CONTRACTILE EFFECT OF ENDOTHELIN IN HUMAN PLACENTAL VEINS - ROLE OF ENDOTHELIUM PROSTAGLANDINS AND THROMBOXANE

OBJECTIVE: The aim was to study the effects of endothelin-1 on human p lacental veins and the role of cyclooxygenase products as mediators of these effects. STUDY DESIGN: Rings of placental veins with and withou t endothelium were suspended in organ chambers filled with physiologic salt solution. After a period of stabilization at Optimal basal tensi on, isometric tensions in the control group were recorded at increasin g concentrations of endothelin-1 (10(-10) to 10(-7) mol/L). Rings in t he experimental groups were treated with either indomethacin (cyclooxy genase inhibitor, 10(-5) mol/L), dazoxiben (thromboxane synthetase inh ibitor, 10(-4) mol/L), or SQ29548 (thromboxane receptor antagonist, 10 (-6) mol/L) before addition of endothelin-1 To demonstrate the presenc e of functional thromboxane receptors in the rings, contractile respon ses 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 tr eated with SQ29548 (10(-6) mol/L) before addition of U-46619. The conc entration-response curves of the treated and control groups were compa red 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 ve ins with endothelium to low endothelin-1 concentrations (10(-9.5) to 1 0(-9) mol/L). (p < 0.05). However, it had no effect at higher endothel in-1 concentrations or in vessels without endothelium. The presence of functional thromboxane A2 receptors was confirmed by the vasoconstric tor effect of U-46619 and its blockade by treatment with SQ29548. Neit her SQ29548 nor the thromboxane A2 synthesis inhibitor dazoxiben signi ficantly influenced the response to endothelin-1. CONCLUSIONS: These r esults demonstrated that endothelin-1 is a potent vasoconstrictor in t he human placental vein. Although functional thromboxane A2 receptors exist in this vessel, endothelin-1's action is independent of thrombox ane A2. Prostaglandins may mediate part of the endothelin-1-induced pl acental vasoconstriction. However, endothelin-1 acts primarily by a di rect effect on vascular smooth muscle cells.