IGF-I stimulation of luteinizing hormone secretion, IGF-binding proteins (IGFBPs) and expression of mRNAs for IGFs, IGF receptors and IGFBPs in the ovine pituitary gland
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Circulating concentrations of insulin-like growth factor-I (IGF-I) are reduced in juvenile sheep during nutritional growth restriction and the associated delay in puberty. Since exogenous IGF-I has been shown to stimulate luteinizing hormone (LH) secretion, it is postulated that endogenous IGF-I may act as a stimulatory metabolic signal to the pubertal ovine hypothalamo-pituitary axis, yet its site of action is unknown. Using coronal hypothalamic and pituitary sections from pubertal ewe lambs, in vitro autoradiography was used to localise 125I-labelled IGF-I binding, and gene expression for components of the IGF system was localised by in situ hybridisation using oligonucleotide probes. High concentrations of 125I-IGF-I binding were seen in the pars tuberalis (PT) and pars distalis (PD) of the pituitary, and relatively little in the hypothalamus; binding in the PT but not the PD was displaced by excess unlabelled IGF-I. Large amounts of mRNA were detected for the type-1 receptor (IGF-1R) and for IGF-binding protein (IGFBP)-5, localised to the PT and PD, and less intense specific hybridisation signals were obtained with mRNAs for IGF-II, type-2 receptor (IGF-2R) and IGFBP-3. There was some evidence for specific hybridisation to IGFBP-4 mRNA in the PT. IGF-I, IGFBP-1 and IGFBP-2 mRNAs were not detected in PT and PD. None of the genes were expressed in hypothalamic tissue. Western-ligand binding on PD extracts from male castrates revealed by their molecular weights the likely presence of IGFBPs-2, -3, and -5. Finally, cultured PD cells from abattoir-killed sheep were challenged with IGF-I (0.1, 1, 10 or 30 nM) or luteinizing hormone-releasing hormone (LHRH, 10 nM) alone, or both together. Basal LH output was stimulated by 10 nM IGF-I (120+/-11.2%, P>0.05), 30 nM IGF-I (148+/-12.8%, P<0.01), and LHRH alone (200+/-16.1%, P<0.001); there was no additive or subtractive effect of LHRH and IGF-I given together. Thus, an intrapituitary IGF system exists in sheep and the present results are consistent with an endocrine role for IGF-I in nutritional modulation of LH secretion at the level of the pituitary gland.
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