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 sk eletal muscle fibres by 50% of their optimum length (Lb) during ten co nsecutive 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 mi croscopy the striation pattern was examined by fluorescently staining F-actin with rhodamine-phalloidin. 2. Electron microscopy revealed tha t fibres which had been stretched during contraction contained areas o f 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 sarc omeres along the length of fibres showed regions of sarcomere inhomoge neity 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 im aging microscopy showed that the tetanic [Ca2+](i) was reduced after s tretching but remained uniformly distributed. 5. This study confirms t he finding that stretch-induced muscle injury has components caused by disorganization of the myofibrillar array and by failure of tetanic C a2+ release. The structural damage is spatially heterogeneous whereas the changes in Ca2+ release appear to be spatially homogeneous.