In rat inner medullary collecting duct, NH4+ uptake by the Na,K-ATPase is increased during hypokalemia

In rat terminal inner medullary collecting duct (tIMCD), the Na,K-ATPase mediates NH4+ uptake, which increases secretion of net H+ equivalents. K+ and NH4+ compete for a common binding site on the Na,K-ATPase. Therefore, NH4+ uptake should increase during hypokalemia because interstitial K+ concentration is reduced. We asked whether upregulation of the Na,K-ATPase during hypokalemia also increases basolateral NH4+ uptake. 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 expression increased although NK-alpha (1) protein expression and Na,K-ATPase activity were unchanged relative to K+-replete controls. However, after 7 days of K+ restriction, both NK-alpha (1) and NK-beta (1) subunit protein expression and Na,K-ATPase activity increased. The magnitude of Na,K-ATPase- mediated NH4+ uptake across the basolateral membrane (J(NH 4)(+)) was determined in tIMCD tubules perfused in vitro from rats after 3 days of a normal or a K+-restricted diet. J(NH 4)(+) was the same in tubules from rats on either diet when measured at the same extracellular K+ concentration. However, in either treatment group, increasing K+ concentration from 10 to 30 mM reduced J(NH 4)(+) >60%. In conclusion, with 3 days of K+ restriction, NH4+ uptake by Na,K-ATPase is increased in the tIMCD primarily from the reduced interstitial K+ concentration.