Distribution of sarcomere length and intracellular calcium in mouse skeletal muscle following stretch‐induced injury

1 The effect on sarcomere organization of stretching intact single skeletal muscle fibres by 50% of their optimum length (Lo) during ten consecutive short tetani was investigated. Stretch reduced tetanic force to 36 ± 4% of the pre‐stretch condition. Sarcomere organization was analysed using both electron and confocal microscopy. For confocal microscopy the striation pattern was examined by fluorescently staining F‐actin with rhodamine–phalloidin. 2 Electron microscopy revealed that fibres which had been stretched during contraction contained areas of severe sarcomere disorganization, as well as adjacent sarcomeres of normal appearance. 3 Confocal images of stretched fibres, which had been fixed and stained with rhodamine–phalloidin, showed focal regions of overstretched sarcomeres and regions where sarcomeres of adjacent myofibrils were out of alignment with each other. Analysis of all sarcomeres along the length of fibres showed regions of sarcomere inhomogeneity were distributed throughout the fibre length and cross‐section. 4 Fibres were microinjected with the fluorescent [Ca2+]i indicator fura‐2 before being stretched. Conventional wide‐field fluorescence imaging microscopy showed that the tetanic [Ca2+]i was reduced after stretching but remained uniformly distributed. 5 This study confirms the finding that stretch‐induced muscle injury has components caused by disorganization of the myofibrillar array and by failure of tetanic Ca2+ release. The structural damage is spatially heterogeneous whereas the changes in Ca2+ release appear to be spatially homogeneous.