Orthovanadate and fluoroaluminate stimulate inositol phosphate production and in vitro ovulation in goldfish (Carassius auratus) follicles

Both sodium orthovanadate and fluoroaluminate were found to stimulate in vitro ovulation in intact goldfish follicles, suggesting the involvement of G-proteins in ovulation. Although orthovanadate was able to stimulate cAMP production, it probably stimulates ovulation by some other mechanism since cAMP blocks ovulation in this species. These agents also stimulated the accumulation of labeled inositol phosphate in follicle walls. The time course of inositol phosphate production showed a slightly delayed and continuous accumulation for isomers of inositol mono-, bis-, and trisphosphates. No change was observed in inositol tetrakisphosphate levels over time. The accumulation of inositol phosphates in response to orthovanadate was also dose-dependent. Lithium chloride (10 mM) caused varying increases in the levels of most isomers and a decrease in ins-3,4-P2. Inositol phosphate production varied significantly with changes in the maturational stage of follicles. Peak production was observed in follicles 7-8 h after hCG treatment, which corresponds almost exactly with the time of ovulation. This correlation of maximal inositol phosphate production with the time of ovulation, along with the stimulation of ovulation by diacylglycerols, a phorbol ester, and the G-protein-stimulating agents, orthovanadate and fluoroaluminate, suggests a role for polyphosphatidylinositol hydrolysis in ovulation.