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 approximately 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 < or = pCa < or = 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 submaximal Ca2+ activations and suggest that the rate of tension recovery is limited primarily by the availability of actin binding sites.
Campbell
Kenneth S
KS
Department of Physiology, University of Kentucky, Lexington, Kentucky, USA. k.s.campbell@uky.edu
D000818:Animals; D002118:Calcium / Q000502:physiology*; D005260:Female; D007537:Isometric Contraction / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010539:Permeability; D051381:Rats; D017207:Rats, Sprague-Dawley; D012518:Sarcomeres / Q000502:physiology
D002118:Calcium