Recovery of mitochondrial function and endogenous antioxidant systems in vitrified bovine oocytes during extended in vitro culture

Abstract

This study was designed to examine the recovery of mitochondrial function and endogenous antioxidant systems in vitrified oocytes during extended incubations. After 16 hr of in vitro maturation, bovine meiosis-II oocytes were vitrified, and then surviving oocytes were cultured an additional 8 hr. ATP content, ATP synthase activity, expression of ATP synthase F0 subunit 6 (ATP6) and 8 (ATP8) genes, and reactive oxygen species (ROS) levels were investigated in the vitrified oocytes during this additional period (4 or 8 hr). The results showed that: (1) the ATP content and ATP synthase activities in vitrified oocytes at 8 hr post-warming (754.6 fmol, 25.9 nmol NADH/min/mg) were significantly higher than in oocytes immediately warmed (568.3 fmol, 8.7 nmol NADH/min/mg), but still lower than in control oocytes (901.5 fmol, 30.7 nmol NADH/min/mg); (2) the relative expression of ATP6 and ATP8 was initially down-regulated in oocytes when they were first warmed, increased by 4 hr post-warming, and were again down-regulated by 8 hr post-warming; (3) ROS levels in oocytes at 0, 4, and 8 hr post-warming were significantly higher than in control oocytes; and (4) after parthenogenetic activation, the blastocyst rate of oocytes at 8 hr post-warming (26.7%) was significantly higher than that of oocytes immediately warmed (16.9%). These results indicated that mitochondrial function and endogenous antioxidant systems recovered significantly better in vitrified-thawed bovine oocytes with 8 hr of additional incubation, but they did not achieve the activity levels found in fresh oocytes.