The hypothesis of this study was that yearlings would have higher levels of oxidative stress in the blood and muscle as measured by malondialdehyde (MDA), total glutathione (GSH-T), glutathione peroxidase (GPx), nitric oxide (NO), cortisol, and creatine kinase (CK) than mares after a single bout of intense exercise; after training, the horses would have similar levels of oxidative stress after a second bout of intense exercise. Ten Standardbred fillies (18 ± 2.4 months) and 10 Standardbred mares (13 ± 2.1 years) ran a repeated sprints exercise test (RSET1) on a treadmill before (RSET1) and after (RSET2) 8 weeks of training. Blood and muscle samples were taken before, during, and after each RSET. Blood samples were analyzed for hematocrit (Hct), total protein (TP), MDA, NO, GPx, GSH-T, cortisol, and CK; muscle samples were analyzed for MDA, NO, GPx, and GSH-T. Analyses were done in the context of a linear mixed model, and data are presented as mean ± SE (P < .05). In RSET1, yearlings had lower TP, Hct, plasma MDA and higher muscle MDA and GSH-T, erythrocyte GSH-T, and higher erythrocyte GPx activity than the mares (P < .05). In RSET2, the trained mares had lower plasma TP and MDA, muscle GSH-T, and higher muscle MDA and NO as compared to RSET1. Trained yearlings had higher muscle NO and erythrocyte GPx in RSET2 as compared to RSET1. Yearlings started with lower oxidative stress and a higher antioxidant status, but after training mares had improved so that few differences were seen between groups in RSET2.
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