Physiological sensing of O ₂ tension (partial O ₂ pressure, pO ₂ ) plays an important role in some mammalian cellular systems, but striated muscle generally is not considered to be among them. Here we describe a molecular mechanism in skeletal muscle that acutely couples changes in pO ₂ to altered calcium release through the ryanodine receptor–Ca ²⁺ -release channel (RyR1). Reactive oxygen species are generated in proportion to pO ₂ by NADPH oxidase 4 (Nox4) in the sarcoplasmic reticulum, and the consequent oxidation of a small set of RyR1 cysteine thiols results in increased RyR1 activity and Ca ²⁺ release in isolated sarcoplasmic reticulum and in cultured myofibers and enhanced contractility of intact muscle. Thus, Nox4 is an O ₂ sensor in skeletal muscle, and O ₂ -coupled hydrogen peroxide production by Nox4 governs the redox state of regulatory RyR1 thiols and thereby governs muscle performance. These findings reveal a molecular mechanism for O ₂ -based signaling by an NADPH oxidase and demonstrate a physiological role for oxidative modification of RyR1.

Vascular endothelial growth factor (VEGF) is exercise responsive, pro-angiogenic, and expressed in several muscle cell types. We hypothesized that in adult mice, VEGF generated within skeletal myofibers (and not other cells within muscle) is necessary for the angiogenic response to exercise training. This was tested in adult conditional, skeletal myofiber-specific VEGF gene-deleted mice (skmVEGF−/−), with VEGF levels reduced by >80%. After 8 wk of daily treadmill training, speed and endurance were unaltered in skmVEGF−/− mice, but increased by 18% and 99% (P < 0.01), respectively, in controls trained at identical absolute speed, incline, and duration. In vitro, isolated soleus and extensor digitorum longus contractile function was not impaired in skmVEGF−/− mice. However, training-induced angiogenesis was inhibited in plantaris (wild type, 38%, skmVEGF−/− 18%, P < 0.01), and gastrocnemius (wild type, 43%, P < 0.01; skmVEGF−/−, 7%, not significant). Capillarity was maintained (different from VEGF gene deletion targeted to multiple cell types) in untrained skmVEGF−/− mice. Arteriogenesis (smooth muscle actin+, artery number, and diameter) and remodeling [vimentin+, 5′-bromodeoxycytidine (BrdU)+, and F4/80+ cells] occurred in skmVEGF−/− mice, even in the absence of training. skmVEGF−/− mice also displayed a limited oxidative enzyme [citrate synthase and β-hydroxyacyl CoA dehydrogenase (β-HAD)] training response; β-HAD activity levels were elevated in the untrained state. These data suggest that myofiber expressed VEGF is necessary for training responses in capillarity and oxidative capacity and for improved running speed and endurance.