Ouabain-sensitive H+-K+ exchange mechanism in the apical membrane of guinea pig colon

Abstract

The K+ uptake process across the apical membrane was investigated in relation to the ouabain-sensitive H+ secretion and the active transepithelial K+ absorption in guinea pig distal colon. The isolated colon was mounted between Ussing chambers or everted and fixed over a polyethylene tube (86Rb uptake measurements). The H+ secretion rate was determined from changes in pH of the weak-buffered mucosal bathing solution. K+ and Rb+ in the mucosal bathing solution powerfully stimulated H+ secretion. Cs+ also activated H+ secretion, though less than K+ or Rb+, whereas Na+, Li+, or choline scarcely did so. The increase in K+ concentration on the mucosal side reduced the degree of inhibition of H+ secretion by mucosal ouabain. 86Rb influx across the apical membrane (5.4 mM Rb+ in mucosal solution) was inhibited by approximately 90% by mucosal ouabain with the half-maximal effective dose being 3.9 +/- 1.0 X 10(-6) M; the half-maximal dose is not different from that for the inhibition of H+ secretion under similar conditions. The ouabain-sensitive component of 86Rb influx and the ouabain-sensitive H+ secretion rate were closely correlated when these rates were altered with either changes in the mucosal Rb+ concentration or addition of inhibitors such as acetazolamide, indomethacin, or 2,4-dinitrophenol. In the presence of K+ on the mucosal side, the ouabain-sensitive 86Rb influx was significantly reduced. Finally, the active transepithelial 42K absorption under short-circuit conditions was inhibited by approximately 50% with 4 X 10(-6) M and almost abolished with 10(-4) M ouabain on the mucosal side. It is concluded that the ouabain-sensitive K+ (Rb+) uptake mechanism, which is closely associated with H+ secretion, is present in the apical membrane of the colonic epithelial cells; the molecular mechanism of this uptake may be a H+-K+-ATPase. This K+ uptake is likely to mediate the apical membrane step of active transepithelial K+ absorption.