Dynamics of circulating concentrations of gonadotropins and ovarian hormones throughout the menstrual cycle in the bonnet monkey: role of inhibin A in the regulation of follicle-stimulating hormone secretion

Abstract

In higher primates, increased circulating follicle-stimulating hormone (FSH) levels seen during late menstrual cycle and during menstruation has been suggested to be necessary for initiation of follicular growth, recruitment of follicles and eventually culminating in ovulation of a single follicle. With a view to establish the dynamics of circulating FSH secretion with that of inhibin A (INH A) and progesterone (P(4)) secretions during the menstrual cycle, blood was collected daily from bonnet monkeys beginning day 1 of the menstrual cycle up to 35 days. Serum INH A levels were low during early follicular phase, increased significantly coinciding with the mid cycle luteinizing hormone (LH) surge to reach maximal levels during the mid luteal phase before declining at the late luteal phase, essentially paralleling the pattern of P(4) secretion seen throughout the luteal phase. Circulating FSH levels were low during early and mid luteal phases, but progressively increased during the late luteal phase and remained high for few days after the onset of menses. In another experiment, lutectomy performed during the mid luteal phase resulted in significant decrease in INH A concentration within 2 hr (58.3+/-2 vs. 27.3+/-3 pg/mL), and a 2- to 3-fold rise in circulating FSH levels by 24 hr (0.20+/-0.02 vs. 0.53+/-0.14 ng/mL) that remained high until 48 hr postlutectomy. Systemic administration of Cetrorelix (150 microg/kg body weight), a gonadotropin releasing hormone receptor antagonist, at mid luteal phase in monkeys led to suppression of serum INH A and P(4) concentrations 24 hr post treatment, but circulating FSH levels did not change. Administration of exogenous LH, but not FSH, significantly increased INH A concentration. The results taken together suggest a tight coupling between LH and INH A secretion and that INH A is largely responsible for maintenance of low FSH concentration seen during the luteal phase.