SCH-28080 INHIBITS BAFILOMYCIN-SENSITIVE H+ SECRETION IN TURTLE BLADDER INDEPENDENTLY OF LUMINAL [K+]

To explore the possible contribution of an H-K-adenosinetriphosphatase (H-K-ATPase) to H+ secretion (J(H)) in the isolated turtle bladder, w e measured electrogenic J(H) (J(H)e) as short-circuit current and tota l J(H) (J(H)T) by pH stat titration in the presence of ouabain at diff erent ambient K+ concentration ([K+]) and during luminal addition of a known gastric H-K-ATPase inhibitor, Schering (Sch)-28080. J(H) was no t reduced by decreasing ambient [K+] to undetectable or very low level s (<0.05 mM by atomic absorption) and luminal BaCl2 addition to furthe r reduce local [K+] at the apical membrane. These K+-removal studies i ndicate that H+ transport is not coupled to countertransport of K+. J( H)T did not exceed J(H)e at any point: in K+-free solutions J(H)T was 0.73 +/- 0.05, and J(H)e, was 0.95 +/- 0.08 mumol/h; in standard (3.5 mM) K+ solutions J(H)T was 0.72 +/- 0.05 and J(H)e 0.98 +/- 0.06 mumol /h; in high (118 mM) K+ solutions J(H)T was 0.65 +/- 0.07 and J(H)e 0. 94 +/- 0.08 mumol/h. Sch-28080 caused a rapid inhibition of J(H), with similar half-maximal inhibitory concentrations (IC50) in K+-free, sta ndard [K+], and high [K+] solutions. Bafilomycin inhibited J(H)e and J (H)T with an IC50 of approximately 100 nM. The observed non-potassium- competitive inhibition Of J(H) by Sch-28080 and the bafilomycin sensit ivity distinguish the H-ATPase of the turtle bladder from the gastric H-K-ATPase. The rapidity of the inhibition by Sch-28080 suggests that it acts at an accessible luminal site of the ATPase.