Effects of exercise or oocyte heat shock on embryo development and gene expression in the horse

Abstract

Horse owners commonly maintain their broodmares in training and competition during the breeding season. The effect this has on mare reproductive efficiency has received limited attention. Heat stress has shown to be detrimental to oocyte competence in other species and heat shock protein 70 has been shown to be an important gene in regulating cellular response to heat. Mares were exercised in a hot humid environment to determine the effects on reproductive efficiency. Embryos were collected at d 7 after ovulation from exercised and control mares. Oocyte developmental competence was measured after oocytes were subjected to a one time heat shock, 42 ºC for 2 or 4 h, at the onset or near completion of in vitro maturation. Embryos from both previous experiments were examined for HSP70 gene expression by real time RT-PCR. Exercised mares ovulated significantly smaller follicles, 39.8 vs. 41.5 mm diameter, and ovulated later after being given PGF2α, 8.5 vs. 9.2 d. Twenty-two embryos (22/35) were recovered from control mares, recovery rate of 63%. Significantly fewer embryos were recovered in exercised mares (11/32), recovery rate of 34%. A lower proportion of grade 1 embryos were recovered from exercised versus control mares (4/11 vs.16/22,respectively). No effect was observed on oocyte nuclear maturation or embryonic development after ICSI when oocytes were exposed to heat shock at the onset of IVM. A heat shock of 42 ºC for 2 or 4 h on oocytes during late IVM resulted, however, in a significantly lower rate of nuclear maturation, and a significant decrease in advanced embryo development (morulae plus blastocysts). Heat shock protein 70 gene expression was shown to be related to quality score of in vivo-recovered embryos, with lower quality embryos recording a significantly higher relative expression. Heat shock of late stage IVM oocytes for 4 h resulted in significantly higher blastocyst HSP70 expression. Results of this study indicate that exercise in a hot humid environment is detrimental to mare reproductive efficiency, late-stage maturing oocytes are sensitive to heat, and HSP70 expression in equine embryos is related to embryo quality score and oocyte quality.