Changes in serum concentrations of immunoreactive inhibin during the oestrous cycle of the rat

Serum and ovarian concentrations of inhibin during the oestrous cycle of rats were determined by a radioimmunoassay (RIA) based on a porcine inhibin RIA. Serum concentrations of LH, FSH, oestradiol-17 beta, progesterone and testosterone were also determined during the cycle. Serum concentrations of inhibin were high (8.47 +/- 0.58 micrograms/l) during the morning of pro-oestrus and then dropped sharply to the lowest level (3.21 +/- 0.38 micrograms/l) at 24.00 h on the day of pro-oestrus after the preovulatory LH and FSH surge. Inhibin concentrations then recovered rapidly, with a peak (7.94 +/- 0.97 micrograms/l) at 15.00 h on the day of oestrus. Inhibin concentrations in serum dropped again to their second lowest value (4.00 +/- 0.12 micrograms/l) at 24.00 h on the day of oestrus. The largest FSH surge observed from midnight of pro-oestrus to the morning of oestrus (the so-called second FSH surge) was shown to be inversely related to the changes in serum concentrations of inhibin, suggesting a close relationship between the serum FSH and inhibin concentrations in cyclic rats.