Nuclear DNA fragmentation and morphological alterations in adult rabbit skeletal muscle after short-term immobilization

Nuclear DNA fragmentation and ultrastructural changes, indicative of myonuc lear apoptosis, were examined in adult skeletal muscle in response to short -term immobilization. Adult rabbits were allocated to 2 days (n=5) or 6 day s (n=5) of unilateral casting of the ankle in full plantar flexion or were used as untreated controls (n=2). Atrophy of the soleus muscle was apparent by significant reductions in wet mass of 15% and 26% after 2 days and 6 da ys of casting (P less than or equal to0.05), respectively. Mean fibre cross -sectional area and myonuclear number per section were also lower (17% and 9.1%, respectively) after 6 days of casting, in comparison with contralater al control muscles (P less than or equal to0.05). Electron-microscopic exam ination showed condensed chromatin and irregularly shaped myonuclei in musc les immobilized for either 2 days or 6 days. Myofibrillar disruption and ab normalities of the subsarcolemmal mitochondria were also apparent in the ab sence of inflammation or plasma membrane alterations in cast muscles. Longi tudinal and transverse sections showed abundant in situ end-labelling of DN A strand breaks (TUNEL) after 2 days, with less after 6 days, of immobiliza tion. Positive labelling corresponded to myonuclear locations within fibres , yet the number of TUNEL-positive nuclei indicated DNA fragmentation in ad ditional cell types such as capillary endothelial cells or fibroblasts. The data indicate that the immobilization of slow-twitch skeletal muscle in a shortened position rapidly induces morphological alterations consistent wit h mitochondrial injury and apoptotic myonuclear elimination.