CIRCULATING BIOACTIVE FOLLICLE-STIMULATING-HORMONE AND IMMUNOREACTIVE INHIBIN LEVELS DURING THE NORMAL HUMAN MENSTRUAL-CYCLE

To examine the relationship between circulating levels of bioactive FSH (B‐FSH) and immunoactive inhibin and oestradiol we studied five women during ovulatory cycles. Daily blood samples were collected from each subject during one menstrual cycle. B‐FSH was measured using a modified, highly sensitive in‐vitro rat granulosa cell bioassay. The inclusion of IGF‐1 (10 μg/1) and transferrin (50 mg/I) in the assay system enhanced granulosa ceil responsiveness to FSH and resulted in increased assay sensitivity. Inhibin was measured by a heterologous radioimmunoassay (RIA) using an antibody raised against 31 kDa bovine inhibin. Bioactive FSH (B‐FSH) levels were closely correlated to those of immunoactive FSH (I‐FSH, r = 0.79, P<0001) throughout the cycle. Peak levels of B‐FSH were observed during the early follicular phase (day –13, 44.7 ± 9.6 IU/1, mean ± SEM) and during the midcycle surge (35.2 ± 6.2 IU/1); lowest levels occurring during the luteal phase (nadir 3.9 ± 0.27 IU/1). Plasma oestradiol levels increased significantly during the follicular phase (P<0.001) to a peak on day – 1 and were negatively correlated with B‐FSH during the late follicular phase (day –8 to – 1; r= –0.45. P<002). There was no change in the concentration of inhibin (range 55.3–72.3 U/1) during the follicular phase until day –2 after which an increase to a midcycle peak of 139± 10.6 U/1 was observed. No correlation was observed between inhibin and B‐FSH during the follicular phase. A second increase in the concentration of inhibin was seen during the luteal phase; peak levels occurred by day 6 (311 ±25.8 U/1), remained elevated until day 12, and were negatively correlated with B‐FSH (r=–0.53, P<0001). No correlation was observed between oestradiol and inhibin or B‐FSH during the luteal phase. We conclude that (1) © 1990 Blackwell Scientific Publications Ltd.