THE DISRUPTION OF MYOFIBER STRUCTURES IN RAT SKELETAL-MUSCLE AFTER FORCED LENGTHENING CONTRACTIONS

Specific antibodies against structural proteins (actin, desmin, dystro phin, fibronectin) of muscle fibres were used to study the effect of f orced lengthening contractions on muscle microarchitecture. Tibialis a nterior (TA) muscle of male Wistar rats were subjected to 240 forced l engthening contractions. At consecutive time points (0, and 6 h, 2, 4, and 7 days) after stimulation, the TA muscle was excised for biochemi cal and histological assays. beta-Glucuronidase activity, a quantitati ve indicator of muscle damage, showed increased values 2-7 days after the lengthening, peaking on day 4 (11.7-fold increase). A typical cour se of histopathological changes (myofibre swelling, necrosis and regen eration) was observed. In immunohistochemistry, the earliest abnormali ty observed was discontinuous dystrophin staining in some swollen fibr es immediately after commencement of exercise, while at the same time no alterations occurred in the staining of the other antibodies studie d. Six hours later, all the swollen fibres were uniformly desmin as we ll as dystrophin negative. The great majority, but not all, of the swo llen fibres showed disorganized actin staining and intramyocellular lo calization of fibronectin. The early phase disruption of myofibre stru ctures as measured in this study provides evidence of their central ro le following damage in skeletal muscle. These results suggest that the sequence of structural changes in the route to muscle fibre necrosis in injury induced by forced lengthening contraction originates in the disruption of the plasma membrane and the intermediate filament, which leads to disturbances in the myofibrillar system.