Impact of Heat Stress on Germinal Vesicle Breakdown and Lipolytic Changes during In Vitro Maturation of Bovine Oocytes

The main objective of this research was to examine the lipolytic changes in triglyceride and phospholipid as well as the incidence of germinal vesicle breakdown during IVM of heat-stressed oocytes compared to non-stressed oocytes. To this end, cumulus-oocyte complexes were matured for 0, 2, 4, 6 or 24 hIVM at 38.5 or 41.0°C (first 12 h only, then transferred to 38.5°C). Triglyceride and phospholipid levels decreased by 2 hIVM (P = 0.0009 and P = 0.0005, respectively) but remained fairly constant to 24 hIVM; lipid decline was not affected by maturation temperature. Elevated maturation temperature hastened meiotic progression by 4 hIVM (P < 0.0001). Incidence of germinal vesicle breakdown was associated, though not directly related, to lipolytic changes in oocyte triglyceride and phospholipid content (R² [R-squared] = 0.2123 and P = 0.0030; R² = 0.2243 and P = 0.0026, respectively). Oocyte ATP content was measured as an indirect indicator of lipolysis (i.e., mitochondrial fatty acid β-oxidation [beta-oxidation] of fatty acids freed during lipolysis of triglyceride/phospholipid for ATP production). The ATP content of oocytes increased during IVM and was greater in heat-stressed oocytes at 24 hIVM compared to controls (P = 0.0082). Levels of ATP were associated, though not directly related, to changes in oocyte triglyceride and phospholipid content (R² = 0.1086 and P = 0.0184; R² = 0.1252 and P = 0.0096, respectively). In summary, heat stress-induced hastening of oocyte germinal vesicle breakdown was not directly explained by lipolytic changes in triglycerides and phospholipids nor the oocyte ATP content.