Effect of calcium on in vitro contractility of human umbilical artery

The action of calcium on human umbilical artery was studied by an in vitro perfusion method. An increase in tonus and intense spontaneous activity were observed in the preparations at high calcium concentrations. A marked relaxation of the umbilical artery occurred in the absence of calcium. Estriol had a relaxing effect on the umbilical vessels which was more intense at calcium concentrations near physiological levels. Estriol also abolished spontaneous contractions. In the absence of calcium, estriol showed no relaxing effect. The results suggest a participation of calcium in the mechanism of action of estriol on human umbilical artery in vitro.