Volume changes of the myosin lattice resulting from repetitive stimulation of single muscle fibers.

Single muscle fibers at 1 degreesC were subjected to brief tetani (20 Hz) at intervals of between 20 s and 300 s over a period of up to 2 h. A band lattice spacing increased during this period at a rate inversely dependent on the rest interval between tetani. Spacing increased rapidly during the first 10 tetani at a rate equivalent to the production of 0.04 mOsmol.liter-1 of osmolyte per contraction, then continued to expand at a much slower rate. For short rest intervals, where lattice expansion was largest, spacing increased to a limiting value between 46 and 47 nm (sarcomere length 2.2 micrometer), corresponding to accumulation of 30 mOsmol.liter-1 of osmolytes, where it remained constant until repetitive stimulation was terminated. At this limiting spacing, force was reduced by up to 30%. The effect of lattice swelling on the lattice compression that accompanies isometric force recovery from unloaded shortening was to increase the compression, similar to that observed in hypotonic media at a similar spacing. During recovery from repetitive stimulation, spacing recompressed to its original value with a half-time of 15-30 min. These findings suggest that mechanical activity produces an increase in osmotic pressure within the cell as a result of product accumulation from cross-bridge and sarcoplasmic reticulum ATPases and glycolysis.