Effect of Nitric Oxide (NO)-Mediated Mitochondrial Apoptosis on the Tenderness of Tan Sheep Meat during Postmortem Aging

The aim of this study was to assess the effect of nitric oxide (NO)-mediated mitochondrial apoptosis on Tan sheep meat tenderness during postmortem aging. The hind leg muscle of Tan sheep was injected with 0.8% NaCl (control), 20 mmol/L aminoguanidine hydrochloride (AH) or 90 mmol/L L-arginine (L-Arg) and aged at 4 ℃ for 0, 6, 12, 24, 48, 96, and 192 h. NO and inducible nitric oxide synthase (iNOS) contents, cytochrome c (Cyt c) redox state, oxidative stress level, mitochondrial apoptotic pathways, and tenderness were measured during postmortem aging. The results indicated that at 12 and 24 h after slaughter, NO and iNOS levels were significantly higher, and ATP levels were significantly lower in the L-Arg group than in the control group (P < 0.05). At 24 and 48 h, Cyt c reduction levels and superoxide dismutase (SOD) activity were significantly higher, while the level of reactive oxygen species (ROS) was significantly lower in the AH group than in the control group (P < 0.05). From 12 to 96 h, ATP contents were significantly lower in the L-Arg group than in the control group (P < 0.05). In addition, at 24 h, Ca2+ concentration significantly increased in the L-Arg group compared with the control group (P < 0.05). At all time points except 24 h, the caspase-3 activity in the L-Arg group was significantly higher than that in the control group (P < 0.05). Moreover, it was found that the Bax/Bcl-2 ratio, pH, and myofibril fragmentation index (MFI) in the L-Arg group were significantly higher than those in the control group at all time points after slaughter (P < 0.05). Shear force was significantly lower in the L-Arg group but significantly higher in the AH group than in the control group (P < 0.05). In conclusion, NO exacerbates mitochondrial oxidative stress and directly upregulates ROS levels during the postmortem aging of Tan sheep meat, leading to a decrease in SOD activity, irreversible opening of mitochondrial permeability transition pore (MPTP), and ultimately mitochondrial damage. In addition, by controlling the redox state of Cyt c, NO indirectly activates caspase-3 activity to accelerate apoptotic energy metabolism, deplete ATP and consequently maintain myocyte homeostasis. Furthermore, NO affects the release of Ca2+, up-regulates the Bax/Bcl-2 ratio, and induces the occurrence of mitochondrial apoptotic cascade, ultimately resulting in an increase in MFI and a decrease in shear force and, consequently, improving the tenderness of Tan sheep meat.