Ouabain-sensitive H,K-ATPase functions as Na,H-ATPase in apical membranes of rat distal colon

Na,K-ATPase activity has been identified in the apical membrane of rat distal colon, whereas ouabain-sensitive and ouabain-insensitive H,K-ATPase activities are localized solely to apical membranes. This study was designed to determine whether apical membrane Na,K-ATPase represented contamination of basolateral membranes or an alternate mode of H,K-ATPase expression. An antibody directed against the H,K-ATPase alpha subunit (HKc alpha) inhibited apical Na,K-ATPase activity by 92% but did not alter basolateral membrane Na,K- ATPase activity. Two distinct H,K-ATPase isoforms exist; one of which, the ouabain-insensitive HKc alpha, has been cloned. Because dietary sodium depletion markedly increases ouabain-insensitive active potassium absorption and HKc alpha mRNA and protein expression, Na,K-ATPase and H,K-ATPase activities and protein expression were determined in apical membranes from control and sodium-depleted rats. Sodium depletion substantially increased ouabain-insensitive H,K-ATPase activity and HKc alpha protein expression by 109-250% but increased ouabain-sensitive Na,K-ATPase and H,K-ATPase activities by only 30% and 42%, respectively. These studies suggest that apical membrane Na,K-ATPase activity is an alternate mode of ouabain-sensitive H,K-ATPase and does not solely represent basolateral membrane contamination.