Bicarbonate-stimulated ATPase in the renal proximal tubule luminal (brush border) membrane

Abstract

Brush border membranes of the rabbit renal tubule have an ATPase which was stimulated 60% by 50 m m HCO 3 −. The K a for HCO 3 − was 36 m m. Kinetic studies of the “HCO 3 −-ATPase” indicate that HCO 3 − had no effect on the K m for ATP and ATP did not alter the K a for HCO 3 −. Several anions, notably SO 3 2−, also accelerated the rate of dephosphorylation of ATP. The V for “SO 3 2−-ATPase” was fivefold greater than that for “HCO 3 −-ATPase.” The K a for SO 3 2− was 0.78 m m. Other anions including Cl − and phosphates, did not enhance ATPase activity. Thus, of the anions present in the glomerular filtrate in appreciable concentrations only HCO 3 − stimulated the luminal membrane enzyme. The anion-stimulated ATPase activity increased sharply from pH 6.1 to 7.1 and moderately with higher pH. The renal ATPase was not inhibited by SCN − nor methyl sulfonyl chloride and was relatively insensitive to oligomycin and quercetin. Carbonyl cyanide p-trifluoromethoxy phenylhydrazone increased the basal rate of the membranal ATPase, suggesting that the ATPase activity is limited by transmembrane H + flux. Carbonic anhydrase significantly increased the HCO 3 −-stimulated ATPase activity. This increment was blocked by Diamox. These findings provide evidence consistent with the hypothesis that the brush border membrane ATPase is involved in the extrusion of H + from tubular cell to lumen and support suggested interrelationships between HCO 3 −-stimulated ATPase, H + secretion, and bicarbonate transport in the kidney.