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Active K absorption in the rat distal colon is energized by an apical membrane H-K-ATPase, whereas K absorption in the distal collecting duct is generally believed to be modulated by a related renal H-K-ATPase. Experiments were performed to establish the mechanism(s) by which dietary Na depletion (with resulting elevated aldosterone levels) and K depletion stimulate K absorption. A colonic H-K-ATPase-specific cDNA probe and a polyclonal antibody were utilized to measure mRNA (Northern blot analyses) and protein (Western blot and immunofluorescence studies) abundance in the distal and proximal colon and renal collecting ducts and cortex of dietary Na- and K-depleted rats. Dietary Na depletion, but not K depletion, upregulated H-K-ATPase-specific mRNA and protein expression in the distal and proximal colon; Na depletion also stimulated H-K-ATPase activity in the distal colon. In contrast to the distal …