MEASUREMENT OF INHIBIN-A (ALPHA-BETA-A DIMER) DURING THE ESTROUS-CYCLE, AFTER MANIPULATION OF OVARIAN ACTIVITY AND DURING PREGNANCY IN EWES

A new two-site ELISA was validated for ovine plasma and used to measur e circulating inhibin-A concentrations during a synchronized oestrous cycle in four ewes and throughout pregnancy in six ewes. Inhibin A con centrations 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 determin ed by radioimmunoassay. The detection limit of the inhibin-ii ELISA wa s approximately 50 pg ml(-1) and no significant crossreaction was obse rved with a range of related molecules including activin-A, inhibin-B, activin-B, follistatin and alpha(2)-macroglobulin. Inhibin-A concentr ations were below the detection limit in plasma from hypophysectomized and ovariectomized ewes. During the oestrous cycle, plasma inhibin-A concentrations (approximately 0.3-0.3 ng ml(-1)) did not vary during t he follicular phase whereas plasma oestradiol increased approximately tenfold. After the preovulatory LH/FSH surge, inhibin-A fell to a nadi r (approximately 0.15 ng ml(-1)) coincident with the peak of the posto vulatory FSH rise. During the next 2 days, FSH concentrations fell to basal values as inhibin-A concentrations increased (P < 0.05) to a pea k (approximately 0.5 ng ml(-1)) 3 days after the preovulatory LH/FSH s urge. 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 b y about 50%, while inhibin-A and oestradiol concentrations fell below detection limits. Within 2 days after the PMSG injection, concentratio ns of inhibin-A (approximately 4.5 ng ml(-1)) and oestradiol (approxim ately 20 pg ml(-1)) had increased to very high values, while FSH conce ntrations 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 (seven fold; P < 0.01) between days 60 and 90, coincident with a twofold decr ease In FSH (P < 0.05). Inhibin A and FSH concentrations remained low for the remainder of gestation and were positively correlated througho ut pregnancy (r = 0.48; P < 0.005). These observations support an endo crine feedback role for ovarian inhibin-A and oestradiol in controllin g the secondary (postovulatory) FSH surge in ewes, but indicate that a n increase in oestradiol is responsible for the characteristic reducti on in FSH during the early to mid-follicular phase. The reduced secret ion of FSH from mid- to late pregnancy cannot be attributed to increas ed inhibin-A secretion by the fete-placental unit, but most likely ref lects increased steroid secretion from this source.