The role of mitochondrial transition pores on bovine oocyte competence after heat stress, as determined by effects of cyclosporin A.

This study was undertaken to investigate the effects on the nuclear maturation and subsequent embryonic development of bovine oocytes exposed to heat stress (HS) when treating bovine oocytes before in vitro maturation (IVM) with 1 µM cyclosporin A (CsA), an inhibitor of mitochondrial permeability transition pore opening. Mitochondrial activity, reactive oxygen species (ROS), and apoptosis levels of the oocytes were also assessed. Nuclear maturation rates of both the HS-exposed oocytes treated with or without CsA groups (HS + CsA or HS group) were significantly lower (P<0.05) than that of the control group, while the rate of the HS + CsA group was significantly higher (P<0.05) than that of the HS group. Furthermore, although the cleavage and blastocyst formation rates of the HS group were significantly lower than those of the control groups (P<0.05), both rates of the HS + CsA group recovered to the same level as those of the control group. The HS group showed a significantly higher ROS level, lower mitochondrial activity in the oocytes, and TUNEL-positive cumulus cells, but not oocytes, compared with those of the control group (P<0.05), whereas the TUNEL-positive and mitochondrial activity levels of the HS + CsA group recovered to those of the control group. These results indicate that 1 µM CsA treatment before IVM may mitigate reduced mitochondrial activity, increase number of apoptotic cumulus cells under HS, and improve the nuclear maturation and developmental competence of bovine oocytes.