Vasotocin stimulates maturation-inducing hormone, oocyte maturation and ovulation in the catfish Heteropneustes fossilis: Evidence for a preferential calcium involvement.

Arginine vasotocin (VT) is the basic neurohypophysial nonapeptide hormone in teleosts. VT is also distributed in the ovary of the catfish Heteropneustes fossilis and induces final oocyte maturation (FOM) and ovulation by stimulating the maturation-inducing hormone (MIH). The present study reports the effects of cAMP (0.5 mM), phosphodiesterase inhibitors (IBMX -0.5 mM and theophylline- 0.5 mM), the inositol triphosphate (IP3) receptor inhibitor heparin (10 μg/mL) and the Ca2+ chelator BAPTA-AM (25 μM) on VT (100 nM) - induced progestin stimulation, FOM and ovulation. Incubation of post-vitellogenic follicles with cAMP, IBMX and theophylline for 0, 8, 16 and 24 h stimulated basal secretion of progesterone (P 4), 17-hydroxyprogesterone (17-P) and 17, 20β-dihydroxy-4-pregnen-3-one (MIH) in a duration-dependent manner. The incubation of the follicles with heparin stimulated P 4 modestly, and 17-P and MIH levels in a duration-dependent manner. The incubation of the follicles with BAPTA-AM stimulated P 4 and MIH levels marginally and 17-P robustly. The stimulation was in the order cAMP > IBMX > theophylline > heparin > BAPTA-AM. The incubation of the follicles with VT stimulated P 4 , 17-P, MIH, GVBD and ovulation in a duration-dependent manner. The co-incubations with VT and the test compounds inhibited the VT-induced stimulation of P 4 , 17-P and MIH levels in a time-dependent manner in the order heparin > BAPTA-AM > cAMP > IBMX > theophylline. Concurrently, the VT-induced stimulation of GVBD and ovulation were also inhibited by the test compounds in the same order. The results show that VT induces FOM and ovulation preferentially acting through Ca2+ pathway and a crosstalk between Ca2+ and cAMP signaling pathways seems to integrate the processes. • In catfish Vasotocin (VT) stimulated maturation-inducing hormone (MIH), GVBD and ovulation. • The VT effect on MIH was strongly inhibited by Ca++ blockers: heparin > BAPTA-AM. • The VT effect on GVBD and ovulation was inhibited by the Ca++ blockers, similar to MIH. • cAMP and PDE inhibitors elicited a minor role in MIH, GVBD and ovulation inhibition. [ABSTRACT FROM AUTHOR]