Changes in the biological: immunological ratio of basal and GnRH-releasable FSH during the follicular, pre-ovulatory and luteal phases of the human menstrual cycle

BACKGROUND: Significant changes in charge isoform distribution of serum FSH occur throughout the human menstrual cycle. In the present study, we analy sed the impact of the changing endocrine milieu characteristic of the menst rual cycle on the capability of basal and gonadotrophin-releasing hormone ( GnRH)-releasable FSH to trigger intracellular signal transduction via the h uman FSH receptor. METHODS: Seven normal women underwent blood sampling eve ry 10 min for 10 h during the early follicular phase (Fl?), pre-ovulatory p hase (PO) and mid- to late luteal phase (LP) of the menstrual cycle. Serum from successive samples collected across 2 h intervals containing FSH relea sed under baseline and exogenous GnRH-stimulated conditions was tested for bioactivity employing a homologous in-vitro assay. RESULTS: The biological to immunological (B:I) ratio of basal and GnRH-releasable FSH was significa ntly (P < 0.05) higher at LP (range, 0.83 +/- 0.07 to 1.35 +/- 0.30) than d uring the FP (0.43 +/- 0.02 to 0.65 +/- 0.04) and PO (0.49 +/- 0.05 to 0.62 +/- 0.06). In all phases, the B:I FSH ratio in baseline samples was simila r to those exhibited by samples collected after 10 and 90 mug GnRH administ ration. CONCLUSIONS: The selective increase in the capability of the admixt ure of FSH isoforms circulating during the LP to activate the FSH receptor, apparently represents an additional mechanism through which the anterior p ituitary may regulate the maturation of those follicles destined to ovulate during the coming cycle.