Dynamics of insulin and insulin-like growth factor-I (IGF-I) ovarian receptors during maturation in the brown trout (Salmo trutta)

In salmonid fishes, the role of insulin and insulin-like growth factor-I (I GF-I) in the regulation of ovarian function is not well understood. Recentl y, we reported that isolated follicular layers of the preovulatory ovarian follicle of coho salmon (Oncorhynchus kisutch) have specific receptors for insulin and IGF-I and that IGF-I modulates steroid production in the follic ular layers. In the present study we have investigated the structural and f unctional characteristics of insulin and IGF-I receptors in the ovary of br own trout (Salmo trutta) and the changes in insulin and IGF-I binding throu ghout the reproductive cycle of this species. The specific binding for IGF- I was 8- to 15-fold higher than the specific binding for insulin. IGF-I rec eptors were also more specific than insulin receptors because unlabeled ins ulin displaced bound radiolabeled insulin at concentrations 40- to 80-fold lower than unlabeled IGF-I; whereas, unlabeled IGF-I displaced bound radiol abeled IGF-I at concentrations 4000- to 8000-fold lower than unlabeled insu lin. Insulin and IGF-I receptors from the brown trout ovary were composed o f 120 kDa alpha-subunits and 90 kDa beta-subunits, which underwent autophos phorylation in a concentration-dependent manner. Receptor tyrosine kinase a ctivity was also stimulated in a concentration-dependent manner by insulin and IGF-I. When ovarian insulin and IGF-I binding was determined from mid-v itellogenesis (March) until ovulation (November), maximal binding for both peptides was detected in mid-vitellogenesis and gradually decreased until t he end of vitellogenesis (August). In the preovulatory period (October), a small increase of insulin and IGF-I binding was observed. After ovulation, insulin binding was no longer detectable and IGF-I binding was very low. Th ese results suggest that insulin and IGF-I receptors in the salmonid ovary follow the structural pattern described in other vertebrate species and tha t insulin and IGF-I could be involved in the regulation of ovarian function during reproductive stages other than the preovulatory period.