Action of thiocyanate on pH gradient formation by gastric microsomal vesicles

The effects of thiocyanate anion (SCN-) on proton accumulation and ATP hydrolysis by hog gastric microsomal vesicles have been investigated. The addition of SCN- to vesicles in the presence of KC1 and valinomycin reduced ATP-dependent proton accumulation in a dose-dependent manner. Inhibition was most pronounced in the presence of internal SCN-, which was obtained by preincubation of vesicles with SCN- or by the addition of K+-valinomycin, which facilitates entry of SCN-. SCN- does not appear to act by inhibition of the vesicular H+ pump because 1) there were minimal effects of SCN- on the rate of ATP hydrolysis, and 2) the initial rate of pH gradient formation was greater with 20 mM SCN- than with 20 mM Cl-. In the presence of K+ and valinomycin, external SCN- inhibited ATP-dependent pH gradient formation by increasing the rate of proton efflux. Preformed pH gradients (acid interior) were rapidly dissipated by internal, but not external, SCN-. These results suggest that SCN- acts to increase the rate of passive proton loss from the vesicle interior and do not support a direct inhibition of ATP-dependent H+ translocation. The results are consistent with the formation of the permeant hydrothiocyanic acid within the vesicles, increasing the rate of proton loss. The data also lend support to the HSCN backflux hypothesis of Sanders et al. [Am. J. Physiol. 234 (Endocrinol. Metab. Gastrointest. Physiol. 3): E120-E128, 1978] for inhibition of gastric acid secretion by SCN-.