ADAPTATION TO LOW-K-K+-ATPASE BUT NOT H+-ATPASE-MEDIATED PH(I) RECOVERY IN OMCD1 CELLS( MEDIA INCREASES H+)

Studies in rat and rabbit outer medullary collecting duct of inner str ipe origin (OMCDis) suggest that both H+-ATPase and H+-K+ ATPase parti cipate in H+ secretion. However, the relative contributions of these t ransporters, and, in particular, that of H+-K+-ATPase to K+ absorption have not been defined precisely. The present study was designed to de lineate more clearly the response of these two transporters to hypokal emia and acidosis in a newly developed mouse OMCD1 cell line. In cells grown in normal K+ (5 mM) media, intracellular pH (pH(i)) recovery wa s similar either in the presence or absence of K+ in the perfusate (De lta pH(i)/min = 0.014 +/- 0.001 vs. 0.017 +/- 0.003, not significant). The inhibitory effects of Sch-28080 (10 mu M) and bafilomycin A(1) (1 0 nM) on pH(i) recovery were evident only in the presence and absence of K+ in the perfusate, respectively. In cells grown in low-K+ (2.5 mM ) media to simulate chronic hypokalemia, pH(i) recovery was significan tly faster than in cells grown in normal K+ media (Delta pH(i)/min = 0 .045 +/- 0.01 vs. 0.014 +/- 0.001, P < 0.01) and was inhibited specifi cally by Sch-28080, not by bafilomycin A(1). In contrast, in cells pre conditioned to low pH (7.0) to simulate chronic acidosis, the enhanced pH(i) recovery was abolished by bafilomycin A(1) but not by Sch-28080 . Rb-86(+) uptake, when used as a K+ congener, was inhibited by Sch-28 080. The K-m for Rb-86(+) uptake (H+-K+-ATPase activity) and the 50% i nhibitory concentration for Sch-28080 were 270 and 5.0 mu M, respectiv ely. These studies provide evidence that, in morphologically homogeneo us OMCD1 cells, 1) both H+-K+-ATPase and H+-ATPase participate in pH(i ) regulation, 2) the H+-K+ ATPase is selectively upregulated by precon ditioning in low-K+ media, and 3) conversely, preconditioning in low-p H media stimulates only the H+-ATPase. Thus, in OMCDis, the H+-K+-ATPa se and H+-ATPase respond selectively and independently to chronic hypo kalemia and acidosis, respectively.