Expression of the mRNAs for the insulin-like growth factors and their binding proteins during development of porcine ovarian follicles.

We examined the expression of the mRNAs for the insulin-like growth factors (IGFs) and two of their binding proteins (BPs), IGFBP-2 and IGFBP-3, in individual follicles of the porcine ovary. Follicular development was synchronized with a progestin (altrenogest). Individual follicles were isolated on days 1, 3, 5 and 7 after progestin withdrawal. No IGFBP-3 mRNA was detected. While IGF-II mRNA was easily detected, the levels of expression did not change. IGF-I and IGFBP-2 mRNAs increased and decreased, respectively, with follicle development until day 7 when IGF-I expression declined. Regression analysis of IGF-I and IGFBP-2 mRNA expression was performed to assess the relative strength of correlations with day, diameter and steroid concentrations as covariates. IGFBP-2 mRNA was correlated with both day and diameter (r = -.713 and -.705, respectively, n = 24) and neither estrogen (E2) nor progesterone (P4) contributed to the fit. While IGF-I mRNA expression was correlated to both day (r = .483) and diameter (r = .587), the strongest predictor was E2 concentration (r = .694, n = 27). In conclusion, the expression of IGF-I and IGFBP-2 mRNAs in the ovarian follicle are discordantly regulated during follicular growth and maturation. The observed changes in these parameters should result in increased bioavailable IGF-I. This supports a pivotal autocrine/paracrine role for these factors during follicle growth and development.