Skeletal myofibrils isolated either by tryptic digestion at 0° C. or by a colloid mill and suspended in buffer solution (pH 7.0, µ; 0.154) containing 20 per cent glycerin and 0.0025 M adenosinetriphosphate at –5° C. contracted slowly and progressively when the temperature was raised above 0° C. Formalin fixation halted this contraction. With the aid of these procedures myofibrils in progressive stages of contraction were then studied with the electron microscope.

Electron micrographs showed that uncontracted fibrils isolated by the colloid mill were structurally similar to those described by other workers. Treatment of fibrils with trypsin removed the Z bands and disorganized the I bands. This enzymatic modification of structure did not impair the contractile response. The principal structural changes during contraction consisted of a migration of dense material from the A band into the A-I junction or the Z band, a gradual increase in width of the fibril, a gradual decrease in length of sarcomeres, an apparent increase in the mean diameter of filaments, and a disorientation of these latter from their parallel arrangement.

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