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

Abstract

In rat terminal inner medullary collecting duct (tIMCD), the Na,K-ATPase mediates NH[Formula: see text] uptake, which increases secretion of net H+ equivalents. K+ and NH[Formula: see text]compete for a common binding site on the Na,K-ATPase. Therefore, NH[Formula: see text] 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 NH[Formula: see text] 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 β1-subunit (NK-β1) protein expression increased although NK-α1 protein expression and Na,K-ATPase activity were unchanged relative to K+-replete controls. However, after 7 days of K+ restriction, both NK-α1 and NK-β1 subunit protein expression and Na,K-ATPase activity increased. The magnitude of Na,K-ATPase-mediated NH[Formula: see text]uptake across the basolateral membrane ( J [Formula: see text]) was determined in tIMCD tubules perfused in vitro from rats after 3 days of a normal or a K+-restricted diet. J [Formula: see text] 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 [Formula: see text] >60%. In conclusion, with 3 days of K+ restriction, NH[Formula: see text] uptake by Na,K-ATPase is increased in the tIMCD primarily from the reduced interstitial K+ concentration.

Molecular Mechanisms of Renal Ammonia Transport

“…Direct studies examining the effects of hypokalemia on Na + -K + -ATPase-mediated ammonia transport have confirmed this model (63). Moreover, the increase in ammonia uptake is almost totally attributable to changes in interstitial K + concentration and does not involve changes in Na + -K + -ATPase hydrolytic activity (63). Thus, the increased rate of ammonia secretion that occurs during hypokalemia may be due, at least in part, to the decreased interstitial K + concentration, which results in more effective NH 4 + transport by basolateral Na + -K + -ATPase.…”

“…Direct studies show that NH 4 + competes with K + as a substrate for hydrolytic activity and that basolateral Na + -K + -ATPase mediates NH 4 + uptake, increases apical proton secretion, and is important for transepithelial ammonia secretion (49,59,61 (62). Direct studies examining the effects of hypokalemia on Na + -K + -ATPase-mediated ammonia transport have confirmed this model (63). Moreover, the increase in ammonia uptake is almost totally attributable to changes in interstitial K + concentration and does not involve changes in Na + -K + -ATPase hydrolytic activity (63).…”

Intercalated cell-specific Rh B glycoprotein deletion diminishes renal ammonia excretion response to hypokalemia

“…During hypokalemia, Na ϩ -K ϩ -ATPase contributes to increased urinary ammonia excretion in the IMCD by mediating increased cellular uptake of NH 4 ϩ ; this increase is due to changes in the relative amounts of extracellular K ϩ and NH 4 ϩ and does not involve changes in Na ϩ -K ϩ -ATPase protein abundance (59,60).…”

Renal and extrarenal regulation of potassium

“…Since K þ and NH 4 þ are competitive inhibitors for a common extracellular binding site on the Na-K-ATPase, NH 4 þ uptake is inhibited by increased extracellular K þ concentration or through the addition of ouabain, which results in reduced net acid secretion. [43][44][45][46] Alternatively NH 4 þ /K þ uptake can be coupled to the transepithelial transport of a third ion. For example, NKCC1 functions as a Na þ -K þ (NH 4 þ )-Cl -cotransporter that participates in the transepithelial transport of Cl -in the rat outer medullary collecting duct.…”