INCREASED TRANSPLASMA MEMBRANE CALCIUM IN FLUX IN VASCULAR SMOOTH-MUSCLE CELLS FROM SPONTANEOUSLY HYPERTENSIVE RATS AFTER INHIBITION OF THENA-K-ATPASE

The changes of cytosolic free calcium concentration (Ca2+!i) after in hibition of Na-K-ATPase by ouabain were investigated in cultured vascu lar smooth muscle cells (VSMC) from spontaneously hypertensive rats fr om the Munster strain (SHR) and normotensive Wistar-Kyoto rats (WKY) u sing the calcium-sensitive fluorescent dye fura 2. VSMC were cultured on cover slips and incubated with 1 muM fura 2 for 60 minutes. Fluores cence measurements were undertaken at 510 nm using excitation waveleng ths of 340 nm and 380 nm. Addition of 1 mM ouabain significantly incre ased 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 ouab ain induced Ca2+!i increase was significantly reduced, indicating tra nsplasmamembrane calcium influx induced by ouabain. Addition of nifedi pine significantly reduced ouabain induced Ca2+!i increase. Inhibitio n of sodium-calcium exchange by NiCl2 did not inhibit ouabain induced Ca2+!i increase. It is concluded that inhibition of Na-K-ATPase by ou abain produces the transplasmamembrane calcium influx via calcium chan nels and not via sodium-calcium-exchange. That effect of ouabain is ma rkedly enhanced in primary hypertension.