OMEPRAZOLE AND SCH-28080 INHIBIT ACID-SECRETION BY THE TURTLE URINARY-BLADDER

There is now convincing evidence that in addition to the vacuolar-type H+-ATPase, a gastric-type H+/K+-ATPase participates in acidification by the distal nephron. To determine whether a similar pump exists in t he turtle bladder, we examined the dependence of acid secretion on muc osal K+, and the effects of supposedly specific inhibitors of the gast ric H+/K+-ATPase, omeprazole and SCH 28080. In CO2-stimulated bladders both drugs produced dose-dependent inhibition of electrogenic H+ secr etion measured as the reverse short-circuit current (RSCC). At the hig hest concentrations tested, H+ secretion decreased 45+/-16% with mucos al and 20+/-7% with serosal omeprazole (p<0.01). SCH 28080 at 400 muM produced essentially complete inhibition of H+ secretion with either m ucosal or serosal application. When H+ secretion was purposefully inhi bited by DIDS or an adverse mucosal pH gradient, SCH 28080 had no effe ct on RSCC. Removing mucosal K+ (measured K+ <50 muM), with or without mucosal barium, had no effect on RSCC. The inhibition of RSCC by omep razole was reversed by mercaptoethanol. Finally, HCO3 secretion, as me asured by either RSCC or PH-stat titration, increased significantly in response to 400 muM SCH 28080. The results demonstrate that these com pounds inhibit acid secretion by the turtle bladder but stimulate the secretion of base. In view of the total independence of acid secretion on potassium, it is unlikely that any of the bladder's acid secretion is mediated by an H+/K+-ATPase. The most reasonable interpretation of the data is that omeprazole and SCH 28080, previously thought to be s pecific inhibitors of the H+/K+-ATPase, also inhibit the vacuolar H+-A TPase of the turtle bladder. The results also indicate that HCO3 secre tion by the bladder employ a different mechanism of H+ transport than is used for acid secretion; there is no simple reversal of polarity in the acid- versus base-secreting cells.