Exercise increases glucose transport into skeletal muscle via a pathway that is poorly understood. We investigated the role of endogenously produced reactive oxygen species (ROS) in contractionmediated glucose transport. Repeated contractions increased 2-deoxyglucose (2-DG) uptake roughly threefold in isolated, mouse extensor digitorum longus (fast-twitch) muscle. N -Acetylcysteine (NAC), a non-specific antioxidant, inhibited contraction-mediated 2-DG uptake by ∼50% (P < 0.05 versus control values), but did not significantly affect basal 2-DG uptake or the uptake induced by insulin, hypoxia or 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR, which mimics AMP-mediated activation of AMP-activated protein kinase, AMPK). Ebselen, a glutathione peroxidase mimetic, also inhibited contraction-mediated 2-DG uptake (by almost 60%, P < 0.001 versus control values). Muscles from mice overexpressing Mn2+ -dependent superoxide dismutase, which catalyses H 2 O 2 production from superoxide anions, exhibited a ∼25% higher rate of contractionmediated 2-DG uptake versus muscles from wild-type control mice (P < 0.05). Exogenous H 2 O 2 induced oxidative stress, as judged by an increase in the [GSSG]/[GSH + GSSG] (reduced glutathione + oxidized glutathione) ratio to 2.5 times control values, and this increase was substantially blocked by NAC. Similarly, NAC significantly attenuated contraction-mediated oxidative stress as judged by measurements of glutathione status and the intracellular ROS level with the fluorescent indicator 5-(and-6)-chloromethyl-2 ,7 -dichlorodihydrofluorescein (P < 0.05). Finally, contraction increased AMPK activity and phosphorylation ∼10-fold, and NAC blocked ∼50% of these changes. These data indicate that endogenously produced ROS, possibly H 2 O 2 or its derivatives, play an important role in contraction-mediated activation of glucose transport in fast-twitch muscle.

Skeletal muscle often shows a delayed force recovery after fatiguing stimulation, especially at low stimulation frequencies. In this study we focus on the role of reactive oxygen species (ROS) in this fatigue-induced prolonged low-frequency force depression. Intact, single muscle fibres were dissected from flexor digitorum brevis (FDB) muscles of rats and wild-type and superoxide dismutase 2 (SOD2) overexpressing mice. ] i in wild-type mouse fibres, whereas rat fibres and mouse SOD2 overexpressing fibres instead displayed a decreased myofibrillar Ca 2+ sensitivity. The SOD activity was ∼50% lower in wild-type mouse than in rat FDB muscles. Myoplasmic ROS increased during repeated tetanic stimulation in rat fibres but not in wild-type mouse fibres. The decreased Ca 2+ sensitivity in rat fibres could be partially reversed by application of the reducing agent dithiothreitol, whereas the decrease in tetanic [Ca 2+ ] i in wild-type mouse fibres was not affected by dithiothreitol or the antioxidant N -acetylcysteine. In conclusion, we describe two different causes of fatigue-induced prolonged low-frequency force depression, which correlate to differences in SOD activity and ROS metabolism. These findings may have clinical implications since ROS-mediated impairments in myofibrillar function can be counteracted by reductants and antioxidants, whereas changes in SR Ca 2+ handling appear more resistant to interventions.