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.

Creatine kinase (CK) is a key enzyme for maintaining a constant ATP/ADP ratio during rapid energy turnover. To investigate the role of CK in skeletal muscle fatigue, we used isolated whole muscles and intact single fibers from CK-deficient mice (CK(-/-)). With high-intensity electrical stimulation, single fibers from CK(-/-) mice displayed a transient decrease in both tetanic free myoplasmic [Ca(2+)] ([Ca(2+)](i), measured with the fluorescent dye indo-1) and force that was not observed in wild-type fibers. With less intense, repeated tetanic stimulation single fibers and EDL muscles, both of which are fast-twitch, fatigued more slowly in CK(-/-) than in wild-type mice; on the other hand, the slow-twitch soleus muscle fatigued more rapidly in CK(-/-) mice. In single wild-type fibers, tetanic force decreased and [Ca(2+)](i) increased during the first 10 fatiguing tetani, but this was not observed in CK(-/-) fibers. Fatigue was not accompanied by phosphocreatine breakdown and accumulation of inorganic phosphate in CK(-/-) muscles. In conclusion, CK is important for avoiding fatigue at the onset of high-intensity stimulation. However, during more prolonged stimulation, CK may contribute to the fatigue process by increasing the myoplasmic concentration of inorganic phosphate.

Abstract-A decreased exercise tolerance is a common symptom in patients with congestive heart failure (CHF). This decrease has been suggested to be partly due to altered skeletal muscle function. Therefore, we have studied contractile function and cytoplasmic free Ca 2ϩ concentration ([Ca 2ϩ ] i , measured with the fluorescent dye indo 1) in isolated muscles from rats in which CHF was induced by ligation of the left coronary artery. The results show no major changes of the contractile function and [Ca 2ϩ ] i handling in unfatigued intact fast-twitch fibers isolated from flexor digitorum brevis muscles of CHF rats, but these fibers were markedly more susceptible to damage during microdissection. Furthermore, CHF fibers displayed a marked increase of baseline [Ca 2ϩ ] i during fatigue. Isolated slow-twitch soleus muscles of CHF rats displayed slower twitch contraction and tetanic relaxation than did muscles from sham-operated rats; the slowing of relaxation became more pronounced during fatigue in CHF muscles. Immunoblot analyses of sarcoplasmic reticulum proteins and sarcolemma Na ϩ ,K ϩ -ATPase showed no difference in flexor digitorum brevis muscles of sham-operated versus CHF rats. In conclusion, functional impairments can be observed in limb muscle isolated from rats with CHF. These impairments seem to mainly involve structures surrounding the muscle cells and sarcoplasmic reticulum Ca Key Words: heart failure Ⅲ skeletal muscle Ⅲ fatigue Ⅲ intracellular Ca 2ϩ handling D ecreased fatigue resistance and skeletal muscle weakness are important symptoms in humans with congestive heart failure (CHF). The reason for the decreased fatigue resistance is not clear. Often, there is no clear correlation between the degree of heart dysfunction and the decrease in exercise tolerance. 1 This suggests that there could be functional impairments within the skeletal muscles that are due to, for instance, alterations in the local environment with restricted local blood flow. 2 In addition, numerous studies have been focused on possible abnormalities in skeletal muscle cells, and significant changes have been found both at the mRNA and protein levels. Generally observed changes in skeletal muscle cells in CHF include a shift of myosin heavy chain distribution toward more fast-type myosin heavy chain, 3-5 altered expression of sarcoplasmic reticulum (SR) Ca 2ϩ -ATPase (SERCA), 6,7 and in later stages, decrements in mitochondrial enzymes and muscle cell atrophy. 8 Functional studies of limb muscle function in CHF are more sparse. One study on bundles of muscle fibers from the fast-twitch extensor digitorum longus (EDL) muscles of rats showed marked dysfunction in CHF with Ϸ50% reductions in tetanic Ca 2ϩ and force and markedly accelerated fatigue development, which was not due to muscle cell atrophy. 9 There seems to be a discrepancy between these very dramatic functional changes and the relatively subtle changes of muscle protein levels observed in muscles from CHF subjects. In accordance, a more recent study showed more mode...