Regulation of Na+ pump expression by vascular smooth muscle cells

The Na+ pump and its regulation is important for maintaining membrane poten tial and transmembrane Na+ gradient in all mammalian cells and thus is esse ntial for cell survival and function. Vascular smooth muscle cells (VSMC) h ave a relatively low number of pump sites on their membrane compared with o ther cells. We wished to determine the mechanisms for regulating the number of pump sites in these cells. We used canine saphenous vein VSMC cultured in 10% serum and passaged one time. These cells were subcultured in 5% seru m media with low K+ (1 mM vs. control of 5 mM), and their pump expression w as assessed. These VSMC upregulated their pump sites as early as 4 h after treatment (measured by [H-3] ouabain binding). At this early time point, th ere was no detectable increase in protein expression of either alpha (1)-or alpha beta (1)-subunits of the pump shown by Western blots. When the cells were treated with the phosphoinositide 3-kinase (PI-3-K) inhibitor LY-2940 02 (which is known to inhibit cytoplasmic transport processes) in low-K+ me dia, the pump site upregulation was inhibited. These data suggest that the low-K+-induced upregulation of Na+ pump number can occur by translocation o f preformed pumps from intracellular stores.