193 INSULIN-LIKE GROWTH FACTOR-1 EXERTS A THERMOPROTECTIVE ROLE ON MITOCHONDRIAL FUNCTION OF BOVINE OOCYTES EXPOSED TO HEAT SHOCK

Abstract

The series of events associated with oocyte maturation are susceptible to disruption by elevated temperature. These events are regulated by a variety of growth factors, such as insulin-like growth factor-1 (IGF-1). Exposure of bovine oocytes to heat shock compromises oocyte competence and triggers apoptosis. It has been shown that cellular stresses often alter mitochondrial function and activate the mitochondrial apoptotic cascade. Therefore, the objective of this study was to determine the effect of heat shock on bovine oocyte mitochondrial activity and the role of IGF-1 in this context. Slaughterhouse derived cumulus–oocyte complexes (COC) were subjected to control (38.5°C for 22 h) and heat shock (41°C for 14 h, followed by 38.5°C for 8 h) treatments in the presence of 0 or 100 ng mL^–1 of IGF-1 during in vitro maturation (IVM). After 22 h, IVM COC were mechanically denuded and subjected to MitoTracker Red CMX-Ros assay (Invitrogen M-7512) to localize and quantify active mitochondria. Denuded oocytes were incubated in TCM-199-HEPES containing 10 μg mL^–1 of polyvinyl alcohol and 50 nM MitoTracker at 37°C for 15 min. Oocytes were evaluated under fluorescence microscope and digital images were obtained and stored as TIFF files. Mitochondrial activity from each oocyte was quantified using the software Image J 1.43. This experiment was replicated 6 times using 97 to 204 COC/treatment. Data were analyzed by least-squares analysis of variance using the general linear model procedure of SAS. In the absence of IGF-1, heat shock reduced (P < 0.001) mitochondrial activity from 64.31 ± 1.91 to 56.74 ± 1.26 arbitrary units for control and heat shock groups, respectively. Addition of IGF-1 to maturation medium did not affect mitochondrial activity in the control group (66.25 ± 1.56). However, IGF-1 improved (temperature × IGF-1; P < 0.001) mitochondrial activity of bovine oocytes subjected to heat shock (70.32 ± 1.32). In conclusion, heat shock reduced bovine oocyte mitochondrial activity, suggesting activation of mitochondrial apoptotic cascade. Moreover, IGF-1 exerted a thermoprotective role, reducing the mitochondrial damage caused by elevated temperature.