Relaxin mediates uterine artery compliance during pregnancy and increases uterine blood flow.

Normal pregnancy involves dramatic remodeling of the uterine vasculature, with abnormal vascular adaptations contributing to pregnancy diseases such as preeclampsia. The peptide hormone relaxin is important for the renal and systemic hemodynamic adaptations to pregnancy, and has been shown to increase arterial compliance and outward hypertrophic remodeling. Therefore, we investigated the possibility that relaxin acts on its receptor, RXFP1, to mediate uterine artery compliance in late pregnancy and increase uterine blood flow velocity in rats. RXFP1 was predominantly localized to the tunica media vascular smooth muscle cells in the uterine artery, although receptors were also detected in endothelial cells. Highest expression of Rxfp1 in the uterine artery occurred in estrus and early pregnancy. Isolated uterine arteries from late pregnant rats treated with a monoclonal antibody against circulating relaxin (MCA1) had significantly increased vessel wall stiffness compared with controls, with no reduction in wall thickness. Chronic infusion of relaxin (4 μg/h, osmotic minipump) for 5 d in nonpregnant rats significantly increased uterine artery blood flow velocity. Overall, these data demonstrate a functional role for relaxin in mediating uterine artery compliance in pregnant rats, which may be necessary to maintain adequate uterine blood flow to the uterus and placenta.