COLONIC H-K+-ATPASE IS INDUCED AND MEDIATES INCREASED HCO3 - REABSORPTION IN INNER MEDULLARY COLLECTING DUCT IN POTASSIUM-DEPLETION()

Background. Potassium depiction increases HCO3- reabsorption in outer medullary collecting duct (OMCD) by activation of colonic (c) H-K-ATPa se (HKA). The purpose of the current experiments was to examine the ro le of the isoforms of HKA in HCO3- reabsorption by terminal inner medu llary collecting duct (IMCD) cells in potassium depletion. Methods. Sp rague-Dawley rats were fed a potassium-fret diet and studied after 8 t o 10 days. mRNA expression of HKA isoforms in terminal portion of inne r medulla was examined and correlated with HCO3- reabsorption in the t erminal IMCD.Results. Gastric (g) HKA mRNA decreased whereas colonic ( c) HKA mRNA expression was heavily induced in terminal portion of inne r medulla in potassium depleted rats. Net HCO3- flux (JtCO(2)) in term inal IMCD increased in potassium depletion (4.56 to 7.06 pmol/min/mm t ubule length, P < 0.001). In normal rats, 1 mM ouabain in perfusate ha d no effect on JtCO(2), whereas 10 mu M Schering 28080 (SCH) decreased JtCO(2) to 2.4 (P < 0.007). In KD rats, 1 mM ouabain decreased JtCO(2 ) to 4.9 (P < 0.005) and 10 mu M SCH decreased JtCO(2) to 3.3 (P < 0.0 01). However, the inhibitory effects of SCH and ouabain on JtCO(2) in potassium depleted animals were not additive. Conclusions. The data in dicate that gHKA is suppressed whereas cHKA is induced in potassium de pletion and mediates increased HCO3- reabsorption in terminal IMCD. Th e results further indicate that cHKA in vivo is sensitive to both SCH and ouabain.