[Interrelationship between the striated structure of muscle and the distribution of calcium in it. I. The changes in the protofibrillary structure of the frog fast muscle fibers in steady potassium contracture].

The ultrastructure of protofibrillar apparatus of frog fast fibers was studied during a prolonged contracture initiated with KSCN. SCN anion inhibited Ca-pump, which resulted in maintaining a high intracellular Ca2+-concentration. It was suggested that stable connections between action and myosin filaments were formed within the region of overlapping. Under these conditions, thick and thin filaments were able to change their length from 1.3 to 1.9 micrometer, and from 0.83 to 1.2 micrometer, correspondingly, according to the sarcomere length. The structural non-uniformity of I-band was observed in contracted sarcomeres: a low electron density zone 30--40 nm wide was formed at the I/A band boundary.