140 Chronic Heat stress in finishing lambs induced metabolic changes in muscle that indicated greater glycogen utilization and lactate production, which may explain greater prevalence of dark cutters.

Abstract Environmental stress, particularly heat events, increases the frequency of dark cutting in meat animals, leading to high pH meat that is dark, firm, and dry. However, the mechanisms by which heat stress conditions predispose livestock to dark cutting are not known. Therefore, our objective was to identify metabolic pathway components altered by heat stress that might influence the incidence of dark cutting. Commercial Polypay-cross wethers were randomly assigned to heat stress (40 °C; n = 11) or thermoneutral (pair-fed controls; 19 °C; n = 10) conditions for 30 d. Venous blood was sampled on d 3, 9, 21, and 30 of heat stress. Semitendinosus muscle was collected at necropsy for protein quantification by western immunoblot, and flexor digitorum superficialis muscle was collected for assessment of ex vivo glucose oxidation. Semitendinosus content of lactate dehydrogenase (LDH-A) tended to be increased (P = 0.10) for heat-stressed wethers, which indicates greater potential for lactate production from anaerobic metabolism. Similarly, semitendinosus content of glycogen phosphorylase (PYGM) tended to be greater (P = 0.09) in heat stressed wethers, which indicates greater utilization of stored glycogen. Semitendinosus content of citrate synthase (CS), a key component for oxidative glucose metabolism, did not differ between groups. However, ex vivo skeletal muscle glucose oxidation was reduced (P < 0.05) by 33% in heat-stressed wethers despite normal glucose uptake rates. Circulating lactate concentrations were not different between groups, presumably due to rapid clearance by the liver as part of the Cori cycle. However, blood pH was reduced (P < 0.05) by heat stress on d 3, 9, and 21, which corresponded with greater (P ≤ 0.05) circulating HCO3 that indicated upregulation of buffering systems in heat-stressed wethers. These findings indicate that metabolic changes during chronic heat stress produce conditions within skeletal muscle that could contribute to dark cutting issues that undermine muscle quality at slaughter.