Active potassium absorption in the rat distal colon is electroneutral, Na(+)-independent, partially chloride-dependent, and energized by an apical membrane H,K-ATPase. Both dietary sodium and dietary potassium depletion substantially increase active potassium absorption. We have recently reported that sodium depletion up-regulates H,K-ATPase alpha-subunit mRNA and protein expression, whereas potassium depletion up-regulates H,K-ATPase beta-subunit mRNA and protein expression. Because overall potassium absorption is non-conductive, K-Cl cotransport (KCC) at the basolateral membrane may also be involved in potassium absorption. Although KCC1 has not been cloned from the colon, we established, in Northern blot analysis with mRNA from the rat distal colon using rabbit kidney KCC1 cDNA as a probe, the presence of an expected size mRNA in the rat colon. This KCC1 mRNA is substantially increased by potassium depletion but only minimally by sodium depletion. KCC1-specific antibody identified a 155-kDa protein in rat colonic basolateral membrane. Potassium depletion but not sodium depletion resulted in an increase in KCC1 protein expression in basolateral membrane. The increase of colonic KCC1 mRNA abundance and KCC1 protein expression in potassium depletion of the rat colonic basolateral membrane suggests that K-Cl cotransporter: 1) is involved in transepithelial potassium absorption and 2) regulates the increase in potassium absorption induced by dietary potassium depletion. We conclude that active potassium absorption in the rat distal colon involves the coordinated regulation of both apical membrane H,K-ATPase and basolateral membrane KCC1 protein.