Hydrogen peroxide decelerates recovery of action potential after high-frequency fatigue in skeletal muscle

Energy (and Reactive Oxygen Species Generation) Saving Distribution of Mitochondria for the Activation of ATP Production in Skeletal Muscle

Exercise produces oxidants from a variety of intracellular sources, including NADPH oxidases (NOX) and mitochondria. Exercise-derived reactive oxygen species (ROS) are beneficial, and the amount and location of these ROS is important to avoid muscle damage associated with oxidative stress. We discuss here some of the evidence that involves ROS production associated with skeletal muscle contraction and the potential oxidative stress associated with muscle contraction. We also discuss the potential role of H2O2 produced after NOX activation in the regulation of glucose transport in skeletal muscle. Finally, we propose a model based on evidence for the role of different populations of mitochondria in skeletal muscle in the regulation of ATP production upon exercise. The subsarcolemmal population of mitochondria has the enzymatic and metabolic components to establish a high mitochondrial membrane potential when fissioned at rest but lacks the capacity to produce ATP. Calcium entry into the mitochondria will further increase the metabolic input. Upon exercise, subsarcolemmal mitochondria will fuse to intermyofibrillar mitochondria and will transfer the mitochondria membrane potential to them. These mitochondria are rich in ATP synthase and will subsequentially produce the ATP needed for muscle contraction in long-term exercise. These events will optimize energy use and minimize mitochondria ROS production.

BK channel openers NS1619 and NS11021 reverse hydrogen peroxide-induced membrane potential changes in skeletal muscle

Large conductance calcium-activated potassium (BK) channels play a crucial role in the repolarization and after-hyperpolarization phases of the cell membrane. The channel openers are also used in treatment of some diseases, including hypo/hyperkalemic periodic paralysis. However, little is known about the effects of BK channels and the channel activators on membrane potentials in skeletal muscle. In addition, the effects of reactive oxygen species (ROS) on BK channels in skeletal muscle are also unknown. Therefore, the aim of this study was to determine the effects of BK channel openers and ROS on membrane potentials in skeletal muscle fibers. For this purpose, resting membrane potentials and action potentials (AP) of frog gastrocnemius muscles were recorded in the presence of commonly used BK channel openers NS1619 and NS11021, H₂O₂ (a type of ROS), and both using intracellular microelectrode technique. The channel activators significantly and dose-dependently decreased amplitude and increased rise time of AP but did not impact repolarization. The presence of H₂O₂ plus NS1619 or NS11021 resulted in significant change because the channel openers completely reversed the deleterious effects of hydrogen peroxide on the repolarization phase of AP in skeletal muscle fibers. In the present study, the contributions of BK channel activation and the modulatory role of H₂O₂ on membrane potentials was demonstrated in skeletal muscle fibers, for the first time. Moreover, it should be noted that BK channel openers should be used in the treatment of reactive oxygen species-induced skeletal muscle diseases.

Reactive oxygen species formation during tetanic contractions in single isolated Xenopus myofibers

Contracting skeletal muscle produces reactive oxygen species (ROS) that have been shown to affect muscle function and adaptation. However, real-time measurement of ROS in contracting myofibers has proven to be difficult. We used amphibian (Xenopus laevis) muscle to test the hypothesis that ROS are formed during contractile activity in isolated single skeletal muscle fibers and that this contraction-induced ROS formation affects fatigue development. Single myofibers were loaded with 5 μM dihydrofluorescein-DA (Hfluor-DA), a fluorescent probe that reacts with ROS and results in the formation of fluorescein (Fluor) to precisely monitor ROS generation within single myofibers in real time using confocal miscroscopy. Three identical periods of maximal tetanic contractions (1 contraction/3 s for 2 min, separated by 60 min of rest) were conducted by each myofiber (n = 6) at 20°C. Ebselen (an antioxidant) was present in the perfusate (10 μM) during the second contractile period. Force was reduced by ∼30% during each of the three contraction periods, with no significant difference in fatigue development among the three periods. The Fluor signal, indicative of ROS generation, increased significantly above baseline in both the first (42 ± 14%) and third periods (39 ± 10%), with no significant difference in the increase in fluorescence between the first and third periods. There was no increase of Fluor in the presence of ebselen during the second contractile period. These results demonstrated that, in isolated intact Xenopus myofibers, 1) ROS can be measured in real time during tetanic contractions, 2) contractile activity induced a significant increase above resting levels of ROS production, and 3) ebselen treatment reduced ROS generation to baseline levels but had no effect on myofiber contractility and fatigue development.

