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Abstract:The aims of the present study were to show the distribution of individual myosin light chain (MyLC) isoforms in fast-twitch (FT) and slowtwitch (ST) muscles and between FT muscles in order to find differences between MyLC isoforms in these muscles, to identify similarities with the distribution of myosin heavy chain (MyHC) isoforms and to investigate changes in these relations during adaptation to endurance and resistance training. Male Wistar strain rats were used in this study. One-dimensional electrophoresis was used for separation of MyHC and MyLC isoforms and two-dimensional electrophoresis was used for identification of MyLC different isoforms. A difference in the relative content of MyLC isoforms between FT muscles exists only in the case of MyLC 1 slow and 2 slow isoforms. Differences in the relative content of MyHC between FT muscles are considerably larger than differences in the MyLC isoforms. MyHC and MyLC isoforms both participate in the remodelling of contractile proteins during exercise training. In conclusion: The present study shows some discrepancy between the modulation of MyHC and MyLC isoforms in muscles with different oxidative potential during adaptation to endurance and resistance training. In ST muscles, there is full agreement between the increase in the relative content of MyHC IIa isoform and MyLC 2 fast and 3 fast isoforms during resistance training without significant changes during endurance training. The ratio of MyLC 3 and MyLC 2 isoforms increases during both types of exercise training, but it is two times higher during resistance training.(Biol.Sport 26:215-234, 2009)

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Background: Aging leads to changes in skeletal muscle quantity and quality and is accompanied with increase in body mass and fat mass, whereas fat-free mass either decreases or remains unchanged. The body composition of rodents has been an important factor for clinical trials in the laboratory. Glucocorticoids such as dexamethasone are widely used in clinical medicine, but may induce myopathy, characterized by muscle weakness, atrophy, and fatigue. In animals treated with glucocorticoids, a dose-dependent reduction of body weight has been observed. This weight loss is usually followed by muscle atrophy and a reduction of several muscle proteins, contributing to impaired muscle function. This study was designed to describe changes in body composition and BMC of 22-month-old rats during 10- and 20-day recovery period after 10-day dexamethasone administration. Method: Data on body mass, lean body mass, fat mass and bone mineral content of the rats were obtained with dual energy X-ray absorptiometry scan. Result: Significant reduction in body mass, lean body mass, fat mass and fast-twitch muscle mass was observed after dexamethasone treatment. Body mass, fat mass and fast-twitch muscle mass stayed decreased during 20 days after terminating the hormone administration; lean body mass reached the preadministration level after 20-day recovery period. There were no significant changes in bone mineral density during the recovery period. Dexamethasone treatment gradually reduced hindlimb grip strength that also stayed decreased during the 20-day recovery period. Conclusion: his study demonstrated that a 10-day period of overexprosure to glycocorticoids induced longlasting changes in old rats’ body composition and these values did not attain the baseline level even after 20-day recovery period.

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