Sodium-calcium exchange exists across the cell membrane of intact vascular smooth muscle cells.

Ouabain stimulated Ca2+ uptake in rat aortic smooth muscle cells in culture. The maximum uptake of Ca2+ was observed at about 200 mumol/l ouabain. Na+K+-ATPase activity of these cells was also maximally inhibited at about 200 mumol/l ouabain. The ouabain-stimulated Ca2+ uptake was not inhibited by the calcium antagonist diltiazem. The cells treated with ouabain also showed increased steady-state cytosolic Ca2+ concentration as measured by the fluorescent dye indicator Fura 2/AM. The ouabain stimulated Ca2+ uptake and increased cytosolic Ca2+ is explained as follows: ouabain inhibits Na+K+-ATPase with subsequent increase in cytosolic Na+, which is then exchanged with external Ca2+ through the Na+-Ca2+ exchange in intact vascular smooth muscle cells.