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

Single muscle fibers at 1 degrees C 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 l attice spacing increased during this period at a rate inversely dependent o n the rest interval between tetani. Spacing increased rapidly during the fi rst 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 r ate. For short rest intervals, where lattice expansion was largest, spacing increased to a; limiting value between 46 and 47 nm (sarcomere length 2.2 mu m), corresponding to accumulation of 30 mOsmol.liter(-1) of osmolytes, w here it remained constant until repetitive stimulation was terminated. At t his limiting spacing, force was reduced by up to 30%. The effect of lattice swelling on the lattice compression that accompanies isometric force recov ery from unloaded shortening was to increase the compression, similar to th at observed in hypotonic media at a similar spacing. During recovery from r epetitive stimulation, spacing recompressed to its original value with a ha lf-time of 15-30 min. These findings suggest that mechanical activity produ ces an increase in osmotic pressure within the cell as a result of product accumulation from cross-bridge and sarcoplasmic reticulum ATPases and glyco lysis.