L Rombauts

Developmental changes in immunoreactive inhibin and FSH in plasma of chickens from hatch to sexual maturity

1. The relationship between immunoreactive inhibin and follicle-stimulating hormone (FSH) was studied in male and female chickens from hatch to sexual maturity. Plasma inhibin was estimated by a heterologous radioimmunoassay validated for use in the chicken. FSH was measured by a recently developed homologous radioimmunoassay. 2. In a cross-sectional study, blood samples and gonads were collected from chickens of both sexes at 1, 3, 5, 7, 14, 21 and 28 d after hatching and subsequently at 14-day intervals until 182 d of age. 3. In the female, plasma progesterone concentration (P4) progressively increased during sexual development. The plasma luteinising hormone (LH) concentration rose during the first week after hatching, and fluctuated thereafter, with troughs at 6 and 14 weeks and peaks at weeks 10 and 18. The plasma inhibin and FSH concentrations remained low until the start of puberty and increased simultaneously thereafter. However, from week 18 on, plasma inhibin continued to rise while plasma FSH fell. Hence, FSH and inhibin were positively correlated before puberty, but developed a negative correlation during sexual maturation. 4. In the male, plasma testosterone and LH concentrations increased 38- and 3.7-fold respectively over the period studied. Inhibin and FSH followed similar time courses and were consequently positively correlated. 5. These results suggest sex differences in the role of inhibin in regulating FSH secretion during development. The FSH-inhibin feedback loop may become operational at the onset of sexual maturity in the hens. In male chickens, the similar pattern of inhibin and FSH secretion suggests that inhibin secretion is driven by FSH.

Evidence for the presence of immunoreactive inhibin in extragonadal tissues of ovariectomized ewes

Six ewes were ovariectomized to determine the immediate and long-term effects of removal of ovaries on the immunoreactive concentrations of FSH, LH and inhibin. Three months after ovariectomy, ewes were slaughtered and tissue samples of brain, pituitary, spleen, liver, perirenal fat, lung, kidney, adrenals and uterus were collected to determine the immunoreactive inhibin content. Both gonadotrophins, FSH and LH, increased significantly after ovariectomy. The increase of FSH, however, was more pronounced and remarkably faster than the changes of LH after ovariectomy. Immunoreactive concentrations of inhibin decreased sharply as early as 15 min after ovariectomy and subsequently decreased more gradually until 2 weeks after surgery. From this moment on, the level stabilized at 56% of the initial value. In control ewes, a considerable amount of immunoreactive inhibin is found in tissue samples of ovary, lung, kidney, pituitary and spleen. After ovariectomy, the level of immunoreactive inhibin decreased in spleen and lung samples while an important increase of immunoreactive inhibin is found in adrenals and pituitary. These results demonstrate a differential regulation of LH and FSH after ovariectomy and support an involvement of inhibin only in the immediate changes of FSH after ovariectomy in sheep. They further suggest that the adrenals and the pituitary may be extragonadal sources of inhibin. To explore the eventual contribution of the adrenals to circulating inhibin, dexamethasone (1.4 mg/ewe) and ACTH (200 IU/ewe) were in a following experiment injected intravenously in control and ovariectomized ewes. The lack of any effect of dexamethasone or ACTH on the plasma concentration of immunoreactive inhibin indicate that adrenal inhibin probably does not contribute to circulating inhibin.

Circulating levels of inhibin and FSH during the oestrous cycle in 4 genotypes of sheep with different reproductive performances

Abstract Two relatively fecund genotypes (Flemish Milksheep and KU-Lovenaar—an F 2 cross-bred originating from pure-bred Suffolk and Flemish Milksheep) and two meat-type breeds (Texel and Suffolk) were compared regarding their plasma inhibin and FSH levels. Oestrus was synchronised by progestagen-impregnated sponges. Blood sampling, starting 12 h after the sponge removal, continued twice daily for 21 consecutive days. To localise the pre-ovulatory LH peak and to determine the cycle length, sampling at 2 h intervals was carried out during the first 3 days and the last 4 days of the 21-day period. Immunoreactive inhibin was measured by a 5-24 α -subunit inhibin assay as well as by an assay based on an antiserum against native bovine 31-kDa inhibin (Monash assay). Changes in plasma concentration of immunoreactive FSH and LH during the oestrous cycle agree with the literature data. The profiles generated by the two different inhibin RIAs were different and showed a negative correlation during the periovulatory phase and on days 9–11. No characteristic pattern could be observed during the oestrous cycle. Significant breed differences were observed for 5-24 α -subunit inhibin as well as for inhibin, measured by the Monash assay. The significantly higher ( P α -subunit inhibin and lower inhibin (Monash assay) concentration in Texel ewes resulted in a differential ratio of both inhibin measurements for the Texel in comparison with the other breeds. No significant breed differences for FSH were observed during the oestrous cycle. It is concluded that, although significant breed differences in inhibin concentration are observed, the FSH-inhibin interaction is not the only factor responsible for breed differences in ovulation rate.