OUABAIN-INSENSITIVE, VANADATE-SENSITIVE K+-ATPASE OF RAT DISTAL COLON IS PARTLY SIMILAR TO GASTRIC H+,K+-ATPASE

A membrane fraction from rat distal colon contained both ouabain-sensitive and -insensitive K+-ATPase activities, which were measured under Na+-free conditions. About 38% of the ouabain-insensitive K+-ATPase activity was inhibited by vanadate. It was determined whether the ouabain-insensitive, vanadate-sensitive K+-ATPase in the colon is similar or identical to gastric H+,K+-ATPase. This colonic K+-ATPase activity was inhibited completely by monoclonal antibody HK4001, which inhibits the hog gastric H+,K+-ATPase activity but not Na+,K+-ATPase or Ca2+-ATPase. The colonic ATPase activity was inhibited partly by SCH 28080, which is a specific reversible inhibitor of gastric H+,K+-ATPase. The colonic ATPase activity was stimulated by low concentrations of K+ (its half-maximal stimulating concentration was 1 mM) and inhibited by high concentrations of K+ (its half-maximal inhibiting concentration was 10 mM), indicating that high and low K+ affinity sites are present in the colonic enzyme as in gastric H,K+-ATPase and that this enzyme is not fully operative under normal physiological conditions. Two other monoclonal antibodies, which inhibit the gastric H+,K+-ATPase activity, did not inhibit the colonic K+-ATPase activity. The present results suggest that the colonic ouabain-insensitive K+-ATPase is partly similar but not identical to the gastric H+,K+-ATPase.