The contribution of ATP-generating systems to Na+ pump (Na+-K+-ATPase) function was studied in Xenopus laevis A6 kidney epithelia apically permeabilized with digitonin. The ouabain-inhibitable Na+ pump current (I(P)) was measured in the presence of otherwise impermeant inhibitors and/or substrates at Na+ and K+ concentrations that allowed near-maximal pump function. Confocal fluorescence microscopy after apical addition of sulfosuccinimidobiotin (molecular weight of 443) showed that all cells were permeabilized. Less than 15% of the endogenous lactate dehydrogenase and creatine kinase (CK) were released into the apical medium. The I(P) was approximately 5 microA/cm2 in the presence of D-glucose. Blocking glycolysis with 2-deoxy-D-glucose or oxidative phosphorylation with antimycin A decreased it by > or = 50%. Exogenously added ATP prevented these decreases fully or partially, respectively. Two CK isoforms were detected, one likely being mitochondrial and the other corresponding to mammalian B isoform of CK. Phosphocreatine partially restored Na+ pump activity during inhibition of either ATP synthesis pathway. In conclusion, the ATP used by Na+ pumps of apically digitonin-permeabilized A6 epithelia is generated to a similar extent by glycolysis and oxidative phosphorylation. The CK system can partially support the ATP supply to the Na+ pumps.

Short-term aldosterone (10(-6) M, 2.5 h) induces in A6-C1 cell epithelia an increase in Na transport, which is due to the in situ activation of the apical Na channel and, presumably, the basolateral Na pump (Na,K-ATPase). We have now directly measured the effect of aldosterone on the transport activity of endogenous Na pumps and hybrid Na pumps containing an exogenous alpha 1 subunit by measuring the pump current (Ip) across epithelia apically permeabilized with amphotericin B. Aldosterone (2.5 h) had no significant early effect on the maximal Ip, nor on the Na concentration required for half-maximal activation. In contrast, it increased the Ip at physiological intracellular Na concentrations (1.7-fold at 5 mM Na). This effect was blocked by the protein synthesis inhibitor cycloheximide. Hybrid pumps containing the transfected cardiotonic steroid-resistant alpha 1 subunit of Bufo marinus were also stimulated by aldosterone (2.5 h). A long aldosterone treatment (4 days) increased the maximal Ip produced by the endogenous pumps 1.5 to 2.1-fold. In conclusion, aldosterone acts on Na pumps containing an alpha 1 subunit in two ways. During its early phase of action it stimulates their transport activity by increasing their apparent Na affinity at physiological intracellular Na concentrations. In the long term it produces an increase in the maximal transport capacity, which corresponds to the known increase in the number of Na pumps.

The effect of protein kinase C (PKC) stimulation on the pump current (Up) generated by the Na,K-ATPase was measured in A6 epithelia apically permeabilized with amphotericin B. Phorbol 12-myristate 13-acetate (PMA) produced a decrease in I carried by sodium pumps containing the endogenous Xenopus laevis or transfected BuJ'o marinus al subunits (-30% reduction within 25 min, maximum after 40 min) independent of the PKC phosphorylation site (T15A/S16A). In addition to this major effect of PMA, which was independent of the intracellular sodium concentration and was prevented by the PKC inhibitor bisindolylmaleimide GF 109203X (BIM), another BIM-resistant, PKC siteindependent decrease was observed when the Ip was measured at low sodium concentrations (total reduction -50% at 5 mM sodium). Using ouabain binding and cell surface biotinylation, stimulation of PKC was shown to reduce surface Na,K-ATPase by 14 to 20% within 25 min. The same treatment stimulated fluid phase endocytosis sevenfold and decreased by 16.5% the basolateral cell surface area measured by transepithelial capacitance measurements. In conclusion, PKC stimulation produces a decrease in sodium pump function which can be attributed, to a large extent, to a withdrawal of sodium pumps from the basolateral cell surface independent of their PKC site. This

In A6 cells cultured on filters, aldosterone (10(-6) M) induces an early increase in the initial rate of ouabain binding to intact monolayers (in K-free buffer) that parallels an early increase in Na transport. This effect is independent of apical Na influx and precedes an increase in the synthesis of Na-K-adenosinetriphosphatase (ATPase) subunits. In the present study we observed in addition a late aldosterone-induced increase in the rate of ouabain binding (2.5 times from 3 to 120 h aldosterone). The role of Na-K-ATPase accumulation and/or translocation to the cell surface was tested by Western blotting, saturation ouabain binding, and cell-surface labeling using sulfosuccinimidobiotin or enzyme-mediated radioiodination. Only cell-surface beta 1-subunit was detected by blotting with streptavidin or autoradiography, because the alpha 1-subunit was not efficiently labeled. Three hours after hormone addition, none of the three parameters had increased significantly, whereas after 20 and 120 h similar increases (approximately 1.6 and 2.3 times, respectively) were detected by all three methods. In addition, it was shown that increasing intracellular Na with amphotericin B or a K-free preincubation also stimulated the rate of ouabain binding without increasing the surface labeling of beta 1-subunits. Taken together, these results suggest that a short aldosterone treatment or an increase in intracellular Na leads to an increase in the rate of ouabain binding that is due to an in situ activation of cell-surface Na-K-ATPase molecules. In contrast, the late increase in the rate of ouabain binding parallels an increase in the number of pumps.

Mineralocorticoid and glucocorticoid hormones stimulate sodium reabsorption across target epithelia by modulating the transcription rate of a cell-specific set of genes. Unidentified regulated gene products mediate an early activation of preexisting sodium channels and pumps. In a later phase, the supply of proteins belonging to the sodium-transport machinery is increased.