Localization, characterization, and quantification of insulin-like growth factor-I-binding sites in the ewe ovary

To assess a potential role of insulin-like growth factor-I (IGF-I) in the ewe ovary, the presence of IGF-I receptors and IGF-I-binding proteins was studied by binding assays performed on granulosa cell suspensions, in follicular fluid, and on ovarian sections. On the ovarian sections, labeling was quantified after autoradiography by microphotometry. Competition studies with IGF-I and insulin allowed us to estimate the relative proportions of binding proteins and type I receptors in the different compartments of the ewe ovary. Our results clearly show that saturable, specific, and high affinity IGF-I receptors are present on the ovine granulosa cells. At equilibrium for both granulosa cell suspensions and frozen sections, the Kd value was close to 2 nM. IGF-I binding proteins were also present in follicular fluid and stroma, thecal, and granulosa cells. At equilibrium for follicular fluid, the Kd value was 0.91 +/- 0.27 nM (mean +/- SE). Moreover, on frozen sections, it was shown that atresia of small follicles (less than 2 mm) was accompanied by a decrease in the number of IGF-I receptors and an increase in the number of IGF-I-binding proteins on granulosa cells. By contrast, this phenomenon was not observed in large follicles. These data indicate that granulosa cells of ewe ovary possess type I receptors, and IGF-I-binding proteins may modulate IGF-I action in the process of follicular growth and atresia.