An electrogenic H-ATpase sensitive to inhibition by N-ethyl-maleimide has been reported to be present in renal distal tubules. In contrast to another H-ATPase (gastric H-K-ATPase), the renal enzyme is not stimulated by K+ and is not inhibited by vanadate. However, our preliminary observations indicated that a K-stimulated ATPase (K-ATPase) sensitive to inhibition by vanadate is present in renal medullary collecting duct (MCD). To localize and further characterize this renal tubular K-ATPase, we measured K-ATPase activity in eight specific segments of the rabbit nephron. K-ATPase activity was the difference in ATPase activity in the presence and absence of KCl but in the presence of ouabain (to inhibit Na-K-ATPase). ATPase activity was determined by a fluorometric microassay in which ATP hydrolysis is coupled to the oxidation of NADH. There was a significant K-ATPase activity (expressed as pmol.min-1.mm-1) in the connecting tubule (CNT, 17.0 +/- 3.3), cortical collecting duct (CCD, 6.6 +/- 0.7), and MCD (8.8 +/- 1.7), but not in the proximal segments and the thick ascending limbs. The renal tubular K-ATPase was not only inhibited by vanadate but also by omeprazole and SCH 28080 (relatively specific inhibitors of gastric H-K-ATPase). It is concluded that K-ATPase present in the CNT, CCD, and MCD has some properties in common with gastric H-K-ATPase. However, the physiological role of K-ATPase in the distal nephron segments remains to be elucidated.
L C Garg, N Narang