Relationship between ovarian steroids, gonadotrophins and relaxin during the menstrual cycle

Abstract

The circulating levels of relaxin have been measured and their relationship with the plasma levels of oestradiol (E2), progesterone, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) have been investigated during the normal menstrual cycle. In addition, the effect of human chorionic gonadotrophin (hCG) on plasma relaxin levels has been studied. In the first part of the study, blood samples were obtained on days 5, 10 and 15 of the follicular phase and on alternate days from the day of the LH surge (detected in early-morning urine and confirmed by circulating levels of LH) until day 6 of the following follicular phase in nine normally cycling female volunteers. In the second part of the study, a single intramuscular dose of hCG (10 000 IU) was given on day 11 of the menstrual cycle. Relaxin was detectable from the mid-luteal phase until the onset of menstruation. The plasma levels of relaxin on days 10 and 12 of the luteal phase were significantly greater than on day 6. Positive associations between the circulating levels of relaxin and E2 and negative associations between the plasma levels of FSH and those of both relaxin and E2 were found on days 8, 10 and 12 of the luteal phase. The relationship between E2 and FSH was stronger than that between relaxin and FSH. Exogenous hCG had no effect on plasma relaxin levels. The pattern of the relationship between E2 and relaxin suggests that a common mechanism may regulate their release or that plasma relaxin levels are determined by those of E2. Furthermore, the absence of any relationship between endogenous LH levels and those of relaxin and the lack of effect of exogenous hCG on plasma relaxin levels suggest that LH does not influence the circulating levels of relaxin directly. The negative relationship between FSH and relaxin is probably indirect, mediated by E2, although it is possible that relaxin influences FSH release directly.