Enhanced endothelin receptor type B-mediated vasodilation and underlying [Ca2+]i in mesenteric microvessels of pregnant rats

Background and Purpose Normal pregnancy is associated with decreased vascular resistance and increased release of vasodilators. Endothelin-1 (ET-1) causes vasoconstriction via endothelin receptor type A (ETAR), but could activate ETBR in the endothelium and release vasodilator substances. However, the roles of ETBR in the regulation of vascular function during pregnancy and the vascular mediators involved are unclear. Experimental Approach Pressurized mesenteric microvessels from pregnant and virgin Sprague-Dawley rats were loaded with fura-2/AM for simultaneous measurement of diameter and [Ca2+]i. Key Results High KCl (51mM) and phenylephrine (PHE) caused increases in vasoconstriction and [Ca2+]i that were similar in pregnant and virgin rats. ET-1 caused vasoconstriction that was less in pregnant than virgin rats, with small increases in [Ca2+]i. Pretreatment with the ETBR antagonist BQ-788 caused greater enhancement of ET-1-induced vasoconstriction in pregnant rats. ACh caused endothelium-dependent relaxation and decreased [Ca2+]i, and was more potent in pregnant than in virgin rats. ET-1 + ETAR antagonist BQ-123, and the ETBR agonists sarafotoxin 6c (S6c) and IRL-1620 caused greater vasodilation in pregnant than in virgin rats with no changes in [Ca2+]i, suggesting up-regulated ETBR-mediated relaxation pathways. ACh-, S6c- and IRL-1620-induced relaxation was reduced by the NO synthase inhibitor N-nitro-l-arginine methyl ester, and abolished by tetraethylammonium or endothelium removal. Western blots revealed greater amount of ETBR in intact microvessels of pregnant than virgin rats, but reduced levels in endothelium-denuded microvessels, supporting a role of endothelial ETBR. Conclusions and Implications The enhanced ETBR-mediated microvascular relaxation may contribute to the decreased vasoconstriction and vascular resistance during pregnancy.