Objective: The aim was to study the effects of endothelin-1 on human placental veins and the role of cyclooxygenase products as mediators of these effects.
Study design: Rings of placental veins with and without endothelium were suspended in organ chambers filled with physiologic salt solution. After a period of stabilization at optimal basal tension, isometric tensions in the control group were recorded at increasing concentrations of endothelin-1 (10(-10) to 10(-7) mol/L). Rings in the experimental groups were treated with either indomethacin (cyclooxygenase inhibitor, 10(-5) mol/L), dazoxiben (thromboxane synthetase inhibitor, 10(-4) mol/L), or SQ29548 (thromboxane receptor antagonist, 10(-6) mol/L) before addition of endothelin-1. To demonstrate the presence of functional thromboxane receptors in the rings, contractile responses to U-46619 (10(-9) to 10(-6) mol/L), a thromboxane A2 analog were measured. The effectiveness of SQ29548 blockade was tested in rings treated with SQ29548 (10(-6) mol/L) before addition of U-46619. The concentration-response curves of the treated and control groups were compared with the Student paired t test.
Results: Endothelin-1 in doses of 10(-10) to 10(-7) mol/L caused concentration-dependent contraction of placental veins. Indomethacin significantly reduced the response of veins with endothelium to low endothelin-1 concentrations (10(-9.5) to 10(-9) mol/L), (p < 0.05). However, it had no effect at higher endothelin-1 concentrations or in vessels without endothelium. The presence of functional thromboxane A2 receptors was confirmed by the vasoconstrictor effect of U-46619 and its blockade by treatment with SQ29548. Neither SQ29548 nor the thromboxane A2 synthesis inhibitor dazoxiben significantly influenced the response to endothelin-1.
Conclusions: These results demonstrated that endothelin-1 is a potent vasoconstrictor in the human placental vein. Although functional thromboxane A2 receptors exist in this vessel, endothelin-1's action is independent of thromboxane A2. Prostaglandins may mediate part of the endothelin-1-induced placental vasoconstriction. However, endothelin-1 acts primarily by a direct effect on vascular smooth muscle cells.