Muscle glycogen stores and fatigue Ørtenblad, Niels; Westerblad, Håkan; Nielsen, Joachim
The Journal of physiology,
September 2013, Letnik:
591, Številka:
18
Journal Article
Recenzirano
Odprti dostop
Studies performed at the beginning of the last century revealed the importance of carbohydrate as a fuel during exercise, and the importance of muscle glycogen on performance has subsequently been ...confirmed in numerous studies. However, the link between glycogen depletion and impaired muscle function during fatigue is not well understood and a direct cause‐and‐effect relationship between glycogen and muscle function remains to be established. The use of electron microscopy has revealed that glycogen is not homogeneously distributed in skeletal muscle fibres, but rather localized in distinct pools. Furthermore, each glycogen granule has its own metabolic machinery with glycolytic enzymes and regulating proteins. One pool of such glycogenolytic complexes is localized within the myofibrils in close contact with key proteins involved in the excitation–contraction coupling and Ca2+ release from the sarcoplasmic reticulum (SR). We and others have provided experimental evidence in favour of a direct role of decreased glycogen, localized within the myofibrils, for the reduction in SR Ca2+ release during fatigue. This is consistent with compartmentalized energy turnover and distinctly localized glycogen pools being of key importance for SR Ca2+ release and thereby affecting muscle contractility and fatigability.
The contractile function of skeletal muscle declines during intense or prolonged physical exercise, that is, fatigue develops. Skeletal muscle fibers fatigue acutely during highly intense exercise ...when they have to rely on anaerobic metabolism. Early stages of fatigue involve impaired myofibrillar function, whereas decreased Ca
2+
release from the sarcoplasmic reticulum (SR) becomes more important in later stages. SR Ca
2+
release can also become reduced with more prolonged, lower intensity exercise, and it is then related to glycogen depletion. Increased reactive oxygen/nitrogen species can cause long-lasting impairments in SR Ca
2+
release resulting in a prolonged force depression after exercise. In this article, we discuss molecular and cellular mechanisms of the above fatigue-induced changes, with special focus on multiple mechanisms to decrease SR Ca
2+
release to avoid energy depletion and preserve muscle fiber integrity. We also discuss fatigue-related effects of exercise-induced Ca
2+
fluxes over the sarcolemma and between the cytoplasm and mitochondria.
The production of reactive oxygen/nitrogen species (ROS/RNS) is generally considered to increase during physical exercise. Nevertheless, direct measurements of ROS/RNS often show modest increases in ...ROS/RNS in muscle fibres even during intensive fatiguing stimulation, and the major source(s) of ROS/RNS during exercise is still being debated. In rested muscle fibres, mild and acute exposure to exogenous ROS/RNS generally increases myofibrillar submaximal force, whereas stronger or prolonged exposure has the opposite effect. Endogenous production of ROS/RNS seems to preferentially decrease submaximal force and positive effects of antioxidants are mainly observed during fatigue induced by submaximal contractions. Fatigued muscle fibres frequently enter a prolonged state of reduced submaximal force, which is caused by a ROS/RNS‐dependent decrease in sarcoplasmic reticulum Ca2+ release and/or myofibrillar Ca2+ sensitivity. Increased ROS/RNS production during exercise can also be beneficial and recent human and animal studies show that antioxidant supplementation can hamper the beneficial effects of endurance training. In conclusion, increased ROS/RNS production have both beneficial and detrimental effects on skeletal muscle function and the outcome depends on a combination of factors: the type of ROS/RNS; the magnitude, duration and location of ROS/RNS production; and the defence systems, including both endogenous and exogenous antioxidants.
Schematic diagram showing major sources and interactions of ROS/RNS that affect sarcoplasmic reticulum Ca2+ release and force production during skeletal muscle fatigue and recovery.
