Induction of Relaxin Secretion in Nonpregnant Women by Human Chorionic Gonadotropin*

Abstract

Five normal women using barrier forms of contraception had serum samples drawn every2 days during control menstrual cycles. Relaxin was not detectable in their serum. All five subjects then received 2500 IU hCG im every 2 days beginning 8-10 days after presumed ovulation. All had immunoreactive relaxin detectable in serum 2-6 days after the first hCG injection. Relaxin rose to pregnancy levels (range, 0.26–0.62 ng/ml; mean, 0.38 ± 0.17 ng/ml) by cycle day 31 and then declined before menstruation. Progesterone levels and cycle lengths of relaxin-secreting cycles were significantly higher (P < 0.05) than control cycles. Threeof the five subjects subsequently received 2500 IU hCG im every 2 days commencing 2-3 days after presumed ovulation until menstruation. Relaxin was not detectable in these cycles. hCG can induce relaxin secretion in nonpregnant women. A hCG stimulus which can induce relaxinsecretion from an 8- to 10-day-old corpus luteum cannot induce relaxin secretion in a 2- to 3-day-old corpus luteum, suggesting that the age of the human corpus luteum of the menstrual cycle is an important determinant of responsiveness to hCG.