Measurement of rate constants for the contractile cycle of intact mammalian muscle fibers.

Small, random length changes were applied to bundles of intact fibers from rat and mouse extensor digitorum longus (EDL) and soleus muscles, while they were being tetanically stimulated. With increasing frequency of length changes, EDL muscle stiffness (tension change per unit change in length) increased, then decreased and increased again. The decrease was not seen in the soleus muscles. The EDL frequency-response could be well fitted by three exponential components with apparent rate constants of approximately 25, 150, and 500 s-1 at 20 degrees C. All rate constants increased steadily with temperature and for each 10 degrees C increase in temperature, the rates in the mouse EDL increased by a factor (Q10) between 1.8 and 2.4. With tetanic stimulation, force increased nearly exponentially to a steady level with a rate constant of 24 s-1 at 20 degrees C in mouse EDL muscles, and a Q10 of 2.4. These values correspond closely to the lowest frequency rate constant measured with length perturbations, which suggests that this process may limit the rate of rise of force in intact muscle fibers. During fatigue the high frequency and intermediate frequency rate constants declined, but the low frequency rate constant remained unchanged. These results are discussed in relation to current biochemical models for cross-bridge cycling.