Tension recovery in permeabilized rat soleus muscle fibers after rapid shortening and restretch

Permeabilized rat soleus muscle fibers were subjected to rapid shortening/restretch protocols ( 20% muscle length, 20 ms duration) in solutions with pCa values ranging from 6.5 to 4.5. Force redeveloped after each restretch but temporarily exceeded the steady-state isometric tension reaching a maximum value similar to 2.5 s after relengthening. The relative size of the overshoot was < 5% in pCa 6.5 and pCa 4.5 solutions but equaled 17% +/- 4% at pCa 6.0 ( approximately half-maximal Ca2+ activation). Muscle stiffness was estimated during pCa 6.0 activations by imposing length steps at different time intervals after repeated shortening/restretch perturbations. Relative stiffness and relative tension were correlated ( p < 0.001) during recovery, suggesting that tension overshoots reflect a temporary increase in the number of attached cross-bridges. Rates of tension recovery ( k(tr)) correlated ( p < 0.001) with the relative residual force prevailing immediately after restretch. Force also recovered to the isometric value more quickly at 5.7 <= pCa <= 5.9 than at pCa 4.5 ( ANOVA, p < 0.05). These results show that ktr measurements underestimate the rate of isometric force development during submaximal Ca2+ activations and suggest that the rate of tension recovery is limited primarily by the availability of actin binding sites.