Basolateral K-Cl cotransporter regulates colonic potassium absorption in potassium depletion

Active potassium absorption in the rat distal colon is electroneutral, Na+- independent, partially chloride-dependent, and energized by an apical membr ane H,K-ATPase, Both dietary sodium and dietary potassium depletion substan tially increase active potassium absorption. We have recently reported that sodium depletion up regulates H,K-ATPase alpha-subunit mRNA and protein ex pression, whereas potassium depletion upregulates H,K-ATPase beta-subunit m RNA and protein expression. Because overall potassium absorption is non-con ductive, H-CL cotransport (KCC) at the basolateral membrane may also be inv olved in potassium absorption. Although KCC1 has not been cloned from the c olon, we established, in Northern blot analysis with mRNA from the rat dist al colon using rabbit kidney KCC1 cDNA as a probe, the presence of an expec ted size mRNA in the rat colon, This KCC1 mRNA is substantially increased b y 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 KC C1 protein expression in basolateral membrane. The increase of colonic KCC1 mRNA abundance and KCC1 protein expression in potassium depletion of the r at colonic basolateral membrane suggests that K-CI cotransporter: 1) is inv olved in transepithelial potassium absorption and 2) regulates the increase in potassium absorption induced by dietary potassium depletion, We conclud e that active potassium absorption in the rat distal colon involves the coo rdinated regulation of both apical membrane II,K-ATPase and basolateral mem brane KCC1 protein.