Regulation of Na+,K+-ATPase and the Na+/K+/Cl+ co-transporter in the renal epithelial cell line NBL-1 under osmotic stress

The long-term adaptation of the Na+,K+-ATPase to hypertonicity was studied using the bovine renal epithelial cell line NBL-1. Na+,K+-ATPase activity measured in intact cells as the ouabain-sensitive fraction of Rb+ uptake was stimulated (40% above controls) after incubating the cells in hypertonic medium. This stimulation was not correlated with significant changes in the amount of Na+,K+-ATPase α1 subunit protein. Nevertheless, the amount of α1 but not β1 subunit mRNA progressively increased after hypertonic shock (3–4-fold above basal values). These results suggest that the α1 subunit gene is modulated by medium osmolarity, although this does not necessarily involve enhanced translation of the mRNA into active α1 protein. Indeed, the increase in the biological activity of the Na+,K+-ATPase is abolished when the electrochemical Na+ transmembrane gradient is depleted by monensin, which is consistent with a post-translational effect on the activity of the sodium pump. A furosemide-sensitive component of Rb+ uptake, attributable to Na+/K+/Cl- co-transporter activity, was very low when cells were cultured in a regular medium, but was greatly induced after hypertonic shock. This induction could not be blocked by cycloheximide. Colcemide addition slightly reduced the absolute increase in Na+/K+/Cl- co-transporter activity, while cytochalasin B significantly potentiated the effect triggered by hypertonic shock. It is concluded: (i) that in NBL-1 cells the α1 but not the β1 subunit of the Na+,K+-ATPase is encoded by an osmotically sensitive gene, and (ii) that the Na+/K+/Cl- co-transporter, although an osmotically sensitive carrier, is induced by a mechanism that is independent of protein synthesis but may rely, in an undetermined manner, on the structure of the cytoskeletal network.