Dimethylformamide improves the in vitro characteristics of thawed stallion spermatozoa reducing sublethal damage

A total of 42 ejaculates were used in the experiment; six ejaculates per stallion, obtained from seven Pure Spanish stallions (PRE), were split and frozen in freezing media with different concentrations and combinations of cryoprotectant (CPA): (i) Cáceres (skim milk based extender) containing 2.5% glycerol (2.5GL), (ii) Cáceres containing 1.5% glycerol and 1.5% dimethylformamide (1.5%GL-1.5%DMFA), (iii) Cáceres extender supplemented with 1.5% glycerol and 2.5% dimethylformamide (1.5%GL-2.5%DMFA) and (iv) Cáceres extender supplemented with 4% dimethylformamide (4%DMFA). After at least 4 weeks of storage in liquid nitrogen (LN), straws were thawed and semen analysed by computer-assisted sperm analysis and flow cytometry (membrane lipid architecture (Merocyanine 540), integrity and sublethal damage (YoPro-1) and mitochondrial membrane potential (JC-1)). After thawing, better results were observed in samples frozen in 4%DMFA or in combinations of 1.5%GL-2.5%DMFA, in fact total motility increased by 16% in the 4%DMFA group compared to 2.5%GL (P < 0.05). Also, there was an increment in the percentage of progressive motile sperm in the 1.5%GL-2.5%DMFA group (9.8% 2.5GL vs 19% in the 1.5%GL-2.5%DMFA group p < 0.05); also, samples frozen in the 4%DMFA group had more intact (YoPro-1 negative) sperm post-thawing, 29.3% in 2.5%GL vs 36.7% in 4%DMFA group (p < 0.05). Membrane lipid architecture was not affected by any of the cryoprotectants tested, while samples frozen in 4%DFMA had a lower percentage of mitochondria with lower membrane potential. It is concluded that DMFA improves the outcome of cryopreservation of stallion spermatozoa mainly reducing sublethal cryodamage.