Hormonal control of inhibin secretion

Erythroid differentiation factor can modulate follicular granulosa cell functions

The action of human erythroid differentiation factor (EDF) on the functions of rat granulosa cells cultured in a chemically defined medium was investigated. In the presence of FSH that induced LH receptor expression and progesterone synthesis during culture of the cells, EDF augmented both responses in a dose- and time-dependent manner. Unlike FSH, EDF itself did not have such an inducing effect at all. Furthermore, in the absence of FSH, EDF was found to strongly enhance the ability of granulosa cells to produce inhibin. Thus, EDF may play an important role in the regulation of granulosa cell function and differentiation during follicle development.

Regulation of inhibin production by rat granulosa cells

Inhibin production by cultured granulosa cells from immature diethylstilbestrol (DES)-primed rats was studied in relation to estradiol and progesterone production. The inhibin content in culture media was assayed with a specific radioimmunoassay (RIA) using an antibody to porcine 32 kDa inhibin that recognizes rat inhibin as well. Inhibin production was about 10 ng/ml/2 X 10(4) cells/72 h at the basal levels and was maximally stimulated with 25 ng/ml of follicle stimulating hormone (FSH) to 45 ng/ml which was 4.5 times the basal levels, with an ED50 value of 2.0 ng/ml. A cyclic AMP analog (dibutyryl cyclic AMP) or reagents that promote cAMP production were also effective in inhibin production, indicating that FSH stimulates inhibin production through a cAMP-dependent pathway. Luteinizing hormone (LH) was not effective in producing inhibin from freshly prepared granulosa cells, whereas granulosa cells pre-incubated with FSH for 48 h because responsive to LH regarding inhibin production. Testosterone sensitized the granulosa cells to the FSH stimulation, whereas hydrocortisone (4 ng/ml) decreased the sensitivity of granulosa cells by increasing the ED50 value for inhibin production by FSH about 10 times. A similar effect was observed regarding estradiol production, while progesterone production due to stimulation by FSH was enhanced by the hydrocortisone treatment. Insulin and platelet extract both stimulated inhibin production and enhanced the maximal response of inhibin production due to stimulation by FSH without altering, or even increasing the ED50 values. Epidermal growth factor (EGF), (D-Leu6)Des-Gly10-LHRH N-ethylamide (GnRH agonist) and 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent protein kinase C activator, inhibited both inhibin production and estradiol or progesterone production. Consequently, the regulation of inhibin production was similar to that of estradiol production, but markedly different from that of progesterone. However, inhibin and estradiol production were modulated differently by various growth factors and hormones. These phenomena might account for possible discrete changes in the plasma levels of inhibin and estradiol in vivo.

Detection of folliculostatin: a sensitive and specific bioassay using the female rat

To establish a sensitive and specific bioassay for the FSH-suppressing activity present in porcine follicular fluid (pFF), we examined the latency of pFF action when injected IV in the acutely ovariectomized (ovax) metestrous rat. By 2h post injection (5.5h after ovax), FSH was suppressed significantly in pFF vs. porcine serum-injected controls. LH was unaltered. In an experiment establishing a dose-response curve for pFF 4.5h after injection, 1.77 mg of pFF protein significantly suppressed FSH. The index of precision (-0.2188) and precision of slope (1.088) were well within acceptable limits for bioassays. We conclude that the ovax metestrous rat, injected 3.5h after surgery and sacrificed at 4.5 or 5.5h, is a sensitive and specific bioassay for folliculostatin.