Mechanical dissection of single intact mammalian skeletal muscle fibers permits real-time measurement of intracellular properties and contractile function of living fibers. A major advantage of ...mechanical over enzymatic fiber dissociation is that single fibers can be isolated with their tendons remaining attached, which allows contractile forces (in the normal expected range of 300-450 kN/m
) to be measured during electrical stimulation. Furthermore, the sarcolemma of single fibers remains fully intact after mechanical dissection, and hence the living fibers can be studied with intact intracellular milieu and normal function and metabolic properties, as well as ionic control. Given that Ca
is the principal regulator of the contractile force, measurements of myoplasmic free Ca
(Ca
) can be used to further delineate the intrinsic mechanisms underlying changes in skeletal muscle function. Ca
measurements are most commonly performed in intact single fibers using ratiometric fluorescent indicators such as indo-1 or fura-2. These Ca
indicators are introduced into the fiber by pressure injection or by using the membrane-permeable indo-1 AM, and Ca
is measured by calculating a ratio of the fluorescence at specific wavelengths emitted for the Ca
-free and Ca
-bound forms of the dye. We describe here the procedures for mechanical dissection, and for force and Ca
measurement in intact single fibers from mouse flexor digitorum brevis (FDB) muscle, which is the most commonly used muscle in studies using intact single fibers. This technique can also be used to isolate intact single fibers from various muscles and from various species. As an alternative to Ca
indicators, single fibers can also be loaded with fluorescent indicators to measure, for instance, reactive oxygen species, pH, and Mg
, or they can be injected with proteins to change functional properties. The entire protocol, from dissection to the start of an experiment on a single fiber, takes ∼3 h.
Reactive oxygen and nitrogen species (ROS/RNS) are important for skeletal muscle function under both physiological and pathological conditions. ROS/RNS induce long‐term and acute effects and the ...latter are the focus of the present review. Upon repeated muscle activation both oxygen and nitrogen free radicals likely increase and acutely affect contractile function. Although fluorescent indicators often detect only modest increases in ROS during repeated activation, there are numerous studies showing that manipulations of ROS can affect muscle fatigue development and recovery. Exposure of intact muscle fibres to the oxidant hydrogen peroxide (H2O2) affects mainly the myofibrillar function, where an initial increase in Ca2+ sensitivity is followed by a decrease. Experiments on skinned fibres show that these effects can be attributed to H2O2 interacting with glutathione and myoglobin, respectively. The primary RNS, nitric oxide (NO•), may also acutely affect myofibrillar function and decrease the Ca2+ sensitivity. H2O2 can oxidize the sarcoplasmic reticulum Ca2+ release channels. This oxidation has a large stimulatory effect on Ca2+‐induced Ca2+ release of isolated channels, whereas it has little or no effect on the physiological, action potential‐induced Ca2+ release in skinned and intact muscle fibres. Thus, acute effects of ROS/RNS on muscle function are likely to be mediated by changes in myofibrillar Ca2+ sensitivity, which can contribute to the development of muscle fatigue or alternatively help counter it.
Physical exercise has emerged as an alternative treatment for patients with depressive disorder. Recent animal studies show that exercise protects from depression by increased skeletal muscle ...kynurenine aminotransferase (KAT) expression which shifts the kynurenine metabolism away from the neurotoxic kynurenine (KYN) to the production of kynurenic acid (KYNA). In the present study, we investigated the effect of exercise on kynurenine metabolism in humans. KAT gene and protein expression was increased in the muscles of endurance-trained subjects compared with untrained subjects. Endurance exercise caused an increase in plasma KYNA within the first hour after exercise. In contrast, a bout of high-intensity eccentric exercise did not lead to increased plasma KYNA concentration. Our results show that regular endurance exercise causes adaptations in kynurenine metabolism which can have implications for exercise recommendations for patients with depressive disorder.
High-intensity interval training (HIIT) is a time-efficient way of improving physical performance in healthy subjects and in patients with common chronic diseases, but less so in elite endurance ...athletes. The mechanisms underlying the effectiveness of HIIT are uncertain. Here, recreationally active human subjects performed highly demanding HIIT consisting of 30-s bouts of all-out cycling with 4-min rest in between bouts (≤3 min total exercise time). Skeletal muscle biopsies taken 24 h after the HIIT exercise showed an extensive fragmentation of the sarcoplasmic reticulum (SR) Ca²⁺ release channel, the ryanodine receptor type 1 (RyR1). The HIIT exercise also caused a prolonged force depression and triggered major changes in the expression of genes related to endurance exercise. Subsequent experiments on elite endurance athletes performing the same HIIT exercise showed no RyR1 fragmentation or prolonged changes in the expression of endurance-related genes. Finally, mechanistic experiments performed on isolated mouse muscles exposed to HIIT-mimicking stimulation showed reactive oxygen/nitrogen species (ROS)-dependent RyR1 fragmentation, calpain activation, increased SR Ca²⁺ leak at rest, and depressed force production due to impaired SR Ca²⁺ release upon stimulation. In conclusion, HIIT exercise induces a ROS-dependent RyR1 fragmentation in muscles of recreationally active subjects, and the resulting changes in muscle fiber Ca²⁺-handling trigger muscular adaptations. However, the same HIIT exercise does not cause RyR1 fragmentation in muscles of elite endurance athletes, which may explain why HIIT is less effective in this group.
