Diabetes-induced cardiac complications include left ventricular (LV) dysfunction and heart failure. We previously demonstrated that LV phosphoinositide 3-kinase p110alpha (PI3K) protects the heart against diabetic cardiomyopathy, associated with reduced NADPH oxidase expression and activity. Conversely, in dominant negative PI3K(p110alpha) transgenic mice (dnPI3K), reduced cardiac PI3K signalling exaggerated diabetes-induced cardiomyopathy; associated with upregulated NADPH oxidase. AIM: To examine whether chronic supplementation with the antioxidant coenzyme Q10 (CoQ10) could attenuate LV superoxide and diabetic cardiomyopathy in a setting of impaired PI3K signalling. METHODS: Diabetes was induced in 6-week-old non-transgenic and dnPI3K male mice via streptozotocin. After 4 weeks of diabetes, CoQ10 supplementation commenced (10mg/kg i.p., 3 times/week, 8 weeks). At study end (12 weeks of diabetes), markers of LV function, cardiomyocyte hypertrophy, collagen deposition, NADPH oxidase, oxidative stress (3-nitrotyrosine) and concentrations of CoQ9 and CoQ10 were determined. RESULTS: LV NADPH oxidase (Nox2 gene expression and activity, on lucigenin-enhanced chemiluminescence), as well as oxidative stress, were increased by diabetes, exaggerated in diabetic dnPI3K mice, and attenuated by CoQ10. Diabetes-induced LV diastolic dysfunction (prolonged deceleration time, elevated end-diastolic pressure, impaired E/A ratio), cardiomyocyte hypertrophy and fibrosis, expression of atrial natriuretic peptide, connective tissue growth factor and beta-myosin heavy chain, were all attenuated by CoQ10. CONCLUSIONS: Chronic CoQ10 supplementation attenuates aspects of diabetic cardiomyopathy, even in a setting of reduced cardiac PI3K protective signalling. Given CoQ10 supplementation has been suggested to have positive outcomes in heart failure patients, chronic CoQ10 supplementation may be an attractive adjunct therapy for diabetic heart failure.