INHIBIN AND FOLLISTATIN CONCENTRATIONS IN FETAL TISSUES AND FLUIDS DURING GESTATION IN SHEEP - EVIDENCE FOR ACTIVIN IN AMNIOTIC-FLUID

The concentrations of inhibin and follistatin in amniotic fluid and in tissue extracts from the placenta, gonads and adrenals of fetal sheep were measured using radioimmunoassays. These tissue extracts were fro m whole fetuses from days 16 to 45 and from the individual organs from day 46 to 145 (term) and were assayed at multiple dilutions. The capa city of these extracts to alter FSH production of rat anterior pituita ry cells in culture was also assessed at multiple dilutions. Immunoact ive inhibin concentrations in amniotic fluid from both sexes increased during gestation and levels were significantly greater in males than females. Peak concentrations of immunoreactive inhibin of 11.2 +/- 1.9 ng/ml were found in males at 116-125 days of gestation. Follistatin c oncentrations did not change throughout gestation and no significant d ifference was noted between sexes. Mean follistatin levels throughout gestation were 3.0 +/- 0.9 ng/ml for males and 3.7 +/- 0.9 ng/ml for f emales. Despite the potential for FSH inhibition by inhibin and follis tatin, amniotic fluid from both sexes at all stages of gestation stimu lated FSH secretion in the pituitary cell bioassays, suggesting the pr esence of activin which was confirmed by the measurement of immunoacti ve activin (13.3 +/- 2.5 ng/ml) in a specific radioimmunoassay. Maximu m concentrations of immunoactive and bioactive inhibin in placental ex tracts were observed in late gestation (2.2 +/- 0.6 and 3.8 +/- 1.6 ng /g respectively) and there was no significant difference between sexes . Follistatin concentrations in placental cotyledons ranged from 11.5 to 27.1 ng/g with no significant difference between sexes. In view of the higher follistatin concentrations compared with inhibin, it is lik ely that the capacity of placental extracts to suppress FSH production by pituitary cells in culture is due predominantly to follistatin. Im munoactive inhibin was observed in high concentrations in the fetal te stis throughout gestation; with concentrations increasing to a maximum of 1993.0 +/- 519.7 ng/g at 126-135 days of gestation with a ratio of bioactive: immunoactive inhibin of 1:20. Although bioactive and immun oactive inhibin was also observed in fetal ovaries and adrenals from b oth male and female fetuses, concentrations were lower than those obse rved in fetal testes. Follistatin concentrations in the fetal testis w ere elevated between 70 and 95 days (97.6 ng/g) and then declined. Sim ilar concentrations were found in the adrenal glands of both sexes (ma les 83.5-103.3 ng/g: females 55 .3-95.8 ng/g). In both males and femal es, immunoactive inhibin concentrations in fetal adrenals increased du ring gestation peaking at levels of 34.4 +/- 16.5 and 27.8 +/- 9.0 ng/ g respectively. These data suggest that the capacity ofadrenal extract s to suppress FSH production by pituitary cells is due to both inhibin and follistatin. These studies demonstrated that significant concentr ations of immunoactive inhibin and follistatin are present in amniotic fluid, and the fetal gonads, adrenal glands and placenta in sheep. Th e role of these proteins during fetal development requires further stu dy.