The effect of exogenous growth hormone (GH) and insulin-like growth factor I (IGF-I) on brain IGF-I binding sites (IGF-IR), and on the levels of growth hormone-releasing hormone (GHRH) and somatostatin was studied in hypophysectomized and intact juvenile male rats. Animals were injected subcutaneously twice daily (n = 5 each) with recombinant GH (rGH) (2.5 U/kg per day) or rIGF-I (500 μg/kg per day). In the hypophysectomized rats, serum GH and IGF-I levels were markedly suppressed and IGF-I levels were partially restored by GH treatment. There was a significant increase in IGF-IR binding capacity in the IGF-I-treated hypophysectomized rats compared to the saline-treated hypophysectomized animals (150.61 ± 45.66 vs 41.32 ± 12.42 fmol/mg, p < 0.05) but no significant difference in IGF-IR mRNA levels. GHRH levels in the saline-treated hypophysectomized group were significantly lower than in the saline-treated intact rats (31.2 ± 11.2 vs 140.6 ± 48.1 pg/mg tissue, respectively, p < 0.01); no effect was induced by GH or IGF-I (37.5 ± 26.8 and 53.8 ± 22.5 pg/mg tissue, respectively). However, in the intact rats, GH and IGF-I injection led to a decrease in GHRH content, which was significant in the GH-treated compared to the saline-treated animals (33.1 ± 16.2 vs 140.6 ± 48.1 pg/mg tissue, p < 0.01). No difference was found in somatostatin levels between intact and hypophysectomized rats (631.2 ± 81.2 and 625.0 ± 62.5 pg/mg tissue, respectively). However, in the hypophysectomized animals, GH and IGF-I treatment induced a significant increase in somatostatin levels (1300 ± 193.7 pg/mg tissue, p < 0.01, and 912.5 ± 81.2 pg/mg tissue, p < 0.05, respectively). Our findings suggest that the bioavailability of exogenous IGF-I is greater than that of GH-stimulated endogenous IGF-I. Because IGF-I is a potent neurotrophic agent, this effect may have important implications for states of neurodegenerative diseases.
- Growth hormone
- Igf-I receptor