Role of calcium and cross bridges in determining rate of force development in frog muscle fibers.

The influence of Ca2+ and force-generating cross bridges on the kinetics of force development was examined in skinned frog muscle fibers activated by photolytic release of Ca2+ from caged Ca2+ at 10 degrees C. Isometric force development was fit by a double exponential equation with rates of 44.4 s-1 (kc1) and 6.1 s-1 (kc2); kc1 was not significantly different from the rate of force development observed in intact fibers. Maximum activation by caged Ca2+ from preexisting submaximal force produced rates of contraction similar to those observed with maximum activation from zero force. Decreasing the Ca2+ level to an extent that resulted in 50% of maximum force development produced an approximately sevenfold decrease in kc1 and no change in kc2. Partial extraction of troponin C reduced kc1 only slightly (by 16%), whereas decreasing the number of force-generating cross bridges by vanadate did not decrease kc1. Neither treatment altered kc2. Thus the rate of force development increases dramatically with increases in Ca2+ level.