RUBIDIUM ABSORPTION AND PROTON SECRETION BY RABBIT OUTER MEDULLARY COLLECTING DUCT VIA H-K-ATPASE

Multiple lines of evidence support the hypothesis that the outer medullary collecting duct from the inner stripe (OMCD(i)) possesses a functional proton-potassium-activated adenosinetriphosphatase (H-K-ATPase). To examine the effect of inhibition of H-K-ATPase on Rb efflux, we measured the Rb tracer rate efflux coefficient (K(Rb)) across the OMCD(i) of animals adapted to a K-restricted diet using the selective K-competitive H-K-ATPase inhibitor, Sch 28080. Sch 28080 (10 muM) did not significantly alter transepithelial voltage (VT) but significantly decreased K(Rb) by 41%. We further examined the effect of 10% peritubular CO2 on K(Rb) and the subsequent effect of Sch 28080 (10 muM) on K(Rb). After exposure to 10% CO2 for 120 min, vehicle-treated tubules exhibited a small but significant increase in K(Rb) without a significant change in VT. In contrast, 10 muM Sch 28080 significantly decreased K(Rb) by 44% without affecting V(T). The lack of an effect of H-K-ATPase inhibition on V(T) in the presence of either 5% or 10% CO2 was in marked contrast to the effect of carbonic anhydrase inhibition (CAI). CAI consistently and significantly decreased VT either in the presence of 5% or 10% CO2. To address whether H-K-ATPase also participates in proton secretion we examined the effect of Sch 28080 (10 muM) on net bicarbonate absorption by the OMCD(i) of rabbits fed a normal rabbit ration and rabbits adapted to a K-restricted diet. Sch 28080 (10 muM) significantly reduced bicarbonate flux by a similar extent under both dietary conditions (76% inhibition in K-replete animals, 62% inhibition in K-restricted animals) without significantly affecting V(T). We conclude the following: Rb/K and net bicarbonate absorption by the OMCD(i) are dependent on H-K-ATPase. Inhibition of H-K-ATPase does not affect V(T), suggesting that this process is electroneutral. H-K-ATPase inhibition and CAI have dissimilar effects on V(T). Finally, the meager response of the OMCD(i) to 10% CO2 compared with the response of the cortical collecting duct further emphasizes the heterogeneity of the collecting duct.