Changes in Myofibrillar and Mitochondrial Compartments during Increased Activity: Dependance from Oxidative Capacity of Muscle

Striated muscle tissue contains fibers with high oxidative capacity (heart muscle), higher oxidative capacity (type I and IIA fibers of skeletal muscle) and low oxidative capacity (type IIB/X fibers of skeletal muscle). Muscle fibers with higher oxidative capacity contain large mitochondria tightly packed with cristae as well as small forms of mitochondria containing relatively few cristae. The intensive development of the mitochondrial apparatus in the post-activity period reflects the adaptive processes, which is intended to supply the increased energy requirements of muscle fibers with higher oxidative capacity. Muscle fibers with low oxidative capacity contain significantly less mitochondria than fibers with higher capacity. It is typical to type IIB fibers that after intensive muscle activity there are damaged myofibrils in a relatively small area, some myofibrils are twisted and lose the connection with the neighboring structures. It is still not fully known how skeletal muscles with different oxidative capacity respond to an increased functional activity and what differences exist in these fibers between oxidative capacity and function of myofibrils. The aim of the present short review was to compare structural-functional changes in mitochondrial and myofibrillar compartments of heart and skeletal muscle fibers with different oxidative capacity and the effect of increased functional activity on the interaction of these compartments.

Adaptation of Skeletal Muscle to Prolonged Activity: Role of Myosin

The aim of this short review is to describe the role of myosin isoforms during the adaptation of skeletal muscle to prolonged physical activity (for example endurance exercise) and to show the coordination between changes in muscle oxidative capacity and myofibrillar apparatus in slow-twitch and fast-twitch muscles. Adaptational changes in myosin isoforms during long lasting muscle activity (decrease of MyHC IIb isoforms relative content and increase of that MyHC IIa and decrease of MyLC 1 fast isoforms in fast-twitch muscles) are in good coordination with changes of muscle oxidative capacity. These changes show that during regular endurance exercise fast-twitch muscle fibers (type IIA) are also recruited and create the potential source of increase in endurance capacity during the process of adaptation to the prolonged physical activity.

Muscle damage and regeneration: Response to exercise training

Exercise training influences the function of skeletal muscle, modifying fibre structure, metabolism and promoting the release of growth factors and other signalling molecules. The number of satellite cells under the basal lamina of type I and type IIA muscle fibres increases during endurance training and under the basal lamina of both type II fibres during resistance training. An increase in satellite cells is related to several factors expressing different genes and type II muscle fibre hypertrophy. Insulin-like growth factor-I has a role in the hypertrophy of muscle fibres through the stimulation of the differentiation of satellite cells. The increased mitochondrial biogenesis via adenosine myophosphate-activated protein kinase is accompanied by the suppression of myofibrillar protein synthesis through pathways mediated by mitogen-activated protein kinases and the nuclear factor kappa B. Insulin-like growth factor-I expression is higher in type I fibres. Myostatin, the expression inhibitor of muscle hypertrophy, is higher in type II fibres. The proteasome-, lysosome- and Ca2+-mediated protein degradation is more intensive in fibres with higher oxidative capacity. Both, oxidative capacity and satellite cells number in muscle fibres play important roles in skeletal muscle regeneration. In this review, we explore the regeneration capacity changes in different types of skeletal muscle fibres in response to resistance, endurance and overtraining.

Glucocorticoid-induced alterations in titin, nebulin, myosin heavy chain isoform content and viscoelastic properties of rat skeletal muscle

Viscoelastic properties of skeletal muscle are associated with a complex network of cytoskeletal proteins where titin and nebulin play a substantial role. The need for evaluation of muscle viscoelastic properties is widely accepted in clinical use to evaluate the effect of treatment or progression of muscle pathology (atrophy). We tested the hypothesis that the viscoelastic properties (elasticity, tone and stiffness) change in atrophied muscles with concomitant changes in cytoskeletal proteins (titin, nebulin) and contractile protein (myosin heavy chain) proportion. Sixteen 24- week-old male rats of the Wistar strain were randomly allocated to two groups: dexamethasone group treated each day for 10 consecutive days with dexamethasone in order to induce atrophy and control group. Skeletal muscle viscoelastic properties (elasticity, tone and stiffness) were determined using a myotonometer. Titin, nebulin and myosin heavy chain content were quantified using SDS-PAGE electrophoresis. We found that glucocorticoid-induced muscle atrophy is accompanied by reduced elasticity and increased tone and stiffness, with concomitant changes in titin, nebulin and myosin heavy chain con- tent. The elasticity decreased by 10.9% (P P < 0.05), and stiffness was significantly lower in dexamethasone group (627.3 N/m vs 758.6 N/m);(P < 0.05). Compared with the control group, the content of titin, nebulin and myosin heavy chain in atrophied muscle was 76.4%, 70.6% and 82.3%, respectively. Our results may lead to a better understanding of the mechanism of muscle atrophy and provide better guidance for rehabilitation practices and help to find rational therapeutic intervention in the future.

Age-Associated Changes in Skeletal Muscle Regeneration: Effect of Exercise

Aim of the present short review is to provide a comprehensive update on age-associated skeletal muscle damage, regeneration, and effect of endurance and resistance type of exercise training on muscle regeneration. Decrease in muscle quantity and quality leads to disability in the aging population. The degradation rate of muscle proteins during aging increased about two times, and muscle strength and motor activity decreased at the same time. Aging induced sarcopenia is a result of decreased synthesis and increased degradation of muscle proteins, which leads to the slower turnover rate of these proteins, especially contractile proteins, and this, in turn, leads to the decrease in muscle strength. Muscle damage is mainly caused by excessive strain in contracting fibre and aging muscle is particularly sensitive to it. The decreased synthesis and increased degradation rate of contractile proteins are in accordance with the increase destructive processes in muscle and lead to the decrease in the regeneration capacity and development of sarcopenia in the elderly. Exercise training increases muscle mass, oxidative capacity, contracile quality, regeneration capacity and via this, physiological functioning of skeletal muscle is improved in the elderly.

Characteristics of myosin isoforms in mammalian skeletal muscle

The distribution of myosin heavy (MyHC) and myosin light chain (MyLC) isoform pattern in horse, rat and human skeletal muscle was investigated to establish relations between them and the role of myosin isoform patterns in mammalian muscle with different twitch characteristics was studied. These two isoforms were separated in a SDS-PAGE gel system, stained using the coomassie and silver staining procedures, and the results were analyzed using a G:BOX system. The relative content of MyHC I isoform in muscle was 2.6 times higher than in human compared to horse muscle (p < 0.001), and 6.3 times higher than in rat muscle (p < 0.001). The relative content of MyHC IIx/d isoform in horse muscle is 2.7 times, and in rat muscle 2.2 times higher in comparison with human muscle (p < 0.001). The role of the MyLC isoform distribution in mammalian skeletal muscle seems to depend on the oxidative capacity of muscles.