INCREASED TRANSPLASMA MEMBRANE CALCIUM INFLUX IN VASCULAR SMOOTH-MUSCLE CELLS FROM SPONTANEOUSLY HYPERTENSIVE RATS AFTER INHIBITION OF THE NA-K-ATPASE

The changes of cytosolic free calcium concentration ([Ca2+]i) after inhibition of Na-K-ATPase by ouabain were investigated in cultured vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats from the Munster strain (SHR) and normotensive Wistar-Kyoto rats (WKY) using the calcium-sensitive fluorescent dye fura 2. VSMC were cultured on cover slips and incubated with 1 muM fura 2 for 60 minutes. Fluorescence measurements were undertaken at 510 nm using excitation wavelengths of 340 nm and 380 nm. Addition of 1 mM ouabain significantly increased [Ca2+]i in VSMC from SHR compared to WKY (204 +/- 30 nM vs 65 +/- 13 nM, mean +/- SEM, p < 0.01). Using a calcium-free medium, the ouabain induced [Ca2+]i increase was significantly reduced, indicating transplasmamembrane calcium influx induced by ouabain. Addition of nifedipine significantly reduced ouabain induced [Ca2+]i increase. Inhibition of sodium-calcium exchange by NiCl2 did not inhibit ouabain induced [Ca2+]i increase. It is concluded that inhibition of Na-K-ATPase by ouabain produces the transplasmamembrane calcium influx via calcium channels and not via sodium-calcium-exchange. That effect of ouabain is markedly enhanced in primary hypertension.