112 Effect of electrical field during thawing of in vitro-produced bovine embryos cryopreserved for direct transfer

At present, in vitro production (IVP) of embryos and embryo transfer (ET) have had a great impact on animal production. The transfer of cryopreserved embryos is important for commercialization and for dissemination of valuable genetics. It is known that an electrical field generates changes on the cell surface, which affects the functions of membrane proteins, enzymatic activity, membrane receptors, and ion-transporting channels (Zhu et al. 2019 Exp. Neurol. 319, 112963). Normally, when an ET is conducted, the practitioners use an electric thawing unit; however, this action could affect the embryo. The objective of our research was to evaluate the effect of an electrical field during thawing on the pregnancy rates of IVP embryos frozen in ethylene glycol. The research was carried out in the Genetic Resources National Center. The IVP was performed with a continuous in vitro culture system based on IVF-Bioscience media, using oocytes collected by ovum pickup and fertilized with the same bull (proven for IVP). On Day 7 of in vitro culture, 100 expanded blastocyst stage embryos grade 1 (IETS Manual 4th ed.) were subjected to a controlled-rate freezing curve after equilibration for 8–10 min in freezing medium with Ethylene Glycol Freeze Plus ViGRO (Vetoquinol USA), starting at −6°C (seeding), decreasing 0.5°C min⁻¹, and ending at −32°C. The embryos were then plunged directly into liquid nitrogen. For thawing, frozen embryos were divided in two groups: the control group (CG), which included 50 embryos thawing in an electrical thawing unit, and the experimental group (EG), which included 50 embryos thawing in the same unit disconnected from an electrical source. Both groups were thawed 10 s in air and then 20 s in water at 30°C and transferred to the recipients. For ET, 120 crossbred cows with a body score condition of 3 (on a scale of 1–5) were used as recipients and synchronized with an intravaginal progesterone device (CIDR, 1.9 g) plus 2 mg of estradiol benzoate on Day 0. On Day 7, the CIDR was removed and a dose of 400 IU of eCG, 1 mg of estradiol cypionate, and 500 μg of cloprostenol sodium were administered. On Day, 16 ET was conducted only in recipients that had exhibited estrus and had a corpus luteum at ET. Pregnancy rates were evaluated by ultrasonography on Day 40. Statistical analysis was carried out using the Chi-squared procedure on the Jamovi software (version 1.2; The Jamovi Project). As shown in Table 1, the pregnancy rates were 30% and 44% for CG and EG, respectively. There were no significant differences between the groups (P = 0.12), although there was a numerical difference (14%). Perhaps cell proliferation was affected similarly to another study involving cancer cells subjected to an electrical field of 150 mV mm⁻¹ (Chen et al. 2021 Electromagnetic Biol. Med. 40:1, 49–64). In conclusion, under the conditions of this research, the electrical field during thawing of cryopreserved IVP bovine embryos did not have a significant effect on pregnancy rates after ET; however, a larger number of ETs should be evaluated in future research.