H-ATPASES OF RENAL CORTICAL AND MEDULLARY ENDOSOMES ARE DIFFERENTIALLY SENSITIVE TO SCH-28080 AND OMEPRAZOLE()

Adenosinetriphosphatase (ATPase) activity stimulated by K+ and inhibit ed by Sch-28080 (SCH), omeprazole (OME), and vanadate has been measure d in microsomes from mammalian renal medulla and attributed to a kidne y isoform of the H+-K+-ATPase. To determine whether the H+-K+-ATPase i nhibitors could also inhibit the vacuolar (V)-type H+-adenosinetriphos phatase (H+-ATPase, i.e., HC pump) in mammalian intracellular vesicles , we examined their effects on bafilomycin-sensitive acidification in renal cortical vesicles (CEV) and medullary endocytic vesicles (MEV). Rats were injected with fluorescein isothiocya nate-labeled dextran, a nd labeled endosomes were enriched from kidney tissue homogenates by d ifferential and Percoll density gradient centrifugation. In the CEV, t he V-type H+ pump was inhibited 25% by SCH and 30% by OME (100 mu M ea ch). Whereas the inhibition by OME was concentration and time dependen t, the inhibition by SCH was only concentration dependent. Inhibition by these compounds was similar in the presence of 50 mM K+ (in = out) and in the complete absence of K+, thus ruling out a significant invol vement of H+-K+-ATPase-mediated acidification. Inhibition, however, wa s not observed with 10 mu M SCH and OME. The sensitivity of the V-type H+ pump to 100 mu M SCH and OME in CEV was confirmed by the comparabl e inhibitions of intravesicular acidification observed in acridine ora nge fluorescence quench studies and by inhibition of P-i liberation in an ATPase assay. We also found that the V-type H+ pump in isolated ra t liver endosomes is sensitive to 100 mu M SCH and OME to a similar de gree. In the MEV, acidification was only weakly affected by 100 mu M S CH and OME, thus suggesting that H+-ATPases in endosomes from cortical and medullary tubules are different, possibly due to a previously des cribed selective expression of subunit isoforms. Our finding indicates the importance of using low concentrations (<10 mu M) of OME and SCH in studies of H+-K+-ATPase in nongastric tissues to avoid misinterpret ation of the data due to nonspecific inhibition of V-type H+-ATPases.