Relaxin in the marmoset monkey: Secretion pattern in the ovarian cycle andearly pregnancy

Relaxin is a peptide hormone with a broad range of biological activities, r elated not only to parturition and lactation but possibly also to deciduali zation, implantation, and early pregnancy. The present study was designed t o investigate the secretion pattern of relaxin throughout the cycle and ear ly pregnancy in the common marmoset monkey in relation to ovarian function and the systemic hormone milieu. First, a novel relaxin ELISA was developed and validated to confirm the pattern of relaxin secretion during pregnancy . Secondly, serum relaxin profiles were determined through nonconceptive an d conceptive cycles and analyzed in relation to the concentration of other hormones and to the development of ovarian follicles and corpora lutea (CL) . Blood samples were collected 2-3 times per week from the experimental ani mals and analyzed for relaxin, progesterone, and LH. The animals from the c onceptive cycles were also ultrascanned at these time points to determine t he ovarian status up to Day 25 of pregnancy. During early pregnancy, the re laxin levels in serum were approximately 1 ng/ml, increasing up to 15 ng/ml in the second trimester, at a time when progesterone levels had declined. In the third trimester, when progesterone levels were increasing again, the levels of relaxin decreased, returning to basal levels by term of pregnanc y. In early pregnancy there was a parallel increase in both relaxin and LH/ hCG, with the relaxin rise in the conceptive cycle appearing sooner than in the nonconceptive cycle, suggesting that, like chorionic gonadotropin (CC) , relaxin may be a useful and early marker for pregnancy. Unlike the situat ion in the human, there was no correlation between the levels of either hor mone and the number of CL detected, infants born, mother's age, or parity. Relaxin levels increased in early pregnancy before bioactive LH/CG, implyin g that relaxin is not directly regulated by this gonadotropin. Furthermore, hCG applied to nonconceptive females during the expected time of implantat ion caused an increase in progesterone but not in relaxin concentrations. I n summary, the results obtained indicate that relaxin may be a reliable ind icator of early pregnancy status in the common marmoset, but it is independ ent of direct CG influence.