Effects of Vanadate on Meiotic Maturation of Porcine Oocytes in Vitro

Oocyte maturation is triggered by the activation of maturation promoting factor (MPF). In activation of MPF during meiotic maturation of porcine oocytes, a possible involvement of phosphorylation and subsequent dephosphorylation of a inhibitory tyrosine residue in p34cdc2 was examined using vanadate, a potent protein tyrosine phosphatase inhibitor. Porcine oocytes were cultured in porcine follicular fluid supplemented with pregnant mare's serum gonadotrophin with or without vanadate for 48 h. Vanadate blocked germinal vesicle breakdown (GVBD) almost completely at the concentration of 250 and 500 μM. The addition of an effective dose of vanadate into the maturation medium at 24 or 30 h after the start of maturation culture revealed that the porcine oocytes before GVBD were sensitive to vanadate and had their GVBD blocked, but those that had undergone GVBD were not-sensitive to vanadate and reached to the second metaphase normally. When vanadate was removed after 24 h treatment, GVBD occurred in 64% of oocytes but most of them were arrested at the first metaphase with dispersed chromosomes and abnormal spindles. These results suggest that tyrosine dephosphorylation in p34cdc2 takes place during the first meiosis but not in the second meiosis and that a prolonged exposure to vanadate might deteriorate microtubules function of the oocytes.