Reactive oxygen and nitrogen species (ROS/RNS) are involved in numerous aspects of cellular signaling. Classically ROS/RNS have been associated with cellular dysfunction and disease, but it is now ...clear that they are also of integral importance under normal conditions. In this review, we discuss ROS/RNS effects in skeletal muscle, with special focus on changes in contractile function. The review deals with the tentative roles of ROS/RNS for acute changes that can occur during strenuous exercise resulting in muscle fatigue, for the recovery from fatigue, and for the effects of training/overtraining. We also discuss two groups of inherited diseases; muscle dystrophies, where recent data suggest that ROS/RNS may be of unexpectedly large importance, and mitochondrial myopathies, where the role of ROS seems more limited than originally thought.
Key points
We investigated whether intramuscular temperature affects the acute recovery of exercise performance following fatigue‐induced by endurance exercise.
Mean power output was better preserved ...during an all‐out arm‐cycling exercise following a 2 h recovery period in which the upper arms were warmed to an intramuscular temperature of ̴ 38°C than when they were cooled to as low as 15°C, which suggested that recovery of exercise performance in humans is dependent on muscle temperature.
Mechanisms underlying the temperature‐dependent effect on recovery were studied in intact single mouse muscle fibres where we found that recovery of submaximal force and restoration of fatigue resistance was worsened by cooling (16–26°C) and improved by heating (36°C).
Isolated whole mouse muscle experiments confirmed that cooling impaired muscle glycogen resynthesis.
We conclude that skeletal muscle recovery from fatigue‐induced by endurance exercise is impaired by cooling and improved by heating, due to changes in glycogen resynthesis rate.
Manipulation of muscle temperature is believed to improve post‐exercise recovery, with cooling being especially popular among athletes. However, it is unclear whether such temperature manipulations actually have positive effects. Accordingly, we studied the effect of muscle temperature on the acute recovery of force and fatigue resistance after endurance exercise. One hour of moderate‐intensity arm cycling exercise in humans was followed by 2 h recovery in which the upper arms were either heated to 38°C, not treated (33°C), or cooled to ∼15°C. Fatigue resistance after the recovery period was assessed by performing 3 × 5 min sessions of all‐out arm cycling at physiological temperature for all conditions (i.e. not heated or cooled). Power output during the all‐out exercise was better maintained when muscles were heated during recovery, whereas cooling had the opposite effect. Mechanisms underlying the temperature‐dependent effect on recovery were tested in mouse intact single muscle fibres, which were exposed to ∼12 min of glycogen‐depleting fatiguing stimulation (350 ms tetani given at 10 s interval until force decreased to 30% of the starting force). Fibres were subsequently exposed to the same fatiguing stimulation protocol after 1–2 h of recovery at 16–36°C. Recovery of submaximal force (30 Hz), the tetanic myoplasmic free Ca2+ (measured with the fluorescent indicator indo‐1), and fatigue resistance were all impaired by cooling (16–26°C) and improved by heating (36°C). In addition, glycogen resynthesis was faster at 36°C than 26°C in whole flexor digitorum brevis muscles. We conclude that recovery from exhaustive endurance exercise is accelerated by raising and slowed by lowering muscle temperature.
Key points
We investigated whether intramuscular temperature affects the acute recovery of exercise performance following fatigue‐induced by endurance exercise.
Mean power output was better preserved during an all‐out arm‐cycling exercise following a 2 h recovery period in which the upper arms were warmed to an intramuscular temperature of ̴ 38°C than when they were cooled to as low as 15°C, which suggested that recovery of exercise performance in humans is dependent on muscle temperature.
Mechanisms underlying the temperature‐dependent effect on recovery were studied in intact single mouse muscle fibres where we found that recovery of submaximal force and restoration of fatigue resistance was worsened by cooling (16–26°C) and improved by heating (36°C).
Isolated whole mouse muscle experiments confirmed that cooling impaired muscle glycogen resynthesis.
We conclude that skeletal muscle recovery from fatigue‐induced by endurance exercise is impaired by cooling and improved by heating, due to changes in glycogen resynthesis rate.
PDF
