Regulation of Na+ pump expression by vascular smooth muscle cells

The Na+ pump and its regulation is important for maintaining membrane potential and transmembrane Na+ gradient in all mammalian cells and thus is essential for cell survival and function. Vascular smooth muscle cells (VSMC) have a relatively low number of pump sites on their membrane compared with other 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% serum media with low K+ (1 mM vs. control of 5 mM), and their pump expression was 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, there 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-294002 (which is known to inhibit cytoplasmic transport processes) in low-K+ media, the pump site upregulation was inhibited. These data suggest that the low-K+-induced upregulation of Na+ pump number can occur by translocation of preformed pumps from intracellular stores.