Effects of some vasodilators on calcium translocation in intact and fractionated vascular smooth muscle.

The "lanthanum method" was used to measure the effects of vasodilators on the 45Ca influx induced in isolated strips of rabbit main pulmonary artery after replacement of NaCl by KCl in the bathing solution. Two Ca antagonists, verapamil and Ro 11-1781, produced dose-dependent inhibitions of 45Ca influx with IC50 values of 3 X 10(-7) and 4 X 10(-6)M, respectively. Papaverine was virtually equipotent with Ro 11-1781 (IC50 = 2 X 10(-6)M). High concentrations of diazoxide (10(-4) to 10(-3)M) and nitroglycerin (10(-3)M) were required to cause inhibitions which did not exceed 40%. While prazosin 3 X 10(-4)M stimulated 45Ca influx, sodium nitrite and sodium nitroprusside had no effect. A comparison of the effects of vasodilators on contraction and 45Ca influx indicated that, of the drugs studied, only Ca antagonists and papaverine may cause relaxation by interfering with Ca influx. While high concentrations (greater than 10(-4)M) of verapamil, papaverine and nitroglycerin produced an inhibition of 45Ca uptake by membrane fractions from rabbit aorta, the other vasodilators tested were inactive.