Mechanism of action of gastric secretory inhibitors: effects of SCN- OCN-, NO-2, and NH+4 on (H+ + K+)-ATPase-mediated transport of H+ inside gastric microsomal vesicles

The mechanisms of action of the known inhibitors of gastric acid secretion such as SCN^?, OCN^?, NO₂^?, and NH₄⁺ (M. E. LeFevre, E. J. Gohmann, Jr. and W. S. Rehm, 1964, Amer. J. Physiol.207, 613–618) were investigated using isolated pig gastric microsomal vesicles as a model system. The gastric microsomal vesicles enriched in (H⁺ + K⁺)-ATPase have previously been demonstrated to accumulate H⁺ in exchange for K⁺. The vesicular accumulation of acridine orange, which is a measure of H⁺ uptake, shows sigmoidal kinetics in the presence of increasing K⁺ with a Hill coefficient of 2.27 and a S₅₀ of 19.05 mm. None of those agents affects the microsomal (H⁺ + K⁺)-ATPase activity, although they inhibit vesicular H⁺ transport in a dose-dependent manner; the order of efficacy being NH₄⁺ > SCN^? > OCN^? > NO₂^?. The inhibitory effects of NH₄⁺ on vesicular H⁺ transport appear to be due to neutralization of the transported H⁺ by freely permeable NH₃ generated from the dissociation of NH₄⁺ in the bulk medium. SCN^?, OCN^?, and NO₂^? appear to work by a different mechanism. These agents do not act as protonophores. Our data demonstrate that the presence of SCN^?, OCN^?, and NO₂^? within the vesicle interior are essential for exerting their inhibitory effects. Furthermore, the inhibitory effects of SCN^? and OCN^? on vesicular H⁺ transport could be reversed by an elevation of intravesicular K⁺. Our data strongly suggest that the effects of SCN^?, OCN^?, and NO₂^? are exerted by interfering with a low-affinity K⁺ site (S₅₀ = 19.05 mm) within the domain of the gastric ATPase complex. This low-affinity K⁺ site is accessible only from the vesicle interior and appears to be essential for the vectorial transport of H⁺ by the gastric microsomal (H⁺ + K⁺)-ATPase system.