Relaxin in the pregnant baboon: evidence for local production in reproductive tissues

We examined the nonluteal production of relaxin in the pregnant baboon. In experimental animals (n = 5), the corpus luteum-bearing ovary was removed in early pregnancy (days 25-30), and intact pregnant baboons served as controls. In both groups of baboons, peripheral, uterine, and ovarian venous blood samples and amniotic fluid were collected immediately before hysterotomy, which was performed late in pregnancy (days 139-142). After hysterotomy, samples of reproductive and control tissues were obtained (placenta, decidua, myometrium, fetal membranes, and omentum). Relaxin concentrations were determined in all samples using an antiporcine relaxin RIA. In unilaterally oophorectomized pregnant baboons, peripheral plasma levels of relaxin were below the limits of detection (less than 157 pg/ml) for 100 days following corpus luteum removal, while levels in control pregnant baboons were greater than 1000 pg/ml. Relaxin levels in uterine venous plasma were comparable to peripheral plasma levels in each group. Relaxin concentrations in amniotic fluid of both groups were below the limits of detection (less than 157 pg/ml). Relaxin was found in decidua, placenta, and myometrium in those pregnant baboons in which the corpus luteum-bearing ovary had been removed over 100 days earlier. This finding together with the absence of a relaxin gradient in uterine venous plasma and undetectable relaxin levels in amniotic fluid is a strong indication of local relaxin production in reproductive tissues without contribution to peripheral plasma levels.