["Acta Physiologica, Volume 231, Issue 4, April 2021. ", "\nAbstract\n\nAim\nLoss of dystrophin causes oxidative stress and affects nitric oxide synthase‐mediated vascular function in striated muscle. Because tetrahydrobiopterin is an antioxidant and co‐factor for nitric oxide synthase, we tested the hypothesis that tetrahydrobiopterin would be low in mdx mice and humans deficient for dystrophin.\n\n\nMethods\nTetrahydrobiopterin and its metabolites were measured at rest and in response to exercise in Duchenne and Becker muscular dystrophy patients, age‐matched male controls as well as wild‐type, mdx and mdx mice transgenically overexpressing skeletal muscle‐specific dystrophins. Mdx mice were also supplemented with tetrahydrobiopterin and pathophysiology was assessed.\n\n\nResults\nDuchenne muscular dystrophy patients had lower urinary dihydrobiopterin + tetrahydrobiopterin/specific gravity1.020 compared to unaffected age‐matched males and Becker muscular dystrophy patients. Mdx mice had low urinary and skeletal muscle dihydrobiopterin + tetrahydrobiopterin compared to wild‐type mice. Overexpression of dystrophins that localize neuronal nitric oxide synthase restored dihydrobiopterin + tetrahydrobiopterin in mdx mice to wild‐type levels while utrophin overexpression did not. Mdx mice and Duchenne muscular dystrophy patients did not increase tetrahydrobiopterin during exercise and in mdx mice tetrahydrobiopterin deficiency was likely because of lower levels of sepiapterin reductase in skeletal muscle. Tetrahydrobiopterin supplementation improved skeletal muscle strength, resistance to fatiguing and injurious contractions in vivo, increased utrophin and capillary density of skeletal muscle and lowered cardiac muscle fibrosis and left ventricular wall thickness in mdx mice.\n\n\nConclusion\nThese data demonstrate that impaired tetrahydrobiopterin synthesis is associated with dystrophin loss and treatment with tetrahydrobiopterin improves striated muscle histopathology and skeletal muscle function in mdx mice.\n\n"]