Enhanced intracellular sodium concentration in kidney cells recruits a latent pool of Na-K-ATPase whose size is modulated by corticosteroids.

Besides its role in the control of the rate of functioning of each Na-K-ATPase unit (as a substrate of the enzyme), the intracellular sodium concentration also regulates the number of active Na-K-ATPase units, as previously described in cultured cells. To evaluate such a possibility in kidney epithelial cells, the intracellular concentration of sodium in rat cortical collecting tubules (CCT) maintained in vitro was altered by the use of the sodium ionophore nystatin. When CCT were preincubated for 2-3 h at 37 degrees C in the presence of nystatin, the enzymatic activity of Na-K-ATPase was markedly stimulated as compared to tubules preincubated without nystatin or in the presence of the ionophore but in the absence of extracellular sodium. Although nystatin increased both Na-K-ATPase activity and [3H]ouabain specific binding in CCT, its action was independent of de novo synthesis of the pump since neither actinomycin D nor cycloheximide abolished it. It is suggested that increasing the sodium concentration in CCT cells induces the recruitment of a latent pool of Na-K-ATPase units. The size of this latent pool of enzyme is under the control of corticosteroids as it is markedly decreased in CCT from adrenalectomized rats.