In rat inner medullary collecting duct, NH+4 uptake by the Na,K-ATPase is increased during hypokalemia

doi:10.1152/ajprenal.0141.2001

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Am J Physiol Renal Physiol 282: F91-F102, 2002. First published August 21, 2001; doi:10.1152/ajprenal.0141.2001
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Vol. 282, Issue 1, F91-F102, January 2002

In rat inner medullary collecting duct, NH uptake by the Na,K-ATPase is increased during hypokalemia

Susan M. Wall¹, Michael P. Fischer¹, Gheun-Ho Kim², Bich-May Nguyen¹, and Kathryn A. Hassell¹

¹ University of Texas, Medical School at Houston, Houston, Texas 77030; and ² Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892

In rat terminal inner medullary collecting duct (tIMCD), the Na,K-ATPase mediates NH uptake, whichincreases secretion of net H⁺ equivalents. K⁺ and NH compete for a common bindingsite on the Na,K-ATPase. Therefore, NHuptake should increase during hypokalemia because interstitialK⁺ concentration is reduced. We asked whether upregulation of theNa,K-ATPase during hypokalemia also increases basolateral NHuptake. To induce hypokalemia, rats ate a diet with a low K⁺ content. In tIMCD tubules from rats given 3 days of dietary K⁺ restriction, Na,K-ATPase $beta$ ₁-subunit (NK- $beta$ ₁) protein expressionincreased although NK- $alpha$ ₁ protein expression and Na,K-ATPase activitywere unchanged relative to K⁺-replete controls. However, after 7 days of K⁺ restriction, both NK- $alpha$ ₁ and NK- $beta$ ₁ subunit protein expressionand Na,K-ATPase activity increased. The magnitude of Na,K-ATPase-mediatedNH uptake across the basolateral membrane(J) was determined in tIMCD tubules perfusedin vitro from rats after 3 days of a normal or a K⁺-restricted diet. J was the same in tubulesfrom rats on either diet when measured at the same extracellularK⁺ concentration. However, in either treatment group, increasingK⁺ concentration from 10 to 30 mM reduced J>60%. In conclusion, with 3 days of K⁺ restriction, NH uptake by Na,K-ATPaseis increased in the tIMCD primarily from the reduced interstitialK⁺concentration.

sodium,hydrogen-adenosinetriphosphatase; terminal inner medullary collecting duct; potassium; ammonium

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