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Cardiovascular Sciences Section, Departments of 1 Molecular Physiology and 2 Medicine, Baylor College of Medicine, Houston, Texas 77030
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
[3H]ouabain binding). At this early time point, there was
no detectable increase in protein expression of either
1- or
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.
sodium-potassium-adenosinetriphosphatase; short-term regulation; LY-294002; low potassium
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