This study aimed to clarify the changes in ATPase activity and its effect on the microstructure and water-holding capacity (WHC) of Tan sheep meat during postmortem aging. The Longissimus dorsi muscle of Tan sheep slaughtered at 6 months of age was aged at 4 ℃ and evaluated for the activities of Na⁺-K⁺-ATPase, Ca²⁺-ATPase and caspase-3, microstructure, pH and drip loss rate after 0, 1, 2, 4 and 8 days. Results showed that the activities of Na⁺-K⁺-ATPase and Ca²⁺-ATPase increased initially and subsequently decreased with aging time, peaking at day 1. The activity of caspase-3 and drip loss increased with aging time up to 2 days and then decreased. In contrast, the opposite trend was observed for muscle pH. The contents of total proteins, low-salt soluble proteins, and high-salt soluble proteins continuously decreased with aging time. The content of water soluble proteins significantly decreased after day 2 (P < 0.05). On day 8, the myofibrils were broken, and gaps between myofibers, between muscle fiber bundles, and between myofibers and sarcolemma appeared; the Z lines were broken, and the H bands disappeared. Correlation analysis indicated that the activity of Na⁺-K⁺-ATPase was correlated with all other parameters (P < 0.01) and the activity of Ca²⁺-ATPase was significantly correlated with muscle pH and the activities of Na⁺-K⁺-ATPase and caspase-3 (P < 0.01). Overall, during postmortem aging, the variations in the activities of Na⁺-K⁺-ATPase and Ca²⁺-ATPase can lead to downstream caspase-3 activation to hydrolyze cytoskeleton proteins, resulting in the formation of gaps at different muscle sites. The water in muscle flows into the formed gaps due to gravity, causing an increase in the drip loss and a decrease in the WHC of Tan sheep meat.

This study was designed to clarify the changes in the expression level of hypoxia-inducible factor 1 (HIF-1) and its effects on the activity of triose-phosphate isomerase (TPI), the contents of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP), and meat quality traits in Tan sheep meat during a 48-hours postmortem aging period. Results indicated that the expression level of HIF-1 increased significantly between 2 and 24 h, and then decreased. The activity of TPI increased significantly, reaching a maximum value at 6 h, then significantly decreased until 48 h. The contents of ATP, ADP, and AMP significantly decreased with increasing aging time, while muscle pH significantly decreased (P < 0.05). The color values of L*, a*, and b* were significantly increased with increasing aging time. Correlation analysis indicated that the expression level of HIF-1 was correlated with meat quality indicators, TPI activity, and the contents of ATP, ADP and AMP significantly (P < 0.05, P < 0.01). It might be speculated that postmortem hypoxia could induce a rapid increase in the expression level of HIF-1 within 6 h postmortem, and the transfer of the HIF-1 complex to the nucleus to bind with glycolytic genes, thereby promoting the expression of glycolysis-related genes, enhancing postmortem glycolysis, leading to increased activity of glycolysis-related enzymes and decreased muscle pH. To sum up, HIF-1 can affect meat color by regulating postmortem muscle glycolysis and pH.