Adaptation to low-K+ media increases H+-K+-ATPase but not H+-ATPase-mediated pH(i) recovery in OMCD1 cells

Studies in rat and rabbit outer medullary collecting duct of inner stripe origin (OMCDis) suggest that both H+-ATPase and H+-K+ ATPase participate in H+ secretion. However, the relative contributions of these transporters, and, in particular, that of H+-K+-ATPase to K+ absorption have not been defined precisely. The present study was designed to delineate more clearly the response of these two transporters to hypokalemia and acidosis in a newly developed mouse OMCD1 cell line. In cells grown in normal K+ (5 mM) media, intracellular pH (pH(i)) recovery was similar either in the presence or absence of K+ in the perfusate (Delta 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) (10 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 significantly 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 specifically by Sch-28080, not by bafilomycin A(1). In contrast, in cells preconditioned 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-28080. The K-m for Rb-86(+) uptake (H+-K+-ATPase activity) and the 50% inhibitory concentration for Sch-28080 were 270 and 5.0 mu M, respectively. These studies provide evidence that, in morphologically homogeneous OMCD1 cells, 1) both H+-K+-ATPase and H+-ATPase participate in pH(i) regulation, 2) the H+-K+ ATPase is selectively upregulated by preconditioning in low-K+ media, and 3) conversely, preconditioning in low-pH media stimulates only the H+-ATPase. Thus, in OMCDis, the H+-K+-ATPase and H+-ATPase respond selectively and independently to chronic hypokalemia and acidosis, respectively.