Role of Na+ and Ca2+ in stretch-induced Na(+)-K(+)-ATPase alpha-subunit regulation in aortic smooth muscle cells.

This study was designed to test the role of Na+ and Ca2+ entry in the stretch-induced Na(+)-K(+)-ATPase alpha 1- and alpha 2-isoform upregulation observed in our previous studies. We measured intracellular Na+ in cyclically stretched rat aortic smooth muscle cells, with or without gadolinium treatment, for various durations and performed Western blotting to analyze the effects of stretch and the calcium channel blocker isradipine on the expression of alpha-isoforms. Intracellular Na+ was elevated significantly after 1- and 2-h stretch, but returned to baseline after 1-, 2-, and 4-day stretch. This increase in intracellular Na+ was blocked by gadolinium. Both alpha 1- and alpha 2-isoforms were upregulated after either 2 or 4 days of cyclical stretch. Isradipine had no apparent effect on stretch-induced upregulation on either alpha-isoform, thus suggesting that Ca2+ entry through L-type channels is not involved in the stretch-induced upregulation. We therefore conclude that a transient intracellular Na+ elevation during stretch may serve as a signal to mediate the alpha 1- and alpha 2-isoform upregulation.