Estrus cycle-dependent co-variation of insulin-like growth factor-I (IGF-I) messenger ribonucleic acid and protein in the rat ovary

It has been suggested that insulin-like growth factor-I (IGF-I) exerts paracrine or autocrine actions in the ovary and may play a role in the regulation of ovarian function. We have examined ovarian levels of IGF-I mRNA and IGF-I protein throughout the estrus cycle. The lowest levels of IGF-I mRNA were found on proestrus (12.00 h). The mRNA levels on estrus (12.00 h) were significantly (P less than 0.05) higher than on proestrus. The correlation between the levels of IGF-I and IGF-I mRNA was linear and significant (r = 0.706; P less than 0.01). Our observations that the IGF-I gene expression and translation vary during the estrus cycle, with an increase between proestrus and estrus, suggest that the gonadotropin surge could be of importance for the regulation of IGF-I in vivo and that IGF-I may be involved in the cell proliferation and differentiation caused by these hormones.