21 Effect of egg yolk extracted low-density lipoprotein on cryopreserved Nguni bull semen

Abstract

Egg yolk has been reported to have a beneficial effect on sperm quality following cryopreservation, and this led to its widespread use in semen extenders. However, egg yolk contains substances that inhibit respiration of sperm cells and diminish their motility rate. Moreover, it also contains low-density lipoproteins (LDL) that have a protective effect on sperm during the cryopreservation process. The objective of this study was improve cryopreservation of Nguni bull semen using egg yolk low-density protein. A total of 25 ejaculates were collected from 5 Nguni bulls aged 4 to 5 years using an electroejaculator during the natural breeding season. Collected raw semen samples were transported to the laboratory and evaluated for sperm motility, viability, plasma membrane, acrosome, and DNA integrity before dilution. Semen was randomly diluted with a sodium citrate extender supplemented with 20% egg yolk (control) and with 6, 8, 10, and 12% LDL concentrations. The diluted semen sample groups were equilibrated for 4 h at 5°C. Following equilibration, semen was loaded into 0.25-mL straws and frozen in a controlled-rate programmable freezer. The groups of semen straws were then plunged into LN and transferred into LN tanks (−196°C) for storage. The frozen semen straws per treatment group were thawed at 37°C and evaluated for sperm motility, viability, plasma membrane, acrosome, and DNA integrity. Data were analysed with ANOVA using Stata V12 statistical software (StataCorp., College Station, TX), and treatment means were separated using Fisher’s protected t-test at the significant level of P < 0.05. Sperm motility and membrane integrity were significantly higher (P < 0.05) in frozen-thawed semen diluted with 8% LDL compared with the other concentrations. However, 6 and 8% LDL resulted in a significantly higher (P < 0.05) live sperm, DNA, and acrosome integrity. Frozen-thawed semen diluted with 10 and 12% LDL resulted in the lowest percentages of sperm motility, live sperm, plasma membrane, acrosome, and DNA integrity following cryopreservation. In conclusion, extender containing 8% LDL resulted in improved Nguni bull semen parameters such as sperm motility, viability, plasma membrane, acrosome, and DNA integrity following cryopreservation. Further studies are required to determine the fertilizing capacity of semen diluted and frozen with LDL in vitro and in vivo.