Growth hormone (GH) modulates insulin-like growth factor I (IGF-I) and type I IGF receptor mRNA levels in the ovary of prepubertal GH-deficient rats

In order to explore the potential role of growth hormone (GH) in modulating insulin-like growth factor I (IGF-I) gene expression in the prepubertal rat ovary, female rats were rendered GH deficient by neonatal administration of monosodium glutamate (MSG). One group of rats received vehicle and served as the control. At 21 days of age, MSG-treated rats received either GH or vehicle for 2 weeks. On days 21, 24, 28 and 31 animals were weighed and subsets were sacrificed for liver RNA extraction. The remaining animals were sacrificed at day 35 when livers and ovaries were collected, and serum was obtained for GH determinations. The IGF-I mRNA levels were estimated by Northern blots and corroborated further by slot-blot analysis. The MSG-treated rats had lower body weights (p < 0.01) and GH levels (p < 0.05) than controls. Growth hormone replacement significantly accelerated the weight gain of MSG-treated rats. At day 24 and thereafter, three RNA IGF-I species (7.5, 1.8 and 0.8-1.2 kB) were seen in the liver. In the ovary, at age 35 days, two major IGF-I mRNA species (7.5 and 0.8-1.2 kB) were seen. The MSG treatment consistently reduced the levels of both IGF-I mRNA species in the ovary. Growth hormone administration partially restored their expression, both in the liver and in the ovary. In addition, ovarian type I IGF receptor mRNA levels were increased in the MSG-treated rats when compared to controls. This trend was reversed by GH replacement. In summary, we have found that in prepubertal female rats rendered GH deficient with MSG, ovarian IGF-I gene expression is reduced while type I IGF receptor mRNA levels are increased. These findings are reversed with GH replacement. These results suggest a physiological role for GH in modulating IGF-I and type I IGF receptor genes in the ovary.