Calcium-induced weakening of Z-disks in postmortem skeletal muscle.

The reaction mechanism of calcium ion in the postmortem weakening of Z-disks was studied in myofibrils prepared from fresh or stored muscles. The alpha-actinin content in myofibrils remained almost unchanged within 10 days postmortem, showing very limited proteolysis of myofibrils during postmortem storage of muscles at 10 degrees C. The postmortem weakening of Z-disks was markedly dependent on muscle pH, showing a minimum at pH 6.5. These results agree well with the calcium-induced weakening of Z-disks of freshly isolated myofibrils, indicating that no protease participates in the postmortem weakening of Z-disks. Z-Disks of myofibrils prepared from stored muscles split into halves after treatment with 0.1 N NaOH for 5 min. The identical splitting of Z-disks was induced by a calcium ion concentration of 10(-4)M, which is of the same order of magnitude as that in the sarcoplasm in postmortem muscle. We therefore conclude that the postmortem weakening of Z-disks is non-enzymatically induced by the raised sarcoplasmic calcium ion concentration of 10(-4) M. Calcium ions probably solubilize the amorphous cementing material of Z-disks, leaving unchanged the two sets of Z filaments composed of alpha-actinin.