Submitted to: Biology of Reproduction
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 25, 1998
Publication Date: N/A
Interpretive Summary: Failure of domestic animals to reproduce in a timely fashion is a major factor influencing efficiency of meat animal production. Scientific research has demonstrated that reproduction is regulated by a cascade of hormones produced by the hypothalamus and pituitary, which ultimately act to regulate the ovary. Hormones produced by the ovary then act in a feedback system to regulate hormone production by the hypothalamus and pituitary. Recent research indicates that insulin-like growth factor-1 (IGF-1) may regulate production and(or) secretion of reproductive hormones from the hypothalamus and pituitary. The actions of IGF-1 are in turn regulated by a specific group of at least 6 different proteins called IGF binding proteins (IGFBPs). The current study was undertaken as a first step in determining how estrogen, produced by the ovary, may influence IGF-1 and IGFBPs using ovariectomized ewes as a model. Part of the ewes (n = 6) were given an implant containing estrogen; the remaining ewes served as untreated (no estrogen) controls. Treatment with estrogen increased IGFBP-3, IGFBP-4 and IGF-1 in the circulation; IGFBP-2 in the pituitary; and IGFBP-2, -3, and -5 in the stalk median eminence of the hypothalamus. These effects of estrogen occurred in association with the well documented estrogen suppression of the pituitary hormone LH. These results indicate that alterations in IGF-1 and IGFBPs may be involved in estrogen modulation of hypothalamic and pituitary function.
The influence of estradiol on concentrations of insulin-like growth factor I (IGF-I) and IGF binding proteins (IGFBPs) in serum, anterior pituitary glands and stalk median eminences (SMEs) of the ovine was evalauted. Amounts of mRNAs in the anterior pituitary gland for IGFBP-2 and gonadotropin subunits were also determined. Chronically ovariectomized ewes sreceived a single Silastic implant (d 1) containing estradiol (E; n=6) or no implant (C; n=4). On d 80, pituitary glands and SMEs were removed and stored at -70 C. Serum concentrations (ng/ml) of IGF-I were greater (p<.05) in E than C ewes (161 vs 108) while serum concentrations (ng/ml) of LH were greater (p<.05) in C than E ewes (9.26 vs 3.41) on d 75. Relative abundance of IGFBP-3 and -4 in serum was greater (p<.01) in E than C ewes whereas IGFBP-2 and a 29 kDa IGFBP were not influenced by estrogen treatment. Relative amounts of pituitary IGFBP-2 tended (p<.1) to be greater in E than nC ewes. This increase was associated with a twofold increase in mRNA for IGFBP-2 (p<.05) in E vs C ewes. Other IGFBPs detected in the pituitary (IGFBP-3 and -5) were not influenced by estrogen treatment. In the SME, relative amounts of IGFBP-5, -3, and -2 were greater (p<.05) in E compared to C ewes. Relative amounts of mRNA for LH beta subunit were approximately twofold less (p<.01) in E than in C ewes. Relative amounts of mRNA for FSH beta and alpha subunit were not influenced by estradiol (p>.1). In summary, estradiol increased serum IGFBP-3, -4 and IGF-I; pituitary expression of IGFBP-2; relative amounts of IGFBP-5, -3, and -2 in the SME; and decreased expression of LH beta. Estradiol may influence gonadotrope function through mechanism(s) involving IGFBP modulation of IGF action in the SME and(or) pituitary gland in the ovine.