Sch-28080 inhibits bafilomycin-sensitive H+ secretion in turtle bladder independently of luminal [K+]

To explore the possible contribution of an H-K-adenosine-triphosphatase (H-K-ATPase) to H+ secretion (JH) in the isolated turtle bladder, we measured electrogenic JH (JeH) as short-circuit current and total JH (JTH) by pH stat titration in the presence of ouabain at different ambient K+ concentration ([K+]) and during luminal addition of a known gastric H-K-ATPase inhibitor, Schering (Sch)-28080. JH was not reduced by decreasing ambient [K+] to undetectable or very low levels (< 0.05 mM by atomic absorption) and luminal BaCl2 addition to further reduce local [K+] at the apical membrane. These K(+)-removal studies indicate that H+ transport is not coupled to countertransport of K+. JTH did not exceed JeH at any point: in K(+)-free solutions JTH was 0.73 +/- 0.05, and JeH was 0.95 +/- 0.08 mumol/h; in standard (3.5 mM) K+ solutions JTH was 0.72 +/- 0.05 and JeH 0.98 +/- 0.06 mumol/h; in high (118 mM) K+ solutions JTH was 0.65 +/- 0.07 and JeH 0.94 +/- 0.08 mumol/h. Sch-28080 caused a rapid inhibition of JH, with similar half-maximal inhibitory concentrations (IC50) in K(+)-free, standard [K+], and high [K+] solutions. Bafilomycin inhibited JeH and JTH with an IC50 of approximately 100 nM. The observed non-potassium-competitive inhibition of JH by Sch-28080 and the bafilomycin sensitivity 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.