Plasma concentrations of immunoreactive (ir)-inhibin were measured in seven pregnant mares from around Day 140 of gestation to Day 2 after parturition using a heterologous bovine-based radioimmunoassay (RIA). Concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), oestradiol-17 beta, progesterone and relaxin were also measured in the same samples. A marked increase in plasma concentrations of ir-inhibin, FSH and LH occurred between Day 220 and Day 300 of gestation but the concentrations of all three hormones returned to baseline by about Day 320 (three weeks before parturition). In contrast, circulating concentrations of the three placental hormones, oestradiol-17 beta, progesterone and relaxin, increased during the final weeks of pregnancy and then decreased markedly to basal values within two days of parturition. There was a positive correlation between circulating concentrations of ir-inhibin and FSH (r = 0.75, P < 0.01) rather than the expected negative correlation. ir-inhibin was not detected in homogenates obtained at Day 190 of pregnancy and form term placenta, but high concentrations of ir-inhibin were present in homogenates of fetal and newborn gonads. Despite the high concentrations of ir-inhibin in these homogenates, they failed to exert any suppressive bioactivity on FSH secretion by rat pituitary cells cultured in vitro. Furthermore, immunohistochemical staining revealed the presence of inhibin in the interstitial cells of equine fetal gonads at Day 190 of gestation. These findings demonstrate for the first time that high concentrations of ir-inhibin, LH and FSH are secreted into the peripheral circulation of the mare during the second half of pregnancy. However, ir-inhibin present in the plasma of pregnant mares appears to be biologically inactive. This hormone is not presumed to be of placental origin but it is proposed that either the enlarged fetal gonads or the maternal ovaries, or both of these organs, may be a source of inhibin in response to the coincident increase in circulating concentrations of LH and FSH.

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Radioimmunoassays for rat FSH and LH have been studied taking advantage of rat FSH and LH radioimmunoassay kits supplied by the NIAMD. Separation of free and antibody bound labeled hormone was accomplished by the double antibody method with pre-precipitation technique. Although rat GH and prolactin did not react in the FSH assay, LH crossreacted slightly to a negligible extent. Hypophysectomized rat serum inhibited the reaction between labeled rat FSH and anti-rat FSH serum. To eliminate the nonspecific interference by serum, hypophysectomized rat serum was added to the diluent for reference preparations in the FSH assay. Rat GH and prolactin did not react in the LH assay. However, FSH crossreacted in the LH assay to a greater degree than LH in the FSH assay. This indicates that LH concentrations may be overestimated when samples with extraordinary large amounts of FSH are measured. Hypophysectomized rat serum did not influence the LH assay. The FSH activity of pituitary extracts determined by bioassay and that by radioimmunoassay agreed well. And there was also a good correlation between LH values obtained both by bioassay and by radioimmunoassay. Although a marked decrease in pituitary FSH and LH levels was found between proestrus and estrus, serum FSH and LH levels were elevated only slightly at 2:00 PM of the proestrus day. Serum and pituitary FSH and LH levels increased after ovariectomy.

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