Cryopreservation is a stressful process for sperm as it is associated with an increased production of reactive oxygen species (ROS). Elevated ROS levels, which create an imbalance with antioxidant capacity, may result in membrane lipid peroxidation (LPO), protein damage and DNA fragmentation. This study aimed to determine whether membrane LPO and DNA fragmentation of frozen-thawed horse sperm relies upon the antioxidant activity, including enzymes (superoxide dismutase (SOD), glutathione peroxidase (GPX), catalase (CAT), and paraoxonase type 1 (PON1)); non-enzymatic antioxidant capacity (Trolox equivalent antioxidant capacity (TEAC), plasma ferric reducing antioxidant capacity (FRAP), and cupric reducing antioxidant capacity (CUPRAC)); and the oxidative stress index (OSI), of their seminal plasma (SP). Based on total motility and plasma membrane integrity (SYBR14+/PI−) after thawing, ejaculates were hierarchically (p < 0.001) clustered into two groups of good (GFE) and poor (PFE) freezability. LPO and DNA fragmentation (global DNA breaks) were higher (p < 0.05) in PFE than in GFE, with LPO and DNA fragmentation (global DNA breaks) after thawing showing a positive relationship (p < 0.05) with SP-OSI levels and ROS production. In addition, sperm motility and membrane integrity after thawing were negatively (p < 0.05) correlated with the activity levels of SP antioxidants (PON1 and TEAC). The present results indicate that LPO and DNA fragmentation in frozen-thawed horse sperm vary between ejaculates. These differences could result from variations in the activity of antioxidants (PON1 and TEAC) and the balance between the oxidant and antioxidant components present in the SP.

ProAKAP4, a precursor of AKAP4 (A-kinase anchor protein) found in the flagellum of mammalian and non-mammalian spermatozoa, serves as a structural protein with established correlations to motility parameters across diverse species. This study aimed to determine the proAKAP4 levels evolution in thawed stallion semen over a 3-hour period, examining its correlation with motility descriptors and mitochondrial membrane potential. Utilizing sixteen ejaculates from four French warmblood stallions, the study involved maintaining thawed samples at 37°C for 3 hours, conducting proAKAP4 enzyme-linked immunosorbent assay (ELISA), computer-assisted sperm analysis (CASA), and mitochondrial membrane potential by JC-1 probe and flow cytometry at 0, 1, and 3 hours post-thawing. The findings indicate significant positive correlations (p≤0.05) between proAKAP4 levels and sperm total or progressive motility at all time points analyzed. Spermatozoa velocity descriptors (VAP, VCL, VSL) and spermatozoa lateral head displacement (ALH) display positive correlations (p≤0.05) with ProAKAP4 at the 0-hour post-thawing. ProAKAP4 concentration exhibits no discernible difference between batches with or without cryoprotectant. Notably, proAKAP4 consumption remains insignificant within the initial hour after thawing but becomes significant (p≤0.05) between 1 and 3 hours post-thawing. In summary, proAKAP4 demonstrates positive correlations with total and progressive motility in stallion semen for up to 3 hours after thawing, albeit showing a noticeable decrease starting from the first hour post-thawing, indicating a progressive consumption as a result of spermatozoa motile activity.