Extracellular matrix and myofibrils during unloading and reloading of skeletal muscle.

The aim of the study was to elucidate the effect of unloading and reloading on the collagen expression and synthesis rate of myofibrillar proteins in fast-twitch (FT) muscle in relation to changes in muscle strength and motor activity. Northern blot analysis was used for testing the specificity of cDNA probes and protein synthesis rate was measured according to incorporation of radioactive leucine into different protein fractions. Unloading depresses collagen type I and III (p<0.001), type IV (p<0.05) and reloading enhances collagen expression in fast-twitch skeletal muscle in comparison with unloading. Enhanced expression of matrix metalloproteinase-2 continued during the first week of reloading (p<0.01) and tissue inhibitor of metalloproteinase-2 during reloading (p<0.05). Changes in collagen expression in FT muscle are in good agreement with changes in myofibrillar protein synthesis during unloading and reloading. In conclusion alterations in extracellular matrix and myofibrillar apparatus in FT skeletal muscle are related to changes in muscle strength and motor activity, are significant in exercise training and determination of recovery periods in the training process as well as in athletes' rehabilitation.