Mechanism of uterine vascular refractoriness to endothelin-1 in pregnant sheep

Endothelin-1 (ET-1) is a potent vasoconstrictor and produces marked pressor responses when given systemically. Studies in sheep have demonstrated that during pregnancy the uterine vasculature is refractory to exogenously admi nistered ET-1. We hypothesize that this pregnancy-dependent refractoriness is due to an upregulation of local uterine metabolism of ET-1 and/or ETB re ceptors and/or downregulation of local uterine ETA receptors. To investigat e these possibilities, 21 nonpregnant and 17 pregnant sheep were used. Dose -response curves to intravenous infusion of ET-1 and phenylephrine were gen erated for pregnant and nonpregnant sheep. ET-1 infused systemically demons trated vasoconstriction in the systemic and renal vasculature of pregnant a nd nonpregnant animals and vasoconstriction in the uterine vasculature of n onpregnant animals. The pregnant animals showed no uterine vascular respons e to ET-1. In contrast, phenylephrine showed vasoconstriction in the system ic, renal, and uterine circulations in both pregnant and nonpregnant sheep. After experimentation, the animals were euthanized, and tissues were harve sted for Western blot and activity analysis of neutral endopeptidase (NEP) or RT-PCR analysis of endothelin-converting enzyme (ECE) and ETA and ETB re ceptors. The content and activity of NEP in the uterine and renal vasculatu re of pregnant and nonpregnant animals were similar. RT-PCR demonstrated th e presence of ECE in the uterine vasculature of pregnant and nonpregnant sh eep. ETA receptor mRNA was significantly reduced in pregnant compared with nonpregnant sheep, whereas ETB receptor mRNA remained unchanged. We conclud e that the uterine vascular refractoriness seen in the pregnant sheep is du e to a downregulation of ETA receptors.