DNA END-LABELLING AND MORPHOLOGICAL CHANGES IN SKELETAL MUSCLE AFTER SHORT-TERM IMMOBILIZATION

Abstract

200 A reduced contractile activity of skeletal muscle induces cellular atrophy and tissue remodelling. It was the objective of this study to examine the nature of the myonuclear loss associated with fibre atrophy, and other morphological changes, induced by short-term immobilization of skeletal muscle. Adult rabbits were allocated to 2 (n = 5), or 6 (n = 5), days of unilateral cast immobilization of the ankle in full plantar flexion, or were used as untreated controls (n = 2). Soleus muscle sections were examined by light microscopy and image analysis following in situ enzyme-mediated end-labelling of nuclear DNA strand breaks (TUNEL), and by transmission electron microscopy. Muscle mass and fibre cross-sectional areas were reduced by 12% and 14%, respectively, after 2 days, and by 26% and 29%, respectively, after 6 days of immobilization, in comparison to non-casted soleii (all p < 0.05). Myonuclear numbers per transverse section were significantly lower after 6 days of casting. Nuclei-to-cross-sectional area ratios did not differ between casted and control muscles. Longitudinal, and transverse, sections showed abundant TUNEL after 2 days, with less after 6 days, of immobilization. Positive labelling corresponded to myonuclear locations within fibres, yet additional TUNEL-positive nuclei indicated DNA fragmentation in endothelial cells, fibroblasts, and/or inflammatory cells. Electron microscopic examination showed condensed chromatin and irregular shape, predominantly in capillary endothelial nuclei, and, less frequently, in myonuclei, of muscles immobilized for either 2 or 6 days. The immobilization of slow-twitch skeletal muscle in a shortened position rapidly induces morphological alterations indicative of endothelial cell and myonuclear apoptosis. Funded by U Auckland Research Committee & Health Research Council (NZ).