Apical K⁺-ATPase and Active Potassium Absorption in the Distal Colon

Abstract

Na,K-ATPase and gastric parietal cell H,K-ATPase are two P-type ATPases that have been extensively studied. P-type ATPases, dimers with distinct α and β subunits, are inhibited by orthovanadate. During the past several years isoforms of both the α and β subunits of Na, K-ATPase and H,K-ATPase have been isolated and sequenced and, at least for the a subunit of Na,K-ATPase, there have been extensive structure-function relationship studies (Lingrel et al, 1990; Horisberger et al, 1991). Na, K-ATPase is essential for both epithelial and non-epithelial cell function by maintaining a low intracellular [Na] and a high intracellular [K] as a result of extrusion of Na in exchange for K. In most epithelial cells Na, K-ATPase is located on the basolateral membrane and until recently ouabain had been considered a specific inhibitor. Na, K-ATPase is not inhibited by either omeprazole or SCH 28080, inhibitors of gastric acid secretion and gastric H,K-ATPase which, in contrast, is located on the apical membrane of parietal cells. Gastric H,K-ATPase functions as a K- H exchange and is inhibited by omeprazole and SCH 28080, but not by ouabain (Wallmark et al, 1987). Despite these pharmacological differences, comparison of its amino acid sequence to that of the Na pump reveals partial (60%) homology (Shull amp; Lingrel, 1986).