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Endocrinology Laboratory, Department of Zoology, University of Kalyani, Kalyani 741235, West Bengal, India¹ Molecular Endocrinology Laboratory, Indian Institute of Chemical Biology, 4, Raja SC Mullick Road, Kolkata 700032, India

(Correspondence should be addressed to D Mukherjee; Email: dilipmukher{at}rediffmail.com)

The effects of salmon calcitonin (sCT) on the secretion of 17β-estradiol (E₂) were examined in female common carp, Cyprinus carpio. Vitellogenic stage fish adapted to high-Ca water were i.p. injected with vehicle, sCT, human chorionic gonadotropin (hCG), or hCG plus sCT. To determine whether ovarian follicles are equipped with CT receptors, a CT binding assay was conducted. In the in vitro experiments, vitellogenic follicles were incubated with stimulators and inhibitors. Administration of sCT increased the basal and hCG-stimulated E₂ release in vivo and in vitro. Binding characteristics of [¹²⁵I]sCT to plasma membrane preparation of carp ovarian follicles showed saturability with high-affinity (K_d=48.48 pmol/l and B_max=1.2 pmol/mg protein). To clarify the mechanism of E₂ production by sCT, in vitro effect of sCT and hCG on aromatase activity (conversion of testosterone to E₂) and cytochrome P450 aromatase (P450arom) gene expression in carp ovarian follicles were investigated. Salmon CT-stimulated both aromatase activity and P450arom gene expression in ovarian follicles of carp. sCT-stimulated E₂ release by the ovarian follicles in vitro was augmented in the presence of dibutyryl cAMP. Inhibitor of protein kinase A (PKA), SQ 22536 inhibited sCT-stimulated steroid production in a dose-dependent manner. Specific inhibitor of protein kinase C (PKC), NPC-15437 dihydrochloride had no inhibitory effects on sCT-induced E₂ release. The present study indicates that sCT binds specifically to carp ovary and stimulates E₂ production by increasing the activity of cytochrome P450 aromatase and P450arom gene expression. The results further suggest that stimulatory action of sCT on E₂ production is mediated through cAMP pathway.