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The goal of this study was to examine the influence of exercise training and type I diabetes on the activity of aconitase and superoxide dismutase (SOD), on levels of reduced glutathione (GSH), on the expression of genes responsible for glutathione synthesis (Gclc, g-glutamylcysteine synthase; Gss, glutathione synthetase), and on the expression of genes responsible for the formation of two potassium channels (Kcnd2, Ito; KCNQl, IKs) in cardiac muscle. Forty female Sprague-Dawley rats were divided into four groups: sedentary controls, sedentary diabetics, trained controls, and trained diabetics. Diabetes was induced via streptozotocin injection, and training consisted of swimming up to 2 hours per day for 11 weeks. Aconitase activity and GSH levels were unchanged, but SOD activity was significantly elevated in trained diabetic rats compared with sedentary diabetic rats. Gss expression was significantly downregulated in diabetic rats compared with control rats. Kcnd2 expression was significantly downregulated in trained diabetic rats compared with sedentary diabetic rats, and KCNQl expression was consistently downregulated in trained rats compared with sedentary rats and in diabetic rats compared with control rats. These results suggest training alone or combined with diabetes increases antioxidant mechanisms and downregulates the expression of genes related to K+ ion channel activity.