Tension recovery in permeabilized rat soleus muscle fibers following rapid shortening and re-stretch

¹ University of Kentucky

^* To whom correspondence should be addressed. E-mail: k.s.campbell{at}uky.edu.

Submitted on May 26, 2005
Revised on July 14, 2005
Accepted on 27 October 2005

	Abstract

Permeabilized rat soleus muscle fibers were subjected to rapid shortening/re-stretch protocols (0.2 l₀, 20 ms duration) in solutions with pCa values ranging from 6.5 to 4.5. Force redeveloped after each re-stretch but temporarily exceeded the steady-state isometric tension reaching a maximum value ~2.5 s after re-lengthening. The relative size of the overshoot was less than 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 following repeated shortening/re-stretch 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 residual force prevailing immediately after re-stretch. 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 k_tr measurements underestimate the rate of isometric force development during sub-maximal Ca²⁺ activations and suggest that the rate of tension recovery is limited primarily by the availability of actin binding sites.

Key Words: cross-bridge, muscle mechanics, stiffness

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