Effect of insulin-like growth factor-I on growth hormone-releasing factor receptor expression in primary rat anterior pituitary cell culture.

We examined the effect of insulin-like growth factor-I (IGF-I) on GH-releasing factor (GRF) receptor expression in the primary rat anterior pituitary cell culture. The levels of GRF receptor mRNA were dose-dependently reduced by IGF-I treatment for 24 h. To clarify whether altered levels of GRF receptor mRNA contribute to GRF receptor concentrations, we examined the GH response to GRF in vitro. There was no difference in basal GH secretion between control and IGF-I pretreated cells, while GRF-stimulated GH secretion in cells pretreated with IGF-I for 24 h was significantly lower than that in control cells. Moreover, specific [125I] Tyr10-human GRF binding to pituitary cells was reduced significantly by IGF-I treatment. These results suggest that IGF-I acts directly on the pituitary and participates in the regulation of GRF receptor expression.