This study was carried out to reveal factors and the mechanism of action by which low‐density lipoproteins (LDLs) protect sperm better than egg yolk (EY) during cryopreservation. We extracted LDL from EY and compared the amount of calcium, progesterone, and antioxidants in EY and LDL. We found a very high concentration of progesterone (1423.95 vs. 10.46 ng/ml) and calcium (29.19 vs. 0.47 mM) in EY as compared with LDL. Antioxidant assays like DPPH (2,2‐diphenyl‐1‐picrylhydrazyl) and the ferric reducing antioxidants power assay revealed that the LDL extender had almost double ability to lose hydrogen than the EY extender. For sperm cryopreservation, 20 ejaculates from four Murrah buffalo bulls were collected. Each ejaculate was divided into four aliquots and extended in 10%, 12%, and 14% LDL (w/v) and EY‐based extenders, followed by cryopreservation. The LDL‐based extender prevented excessive cholesterol efflux, and its high content of antioxidants minimized reactive oxygen species generated during cryopreservation, resulting in a functional CatSper channel. The EY‐based extender promoted excess cholesterol efflux due to the presence of high‐density lipoprotein, resulting in a compromised CatSper channel. High intracellular calcium in a cryopreserved sperm in the EY group as compared with the LDL group indicates that progesterone present in EY activates the CatSper channel, resulting in a heavy calcium influx into the sperm. The greater tyrosine phosphorylation and increased number of F‐pattern in the sperm cryopreserved in the EY extender indicate that high intracellular calcium triggers more capacitation‐like changes in the sperm cryopreserved in EY than LDL extender. In conclusion, we demonstrated the new facts and understandings about LDL and EY for semen cryopreservation.

中文翻译：

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低密度脂蛋白在水牛冷冻保存过程中保护精子：解开作用机制

本研究旨在揭示低温保存过程中低密度脂蛋白 (LDL) 比蛋黄 (EY) 更好地保护精子的因素和作用机制。我们从 EY 中提取了 LDL，并比较了 EY 和 LDL 中钙、黄体酮和抗氧化剂的含量。与 LDL 相比，我们发现 EY 中孕酮（1423.95 与 10.46 ng/ml）和钙（29.19 与 0.47 mM）的浓度非常高。DPPH (2,2-diphenyl-1-picrylhydrazyl) 等抗氧化试验和铁还原抗氧化剂功效试验表明，LDL 补充剂的失氢能力几乎是 EY 补充剂的两倍。对于精子冷冻保存，收集了来自四头墨拉水牛公牛的 20 份精液。每个射精被分成四等份，并在 10%、12% 和 14% 的 LDL (w/v) 和基于 EY 的稀释剂中进行扩展，其次是冷冻保存。基于 LDL 的稀释剂可防止过多的胆固醇外流，其高含量的抗氧化剂最大限度地减少了冷冻保存过程中产生的活性氧，从而形成了一个功能性的 CatSper 通道。由于高密度脂蛋白的存在，基于 EY 的增量剂促进了过量的胆固醇流出，导致 CatSper 通道受损。与低密度脂蛋白组相比，EY 组冷冻保存精子中的高细胞内钙表明 EY 中存在的孕酮激活 CatSper 通道，导致大量钙流入精子。在 EY 扩展剂中冷冻保存的精子中酪氨酸磷酸化程度更高和 F 模式数量增加表明，与 LDL 扩展剂相比，高细胞内钙触发了 EY 中冷冻保存的精子更多的类似获能变化。总之，我们展示了关于 LDL 和 EY 用于精液冷冻保存的新事实和新认识。