Fragmin was able to reduce the isometric tension of Physarum actomyosin threads to 15-30% of the control tension at the Ca2+ concentrations greater than 10(-6) M. However, fragmin had no effect on the tension of threads when the Ca2+ concentration was lowered below 10(-7) M. The tension once reduced by fragmin could not be recovered by the removal of Ca2+. The remaining tension was shown to be still active from the experiment with quick release or stretch of the thread. This tension reduction is parallel to the decrease in viscosity of F-actin solution by fragmin. Electron microscopy showed that F-actin filaments became shorter in the thread after the tension was reduced by fragmin. Therefore, the severing of F-actin by fragmin in micromolar concentration of calcium resulted in the relaxation of tension by actomyosin threads.

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