Synchronous behavior of spontaneous oscillations of sarcomeres in skeletal myofibrils under isotonic conditions

An isotonic control system for studying dynamic properties of single myofibrils was developed to evaluate the change of sarcomere lengths in glycerinated skeletal myofibrils under conditions of spontaneous oscillatory contraction (SPOC) in the presence of inorganic phosphate and a high ADP- to-ATP ratio. Sarcomere length oscillated spontaneously with a peak-to-peak amplitude of about 0.5 μm under isotonic conditions in which the external loads were maintained constant at values between 1.5 x 10⁴ and 3.5 x 10⁴ N/m². The shortening and yielding of sarcomeres occurred in concert, in contrast to the previously reported conditions (isometric or auxotonic) under which the myofibrillar tension is allowed to oscillate. This synchronous SPOC appears to be at a higher level of synchrony than in the organized state of SPOC previously observed under auxotonic conditions. The period of sarcomere length oscillation did not largely depend on external load. The active tension under SPOC conditions increased as the sarcomere length increased from 2.1 to 3.2 μm, although it was still smaller than the tension under normal Ca²⁺ contraction (which is on the order of 10⁵ N/m²). The synchronous SPOC implies that there is a mechanism for transmitting information between sarcomeres such that the state of activation of sarcomeres is affected by the state of adjacent sarcomeres. We conclude that the change of myofibrillar tension is not responsible for the SPOC of each sarcomere but that it affects the level of synchrony of sarcomere oscillations.