Pregnancy increases ET-1-induced contraction and changes receptor subtypes in uterine smooth muscle in humans

The purpose of the present study was to investigate whether pregnancy affects endothelin (ET)-1-induced contraction, the density of ET receptors, and the ratio of receptor subtypes (ET(A) and ET(B)) in uterine smooth muscle in humans. We also investigated which ET receptor subtypes mediate ET-l-induced contraction in the human uterus. In uterine membrane preparations, I-125-labeled ET-1 (I-125-ET-1) binding sites (B-max) in pregnant women did not differ from those in age-matched nonpregnant women (596.2 +/- 107.1 vs. 512.1 +/- 167.7 fmol/mg protein). The dissociation constant (K-d) in pregnant women did not differ from that in nonpregnant women. Competitive displacement experiments with I-125-ET-1 binding to the membranes using BQ-123 (ET(A) receptor antagonist) showed that the percentage of ET(A) receptors in uterine muscle was significantly higher in pregnant women than in nonpregnant women (P < 0.01). The calculated ratios of ET(A) to ET(B) receptors in pregnant and nonpregnant uteri were 92:8 and 68:32, respectively. Combination treatment with BQ-788 (ET(B) receptor antagonist) completely inhibited the BQ-123-resistant component of I-125-ET-1 specific binding. ET-1 caused dose-dependent contractions in isolated human uteri from both pregnant and nonpregnant women. The maximum response was markedly greater in pregnant women than in nonpregnant women, whereas pD(2) (-log[EC(50)]) values did not differ between pregnant and nonpregnant uteri. In pregnant human uterus, BQ-123 (10(-6) M) significantly shifted the dose-dependent curve of ET-1 response to the right, whereas BQ-3020 (ET(B) receptor agonist) did not cause contraction. These results suggested that ET-l-induced contraction of the human uterus is mediated through only ETA receptors and that ET-l-induced uterine contraction in humans is markedly increased during pregnancy. In addition, the present study suggests that, although (125)-ET-1 B-max are not altered during pregnancy, the proportion of ET(A) receptors is increased and that of ET(B) receptors is decreased in the pregnant human uterus.