APICAL MEMBRANE LOCALIZATION OF OUABAIN-SENSITIVE K+-ACTIVATED ATPASE ACTIVITIES IN RAT DISTAL COLON

This study sought to establish the presence of K+-activated adenosinetriphosphatase (ATPase) activity in the colonic mucosa of the rat distal colon. K+-activated ATPase activity was present in apical membranes but not in basolateral membranes. K+-activated ATPase activity in apical membranes represented an approximate 10-fold enrichment compared with that in the homogenate. Na+-K+-activated ATPase activity was also present in homogenate but was enriched less than fourfold in apical membranes. K+-activated ATPase activity in apical membranes had both ouabain-sensitive and ouabain-insensitive components. In contrast, Na+-K+-activated ATPase activity was completely inhibited by ouabain. Similar half-maximal concentrations for K+ and pH activation curves were found for both ouabain-sensitive and ouabain-insensitive fractions. In addition to K+, the ouabain-sensitive fraction of K+-activated ATPase activity was stimulated by Rb+, NH4+, and Cs+, whereas the ouabain-insensitive fraction was activated only by Rb+. K+-activated ATPase activity was significantly inhibited by vanadate but not by N-ethylmaleimide or omeprazole. In the proximal colon, in contrast to the distal colon, active K+ absorption is not present, and K+-activated ATPase is approximately 20% of that in the distal colon. These studies demonstrate that K+-activated ATPase is present in apical membranes of rat distal colon and permit the speculation that this enzyme represent a unique and distinct ATPase (compared with either Na+-K+-ATPase or gastric parietal cell K+-ATPase) and is likely linked closely to the active K+ absorptive process present in this epithelium.