OVARIAN RECEPTORS FOR INSULIN AND INSULIN-LIKE-GROWTH-FACTOR-I (IGF-I) AND EFFECTS OF IGF-I ON STEROID-PRODUCTION BY ISOLATED FOLLICULAR LAYERS OF THE PREOVULATORY COHO SALMON OVARIAN FOLLICLE

In this study, receptors for insulin and insulin-like growth factor I (IGF-I) in isolated theca-interstitial layers and granulosa cells of t he coho salmon preovulatory ovary were characterized, and the effects of IGF-I on ovarian steroidogenesis were examined. Specific receptors for insulin and IGF-I were found in granulosa and theca-interstitial l ayers. In both follicular layers, IGF-I receptors were greater in numb er and higher in affinity than insulin receptors. The effects of IGF-I on in vitro production of testosterone (T) and 17 alpha-hydroxyproges terone (17OH-P) by theca-interstitial layers and of 17 beta-estradiol (E-2) and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) by granulosa cell layers were evaluated during the preovulatory period. Both human and salmon recombinant IGF-I inhibited the basal and GTH II -stimulated T and 17OH-P production by theca-interstitial layers throu ghout the preovulatory period. In contrast, IGF-I stimulated the produ ction of both E-2 and 17,20 beta-P by granulosa cell layers prior to g erminal vesicle breakdown (GVBD) but only stimulated the production of 17,20 beta-P by granulosa cell layers after GVBD. The inhibitory effe cts of IGF-I on steroid production by the theca-interstitial layer and the opposite stimulatory effects on steroid production by the granulo sa cell layer, coupled by the presence of specific IGF-I receptors in both follicular layers, suggest that IGF-I may play a role in the regu lation of steroidogenesis in the preovulatory coho salmon ovary. (C) 1 997 Academic Press.