HCO3- on renal hydrogen transporters: II. H+-ATPase

Effect of acute increases in filtered HCO3- on renal hydrogen transporters: II. H+-ATPase. Adaptive increases in renal bicarbonate reabsorp-tion occur in response to acute increases in filtered bicarbonate (FLHCO3). In a previous study, we showed that an increase in FLHCO3 induced by plasma volume expansion increased the Vmax for Na+/H+ exchange activity in renal cortical brush border membrane vesicles (BBMV), providing a potential mechanism for the adaptive increase in HCO3- reabsorption. The present studies were undertaken to determine whether the increase in FLHCO3 induced by plasma expansion also stimulates the other major H+ transporter in cortical BBMV, the H+-ATPase. H+-ATPase activity was assessed in BBMV obtained from hydropenic and plasma expanded Munich-Wistar rats, using a NADH-linked ATPase assay. H+-ATPase activity was measured as the ouabain and oligomycin-insensitive, bafilomycin A1-sensitive component of total ATPase activity. Acute plasma expansion doubled single nephron FLHCO3, and this change was associated with a 64% increase in the Vmax for H+-ATPase activity, with no change in apparent Km. The Vmax for H+-ATPase activity correlated directly with whole kidney GFR and FLHCO3 (r = 0.68 and 0.72, respectively), and with single nephron GFR and FLHCO3 (r = 0.76 and 0.80, respectively). Thus, the mechanism for the adaptive increase in proximal tubular HCO3- reabsorption that occurs in response to acute increases in FLHCO3 appears to be related to increased activity of both H+-ATPase and Na+/H+ exchange in the apical membrane of the proximal tubule epithelium