Dietary selenium and prolonged exercise alter gene expression and activity of antioxidant enzymes in equine skeletal muscle

Untrained Thoroughbred horses (6 mares and 6 geldings; 11 yr [SE 1] and 565 kg [SE 11]) were used to evaluate antioxidant gene expression and enzyme activity in blood and skeletal muscle in response to prolonged exercise after receiving 2 levels of dietary selenium for 36 d: 0.1 (CON; n = 6) or 0.3 mg/kg DM (SEL; n = 6). Horses were individually fed 1.6% BW coastal bermudagrass hay, 0.4% BW whole oats, and a mineral/vitamin premix containing no Se. Sodium selenite was added to achieve either 0.1 or 0.3 mg Se/kg DM in the total diet. On d 35, horses underwent 2 h of submaximal exercise in a free-stall exerciser. Blood samples were obtained before (d 0) and after 34 d of Se supplementation and on d 35 to 36 immediately after exercise and at 6 and 24 h after exercise. Biopsies of the middle gluteal muscle were obtained on d 0, before exercise on d 34, and at 6 and 24 h after exercise. Supplementation with Se above the NRC requirement (SEL) increased serum Se (P = 0.011) and muscle thioredoxin reductase (TrxR) activity (P = 0.051) but had no effect on glutathione peroxidase (GPx) activity in plasma, red blood cell (RBC) lysate, or muscle in horses at rest. Serum creatine kinase activity increased (P < 0.0001) in response to prolonged exercise but was not affected by dietary treatment. Serum lipid hydroperoxides were affected by treatment (P = 0.052) and were higher (P = 0.012) in horses receiving CON than SEL immediately following exercise. Muscle expression of TrxR1 was unchanged at 6 h but increased (P = 0.005) 2.8-fold 24 h after exercise, whereas muscle TrxR activity remained unchanged. Glutathione peroxidase activity increased in plasma (P < 0.0001) and decreased in RBC lysate (P = 0.010) after prolonged exercise. A Se treatment x time interaction was observed for RBC GPx activity (P = 0.048). Muscle GPx1 and GPx3 expression and GPx activity did not change during the 24-h period after exercise. Level of dietary Se had no overall effect on expression of MT1B, MT3, TrxR1, GPx1, GPx3, SOD1, or SOD2 in muscle following exercise. The impact of prolonged exercise on the activities of antioxidant enzymes varied. Furthermore, changes in enzyme activity did not necessarily align with enzyme gene expression following exercise. A higher level of Se intake elevated Se status of untrained horses, increased GPx activity, and lessened lipid peroxidation following exercise, suggesting that Se may be beneficial for mitigating oxidative muscle damage and aiding in postexercise recovery.