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The interaction between SCH28080 and omeprazole, two specific inhibitors of gastric H+/K+-ATPase, was investigated using gastric glands and isolated gastric membranes. For gastric glands, inhibition of acid formation by SCH28080 was not reversed by washing whereas inhibition by omeprazole was partially reversed after washing. These features are opposite to what is found with isolated membranes. However, if gastric glands were permeabilized with digitonin after exposure to the inhibitors and recovery measured as ATP-dependent acid formation or H+/K+-ATPase activity, inhibition by SCH28080 was completely reversed while inhibition by omeprazole was non-reversible. Using a procedure of pretreatment with inhibitors followed by permeabilization and assay of recovered activity, it was found that a combined treatment with SCH28080 plus omeprazole prevented the irreversible inhibition by omeprazole, i.e. acid forming capability and ATPase activity were fully recovered. In order to test the possibility that SCH28080 prevented activation of omeprazole by dissipating an acid environment, control experiments were performed with SCN, which gave equivalent dissipation of the acid gradient but did not prevent the irreversible inhibition by omeprazole. These results were confirmed in isolated gastric membranes where residual p-nitrophenylphosphatase activity was assayed following exposure of acid transporting vesicles to omeprazole. Compared to control conditions, omeprazole inhibited 48% of the phosphatase activity whereas simultaneous addition of SCH28080 reduced the inhibition to 14%. The results therefore suggest that SCH28080 selectively blocks irreversible inhibition by omeprazole and thus that these two agents interact at a common region of the luminal aspect of the gastric H+/K+-ATPase.
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