DOES THE COLONIC H,K-ATPASE ALSO ACT AS AN NA,K-ATPASE

We previously have demonstrated that the colonic P-ATPase alpha subuni t cDNA encodes an H,K-ATPase when expressed in Xenopus laevis oocytes. Besides its high level of amino acid homology (75%) with the Na,K-ATP ase, the colonic H,K-ATPase also shares a common pharmacological profi le with Na,K-ATPase, because both are ouabain-sensitive and Sch 28080- insensitive. These features raise the possibility that an unrecognized property of the colonic H,K-ATPase would be Na+ translocation. To tes t this hypothesis, ion-selective microelectrodes were used to measure the intracellular Na+ activity of X. laevis oocytes expressing various combinations of P-ATPase subunits. The results show that expression i n oocytes of the colonic H,K-ATPase affects intracellular Naf homeosta sis in a way similar to the expression of the Bufo marinus Na,K-ATPase ; intracellular Na+ activity is lower in oocytes expressing the coloni c H,K-ATPase or the B. marinas Na,K-ATPase than in oocytes expressing the gastric H,K-ATPase or a beta subunit alone. In oocytes expressing the colonic H,K-ATPase, the decrease in intracellular Naf activity per sists when diffusive Na+ influx is enhanced by functional expression o f the amiloride-sensitive epithelial Na+ channel, suggesting that the decrease is related to increased active Na+ efflux. The Na+ decrease d epends on the presence of K+ in the external medium and is inhibited b y 2 mM ouabain, a concentration that inhibits the colonic H,K-ATPase. These data are consistent with the hypothesis that the colonic H,K-ATP ase may transport Na+, acting as an (Na,H),K-ATPase. Despite its molec ular and functional characterization, the physiological role of the co lonic (Na,H),K-ATPase in colonic and renal ion homeostasis remains to be elucidated.