Comparative efficacy of antioxidant retinol, melatonin, and zinc during in vitro maturation of bovine oocytes under induced heat stress

Abstract

Both A and B grade cumulus-oocyte complexes (COCs) aspirated from cattle ovaries at slaughter were matured in vitro under normal (38.5 °C) and elevated temperatures (41 °C). Maturation competence based on cumulus expansion, COC diameter, and nuclear maturation were compared with and without antioxidant supplementation incorporating 7.5 μM retinol, 1 nM melatonin, and 1.5 μg/mL zinc chloride in an oocyte maturation medium. Heat stress significantly reduced cumulus expansion by approximately 20%, while only retinol could bring a significant (P ≤ 0.05) increase. Heat stress also decreased expansion of A (approximately 50%) and B (approximately 40%) grade COC diameter. All antioxidants significantly increased COC diameter at 38.5 °C in grade A COCs, but only retinol could significantly increase grade B COCs. At 41 °C, only retinol in grade A COCs significantly enhanced diameter. Elevated temperature also decreased the metaphase II stage of nuclear maturation by approximately 75%, and no antioxidant was protective, except retinol, which was only marginally so. Retinol (7.5 μM) was further supplemented in maturation and culture medium for in vitro embryonic development at 38.5 °C and demonstrated significantly higher (P ≤ 0.01) maturation, fertilization, and a 2-4 cell cleavage rate. Retinol supplementation not only showed better maturation results, but also was a better antioxidant in overcoming the deleterious effects of elevated temperature.