Models to explain fatigue during prolonged endurance cycling

Much of the previous research into understanding fatigue during prolonged cycling has found that cycling performance may be limited by numerous physiological, biomechanical, environmental, mechanical and psychological factors. From over 2000 manuscripts addressing the topic of fatigue, a number of diverse cause-and-effect models have been developed. These include the following models: (i) cardiovascular/anaerobic; (ii) energy supply/energy depletion; (iii) neuromuscular fatigue; (iv) muscle trauma; (v) biomechanical; (vi) thermoregulatory; (vii) psychological/motivational; and (viii) central governor. More recently, however, a complex systems model of fatigue has been proposed, whereby these aforementioned linear models provide afferent feedback that is integrated by a central governor into our unconscious perception of fatigue. This review outlines the more conventional linear models of fatigue and addresses specifically how these may influence the development of fatigue during cycling. The review concludes by showing how these linear models of fatigue might be integrated into a more recently proposed nonlinear complex systems model of exercise-induced fatigue.

Gold sodium thiomalate improves membrane potential impaired by high-frequency stimulation

Effects of gold sodium thiomalate (GSTM) on membrane potential and tetanus tension were examined to elucidate whether the gold compound improves mechanical and electrical muscle dysfunction produced by continuous repeated stimulation of frog skeletal muscles. Continuous stimulation (50 Hz for 2 min, 0.05 ms pulse duration) to the sartorius muscle depolarized the membrane, decreased action potential amplitude, and prolonged action potential duration. GSTM (0.1 mM), unlike thiomalic acid (0.1 mM), markedly decreased impairment of these electrical parameters produced during the stimulation period. In the presence of 500 units/mL of catalase, fatigue stimulation still lengthened by 1.5-fold the half-duration of the action potential after a 5-min rest. The prolongation was, however, smaller than that in controls (no catalase). Application of both catalase and GSTM led to no further changes in action potential compared with the application of catalase alone. GSTM did not affect resting tension of single toe muscle fibers though it suppressed the maximum tension after continuous stimulation. These findings suggest that GSTM can inhibit excitable dysfunction of skeletal muscles subjected to continuous stimulation and that such protective effects of GSTM may be partially mediated by H2O2.Key words: gold sodium thiomalate, catalase, continuous stimulation, resting and action potentials, force, frog skeletal muscle.

H2O2 activates ryanodine receptor but has little effect on recovery of releasable Ca2+ content after fatigue

We studied whether hydrogen peroxide (H(2)O(2)) at </=10 microM activates the ryanodine receptor and decreases releasable Ca(2+) content in the sarcoplasmic reticulum after fatigue. Exposure of rabbit or frog skeletal muscle ryanodine receptors to 10 microM H(2)O(2) enhanced channel activity in lipid bilayers when the redox potential was defined at cis = -220 mV and trans = -180 mV. Channel activation by 10 microM H(2)O(2) was also observed when cis potential was set at -220 mV without defining trans potential, but the effect was less. Reduction of trans redox potential from -180 to -220 mV did not alter channel activity. H(2)O(2) at 500 microM failed to activate the channel when the redox potential was not controlled. Stimulation of the frog muscle fiber for 2 min (50 Hz, a duty cycle of 200 ms/s) decreased tetanus tension by approximately 50%. After 1 min, tetanus recovered rapidly to approximately 70% of control and thereafter slowly approached the control level. Amplitudes of caffeine- and 4-chloro-m-cresol-induced contractures were decreased after a 60-min rest. The decrease is not enhanced by exposure to 10 microM H(2)O(2). These results suggest that H(2)O(2) markedly activates the ryanodine receptor under the redox control in vitro, but externally applied H(2)O(2) may not play an important role in the postfatigue recovery process.