| Abstract: | Rat stimulated heavy gastric membranes enriched with (H+-K+)-ATPase, a marker for the apical membrane of the parietal cell, displayed a 32P-histone-dephosphorylating activity which appeared to be physically copurified with, but functionally independent of, the ATPase. The protein phosphatase activity was optimal at pH 7.5 and was inhibited by fluoride (50 mM), inorganic phosphate (50 mM), and p-chloromercuribenzoate (0.1 mM), but was insensitive to vanadate (1 mM). The 32P-phosphoproteins in the heavy gastric membranes were also dephosphorylated, apparently by their own membrane-bound phosphatase in the presence of Mg2+ at millimolar concentrations, which is likely to enhance membrane-membrane interaction. Heavy gastric membrane vesicles incubated with Mg2+ (2 mM) exhibited no alterations in K+-dependent ATP-hydrolyzing activity, Cl permeability, and protein and lipid compositions, but irreversibly lost the ATP, K+-dependent H+-pumping activity. Since valinomycin, a K+-specific ionophore, restored the intravesicular acidifying activity and an inhibitor of the protein phosphatase, inorganic phosphate, largely blocked the Mg2+-induced change in the membrane transport function, it is reasonable to propose that the phosphatase action on certain membrane proteins, possibly the putative K+ transporter or regulatory proteins, selectively decreases K+-conductance in the apical membranes of gastric parietal cells. |
