Interactions between oxygen tension and glucose concentration that modulate actions of heat shock on bovine oocytes during in vitro maturation

Abstract

Exposure of oocytes to elevated temperature (i.e. heat shock) during maturation can reduce fertilization rate and development of the resultant embryos. Given the possible role of free radicals in actions of heat shock on cellular function, we tested the hypothesis that a high oxygen environment exacerbates the magnitude of deleterious effects of heat shock on in vitro maturation of bovine oocytes. A preliminary experiment was performed to establish conditions for oocyte maturation that would be independent of oxygen concentration. Oocytes were matured in a modified tissue culture medium-199 (mTCM-199) or modified synthetic oviduct fluid (mSOF) containing 5.6 or 20 mM glucose and under either high (atmospheric oxygen, ∼21%, v/v) or low oxygen (5%, a value approximating oxygen content of the follicle). For oocytes matured in mTCM-199, development was greater in high oxygen than in low oxygen, whereas development was unaffected by oxygen using mSOF (medium × oxygen, P < 0.05). Accordingly, mSOF was used as the maturation medium in a second study to test the effect of oxygen concentration on the magnitude of actions of heat shock during maturation. Maturation was at 38.5 °C for 22 h (control) or 41 °C for 12 h and 38.5 °C for 10 h (heat shock). Heat shock slightly decreased cleavage rate, regardless of the maturation conditions, and decreased blastocyst development under all maturation conditions except for the group matured under high oxygen and high glucose (temperature × glucose for oocytes under low oxygen, P < 0.05). The percentage of oocytes becoming blastocysts for control and heat shocked oocytes was 25.9% versus 22.5% (low oxygen—5.6 mM glucose), 41.6% versus 34.9% (low—20 mM), 41.7% versus 35.0% (high—5.6 mM), and 37.6% versus 37.5% (high—20 mM). In conclusion, under an oxygen tension that approached physiological conditions, heat shock during in vitro maturation reduced oocyte competence for fertilization and subsequent development.