Stimulation of Brook Trout Ovarian Steroidogenesis by Gonadotropins I and II Is Mediated by the Cyclic Adenosine 3',5'-Monophosphate/Protein Kinase A Pathway

Abstract

In teleosts, ovarian steroidogenesis is under the control of two gonadotropic hormones, GTH I and GTH II, that are structurally and functionally homologous to FSH and LH. The intracellular mechanisms by which GTH I and GTH II stimulate steroidogenesis in the teleost ovary are not well understood. The purpose of this study was to investigate the involvement of the _cAMP/protein kinase A (PKA) and protein kinase C (PKC)/Ca ²⁺ signaling pathways in the steroidogenic actions of GTH I and GTH II in the ovary of the brook trout (Salvelinus fontinalis). The _cAMP/PKA pathway mediated the actions of GTH I before germinal vesicle breakdown (preGVBD) and GTH II after GVBD and before ovulation (preOV). Experimental increases in intracellular _cAMP concentration mimicked the steroidogenic effects of GTH I and GTH II, and an antagonistic analog of _cAMP partially blocked them. In addition, GTH I and GTH II stimulated the production of _cAMP in preGVBD and preOV follicles, respectively. Activation of the PKC/Ca²⁺ pathway by a phorbol ester or a Ca²⁺ ionophore blocked the GTH I- and GTH I-induced steroid production, whereas inhibition of PKC by specific inhibitors potentiated the effects of GTH I. These results suggest that the _cAMP/PKA signaling pathway mediates the stimulatory effects of GTH I and GTH II on steroidogenesis, and they also suggest the additional involvement of the PKC/Ca²⁺ signaling pathway in modulating the actions of gonadotropins in brook trout ovarian follicles.