Measurement of inhibin-A (alpha beta A dimer) during the oestrous cycle, after manipulation of ovarian activity and during pregnancy in ewes.

A new two-site ELISA was validated for ovine plasma and used to measure circulating inhibin-A concentrations during a synchronized oestrous cycle in four ewes and throughout pregnancy in six ewes. Inhibin A concentrations were also determined in four ewes during chronic treatment with a GnRH agonist and after subsequent exposure to pregnant mares' serum gonadotrophin (PMSG) to stimulate ovarian follicular development. Concentrations of FSH, LH, oestradiol and progesterone were determined by radioimmunoassay. The detection limit of the inhibin-A ELISA was approximately 50 pg ml-1 and no significant crossreaction was observed with a range of related molecules including activin-A, inhibin-B, activin-B, follistatin and alpha 2-macroglobulin. Inhibin-A concentrations were below the detection limit in plasma from hypophysectomized and ovariectomized ewes. During the oestrous cycle, plasma inhibin-A concentrations (approximately 0.3-0.4 ng ml-1) did not vary during the follicular phase whereas plasma oestradiol increased approximately tenfold. After the preovulatory LH/FSH surge, inhibin-A fell to a nadir (approximately 0.15 ng ml-1) coincident with the peak of the postovulatory FSH rise. During the next 2 days, FSH concentrations fell to basal values as inhibin-A concentrations increased (P < 0.05) to a peak (approximately 0.5 ng ml-1) 3 days after the preovulatory LH/FSH surge. Over the following 3 days, FSH values increased again (P < 0.05) as inhibin-A concentrations fell to approximately 0.25 ng ml-1 (P < 0.05). Chronic GnRH agonist treatment suppressed FSH concentrations by about 50%, while inhibin-A and oestradiol concentrations fell below detection limits. Within 2 days after the PMSG injection, concentrations of inhibin-A (approximately 4.5 ng ml-1) and oestradiol (approximately 20 pg ml-1) had increased to very high values, while FSH concentrations had been reduced by a further 50%. Plasma concentrations of inhibin-A and FSH were similar to those in nonpregnant ewes during the first 60 days of gestation, but inhibin-A values fell markedly (sevenfold; P < 0.01) between days 60 and 90, coincident with a twofold decrease in FSH (P < 0.05). Inhibin A and FSH concentrations remained low for the remainder of gestation and were positively correlated throughout pregnancy (r = 0.48; P < 0.005). These observations support an endocrine feedback role for ovarian inhibin-A and oestradiol in controlling the secondary (postovulatory) FSH surge in ewes, but indicate that an increase in oestradiol is responsible for the characteristic reduction in FSH during the early to mid-follicular phase. The reduced secretion of FSH from mid- to late pregnancy cannot be attributed to increased inhibin-A secretion by the feto-placental unit, but most likely reflects increased steroid secretion from this source.