Low K⁺ increases Na⁺-K⁺-ATPase α- and β-subunit mRNA and protein abundance in cultured renal proximal tubule cells

Studies from this laboratory demonstrate that LLC-PK₁/Cl₄ cells, a cultured renal cell line, respond to incubation in low-K⁺ medium by coordinately increasing abundance of both α- and β-subunits of Na⁺-K⁺- ATPase but increase only β- and not α-mRNA levels (Lescale-Matys et al. J. Biol. Chem. 265: 17935-17940, 1990) and that α-abundance is likely increased as a result of increased efficiency of α-mRNA translation (L. Lescale-Matys and A. A. McDonough. J. Cell Biol. 111: 311A, 1990). The aim of this report was to determine if nontransformed kidney cells would respond to low K⁺ in a similar manner. We incubated primary cultures of rat proximal tubule cells in low K⁺ (0.25 mM) for up to 24 h to address this aim. Na⁺-K⁺-ATPase activity, measured enzymatically, and abundance of α- and β-subunits, measured by immunoblot, were increased significantly and coordinately by 8 h of low K⁺, and, by 24 h of low K⁺, these parameters were increased to 2.17 ± 0.34 (activity), 2.03 ± 0.21 (α), and 2.39 ± 0.48 (β)-fold over control. Pretranslationally, β-mRNA, measured by Northern blot analysis, increased to 1.76 ± 0.35 after 3 h of low K⁺ and to 3.4 ± 0.75-fold over control after 24 h of low K⁺. The increase in α-mRNA was smaller and delayed compared with the β-mRNA response, but it was sufficient to account for the observed increase in α-protein and Na⁺-K⁺-ATPase activity at steady state: α-mRNA increased to 1.27 ± 0.09 after 6 h and to 1.91 ± 0.41-fold over control after 24 h in low K⁺. We conclude that the accumulation of sodium pumps in cultured renal proximal tubule cells, unlike LLC-PK₁ cells, can be accounted for by increases in both α- and β-subunit mRNA levels.