Expression of mRNAs for insulin-like growth factor binding proteins-2, -3 and -4 in the ovine ovary throughout the oestrous cycle.

Abstract

Ovarian localization of the IGFs and their binding proteins is species-specific. In the ewe we have demonstrated previously that IGF-II mRNA is found at high concentrations in the theca, whereas IGF-I mRNA is undetectable in ovine follicles at any stage of development, although both are present in, the corpus luteum. In this study we have investigated the localization of mRNA for IGF-binding proteins (IGFBPs)-2, -3 and -4 using in situ hybridization to further elucidate the roles of the IGFs and their possible modulation during follicular development. Results were quantified by optical density (OD) measurements of autoradiographs using image analysis. There was intense follicular expression of mRNAs for both IGFBP-2 and -4 although IGFBP-3 mRNA was undetectable. Concentrations of IGFBP-2 mRNA varied significantly (P < 0.01) with respect to size, being highest in small (< 2 mm) follicles but with no variation attributable to health, whereas expression of IGFBP-4 mRNA was significantly (P < 0.01) influenced by health and not size, being higher in healthy than atretic follicles. IGFBP-2, -3 and -4 mRNAs were all present at low concentrations in the corpus luteum (OD 0.04 +/- 0.005, n = 6; 0.05 +/- 0.02, n = 5; 0.1 +/- 0.02, n = 8 respectively), whereas only IGFBP-2 and -4 mRNAs were found in ovarian stroma (OD 0.04 +/- 0.006, n = 4; 0.04 +/- 0.007, n = 9 respectively). These data suggest that changes in follicular distribution of IGFBPs-2 and -4 may modulate the actions of the IGFs during follicular growth thus contributing to the process of follicle selection.