Expression, distribution, regulation and function of IGFs in the ovine fetal pituitary

Activation of the fetal pituitary-adrenal axis is crucial for fetal organ maturation and the onset of parturition in sheep. Many factors including corticotrophin-releasing hormone (CRH) and arginine vasopressin secreted from the hypothalamus, and growth factors produced within the pituitary may be involved in the regulation of maturation of the fetal pituitary gland. IGFs have mitogenic and differentiation-promoting capacities in a variety of organs and are synthesized as paracrine factors within developing tissues. However, there is little information concerning the synthesis, distribution, regulation and function of IGFs in the fetal pituitary gland at different times during pregnancy. Therefore, we have localized IGF-I and IGF-II mRNAs and peptides, and determined the effect of cortisol on the level of IGF-II mRNAs in the pituitary glands of developing sheep fetuses. We examined the possible effects of IGFs on corticotroph function in cultures of adenohypophysial cells from term fetuses. Seven species of IGF-II transcripts of 1.2-6.0 kb were identified by Northern blot analysis in the pituitary gland of fetuses between day 60 of gestation and term (day 145). The levels of IGF-II mRNAs did not change significantly during pregnancy, although there was a trend for the presence of higher levels of IGF-II mRNAs at day 60 of gestation. IGF-I mRNA was not detectable. By in situ hybridization, IGF-II mRNA was localized to non-endocrine cells and to cells lining the blood vessels of the pars distalis, to some presumed endocrine cells in the pars distalis and pars intermedia, and to clusters of cells in the pars nervosa. In contrast, IGF-I and IGF-II peptides were detected in the presumed endocrine cells in the pars distalis and pars intermedia but not in the pars nervosa. Incubation of adenohypophysial cells from term fetuses with IGF-I, but not IGF-II, for 48 h increased specific 125I-Tyr-ovine CRH binding. However, neither IGF-I nor IGF-II had any significant effects on the basal or CRH-stimulated immunoreactive (ir)-ACTH output, the level of POMC mRNA or the number of ir-ACTH positive cells. Infusion of cortisol to fetuses starting at day 96 of gestation for 100 h or at days 120-125 of gestation for 84 h did not affect the level of IGF-II mRNAs in the pars distalis but decreased the levels of POMC mRNA. These results are consistent with IGFs having the potential to influence fetal pituitary function, although probably on cell types other than the corticotrophs.(ABSTRACT TRUNCATED AT 400 WORDS)