In rat tIMCD, NH4+ uptake by Na+-K+-ATPase is critical to net acid secretion during chronic hypokalemia

The purpose of this study was to determine the magnitude of Na+ pump-mediat ed NH4+ uptake in the terminal inner medullary collecting duct (tIMCD) at K + and NH4+ concentrations observed in vivo in the inner medullary interstit ium of normal and in K+-restricted rats. Interstitial K+ and NH4+ concentra tions in the terminal half of the inner medulla were taken to be 10 and 6 m M in K+-restricted rats, but 30 and 6 mM in K+-replete rats. In tubules fro m K+-restricted rats, when perfused at a K+ concentration of 10 mM, additio n of ouabain to the bath reduced total bicarbonate flux (J(tCO2)) by 40% an d increased intracellular pH (pH(i)), indicating significant NH4+ uptake by the Na+-K+-ATPase. In tubules from K+-restricted rats, J(tCO2) was reduced with increased extracellular K+. At a K+ concentration of 30 mM, ouabain a ddition neither reduced J(tCO2) nor increased pH(i) in tubules from rats of either treatment group. In conclusion, in the tIMCD from hypokalemic rats, 1) acute changes in extracellular K+ concentration modulate net acid secre tion, and 2) Na+ pump-mediated NH4+ uptake should be an important pathway m ediating transepithelial net acid secretion in vivo.