Molecular Properties of Post‐Mortem Muscle. 3. Electron Microscopy of Myofibrils

SUMMARY— A study was made of the fine structure of myofibril suspensions prepared from seven heifers immediately after death and after various times post‐mortem. Studies on myofibrils sampled immediately after death showed that sucrose isolation gave the best structural preservation as indicated by maintenance of Z‐line structure. Although the appearance of resting muscle was maintained in both sucrose and KCI preparations, several myofibrils from the KCI‐treated preparations showed stretched sarcomeres. Glycerol‐treated myofibrils usually had shorter sarcomere lengths than myofibrils prepared with the other two solvents. Although fibrillar preservation seemed adequate when glycerol was used, Z‐line structure was seldom well‐preserved with glycerol. Myofibrils from muscle sampled 24 hr post‐mortem at 2°C were supercontracted with thick filaments pushed against or through the Z‐line, and no trace of l‐bands remained. Myofibrils from muscle sampled 24 hr post‐mortem at 16°C were contracted, but to a much lesser extent than 2°C‐24 hr myofibrils. Storage at 2°C for 312 hr after death resulted in myofibrils that were contracted and that were structurally in a much poorer state of preservation than their 16°C counterparts. The 16°C‐312 hr myofibrils were slightly contracted as indicated by the absence of H‐zones and the presence of prominent, although narrowed, I‐bands. All observations showed that shortening accompanying rigor mortis caused changes in banding patterns similar, and probably identical, to those predicted by Huxley's sliding filament model for contracting muscle.