OVARIAN AND EXTRAOVARIAN SOURCES OF IMMUNOREACTIVE INHIBIN IN THE CHICKEN - EFFECTS OF DEXAMETHASONE

The present study investigates whether besides the ovary, extragonadal sources contribute to the total amount of immunoreactive inhibin in t he plasma of the domestic hen. A comparison of the inhibin content of different organs shows that, expressed per milligram of tissue, the ad renal ranks second only to the ovarian granulosa layer. To explore the contribution of the adrenals to plasma inhibin, dexamethasone (100 mu g/kg BW) was injected iv into intact, ovariectomized, and sham-operat ed hens. Control animals of each experimental group were injected with saline (0.9% (w/v) NaCl). Dexamethasone significantly (P < 0.05) decr eased plasma inhibin concentrations in the three groups. The suppressi ve effect of dexamethasone in intact hens, however, was caused by a di rect effect of this synthetic glucocorticoid on the gonads. Indeed, de xamethasone decreased the production of inhibin by granulosa cells in vitro and also lowered the immunoreactive inhibin concentration in ova riectomized animals. The decreased plasma inhibin concentration in ova riectomized animals is probably due to a direct effect of dexamethason e on the adrenals. Adrenal cells produced immunoreactive inhibin in vi tro. The inhibin secretion by adrenal cells was significantly (P < 0.0 5) depressed by dexamethasone. In conclusion, the ovary is the major s ource of plasma immunoreactive inhibin in the laying hen. The presence of substantial amounts of immunoreactive inhibin in the adrenal, the secretion of inhibin by cultured adrenal cells, and the decreased immu noreactive inhibin in ovariectomized animals treated with dexamethason e indicate that the adrenal is a likely source of extragonadal inhibin . The nature and the role of this adrenal inhibin remain to be investi gated. (C) 1997 Academic Press.