Impact of an Acute Heat Shock During In Vitro Maturation on Interleukin 6 and its Associated Receptor Component Transcripts In Bovine Cumulus-Oocyte Complexes

An acute heat stress event after LH surge increased intrafollicular interleukin 6 (IL6) levels in cows that became hyperthermic. To examine direct consequences of the physiologically-relevant elevated temperature of 41.0ºC on the cumulus-oocyte complex (COC), IL6 transcript abundance and related receptor components were evaluated throughout in vitro maturation. Because IL6 and LIF share a signal transducer, LIF was also examined. Abundance of IL6 was interactively influenced by temperature and time (P < 0.0001). Heat-induced increases in IL6 were first noted at 4 hIVM; peak levels occurred at 4.67 versus 6.44 hIVM for 41.0 and 38.5ºC COCs, respectively (SEM = 0.23; P < 0.001). Abundance of IL6R (P = 0.05) and IL6ST (P = 0.002) were also interactively influenced by temperature and hIVM. Peak IL6ST levels occurred at 6.95 versus 8.29hIVM for 41.0 and 38.5ºC, respectively (SEM = 0.23; P < 0.01). Transcript for LIF differed over time (P < 0.0001) but was not affected by 41.0ºC exposure. Blastocyst development after performing IVF was not affected by 41.0ºC exposure for 4 or 6 h. Progesterone released into maturation medium by COCs was interactively influenced by temperature and hIVM (P = 0.0116). When limiting analysis to when IL6 was temporally produced, progesterone levels were only impacted by time and temperature. Heat-induced shift in the temporal production of IL6 along with its impact on progesterone likely cooperate in heat-induced hastening of meiotic progression described by others.