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Am J Physiol Renal Physiol 277: F866-F874, 1999;
0363-6127/99 $5.00
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Vol. 277, Issue 6, F866-F874, December 1999

In rat tIMCD, NH+4 uptake by Na+-K+-ATPase is critical to net acid secretion during chronic hypokalemia

Susan M. Wall, Bradley S. Davis, Kathryn A. Hassell, Pramod Mehta, and Stanley J. Park

University of Texas, Medical School at Houston, Houston, Texas 77030

The purpose of this study was to determine the magnitude of Na+ pump-mediated NH+4 uptake in the terminal inner medullary collecting duct (tIMCD) at K+ and NH+4 concentrations observed in vivo in the inner medullary interstitium of normal and in K+-restricted rats. Interstitial K+ and NH+4 concentrations in the terminal half of the inner medulla were taken to be 10 and 6 mM in K+-restricted rats, but 30 and 6 mM in K+-replete rats. In tubules from K+-restricted rats, when perfused at a K+ concentration of 10 mM, addition of ouabain to the bath reduced total bicarbonate flux (JtCO2) by 40% and increased intracellular pH (pHi), indicating significant NH+4 uptake by the Na+-K+-ATPase. In tubules from K+-restricted rats, JtCO2 was reduced with increased extracellular K+. At a K+ concentration of 30 mM, ouabain addition neither reduced JtCO2 nor increased pHi 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 secretion, and 2) Na+ pump-mediated NH+4 uptake should be an important pathway mediating transepithelial net acid secretion in vivo.

collecting duct; sodium-potassium-adenosinetriphosphatase; ouabain; ammonium; potassium


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