Magnitude of sarcomere extension correlates with initial sarcomere length during lengthening of activated single fibers from soleus muscle of rats
Appaji Panchangam 1, Dennis R. Claflin 1, Mark L. Palmer 1 and John A. Faulkner 2*
1 University of Michigan
2 University of Michigan Medical School Inst. of Gerontology
* To whom correspondence should be addressed. E-mail: jafaulk{at}umich.edu.
Submitted on July 24, 2007
Revised on September 10, 2007
Accepted on 16 April 2008
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Abstract |
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A laser-diffraction technique was developed that reported rapidly the lengths of sarcomeres (Ls) in serially-connected sectors of permeabilized single fibers. The apparatus translated a laser beam along the entire length of a fiber segment within 2 ms, with brief stops at each of 20 contiguous sectors. We tested the hypothesis that during lengthening contractions, when maximally activated fibers are stretched, sectors that contain the longer sarcomeres undergo greater increases in Ls than those containing shorter sarcomeres. Fibers (n = 16) were obtained from soleus muscles of adult male rats and middle portions, length 1.05 ± 0.11 mm (mean ± SD), were investigated. Single stretches of strain 27% and a strain rate of 54% s-1 were initiated at maximum isometric stress and resulted in a 19 ± 9 % loss in isometric stress. The data on Ls revealed that: (1) the stretch was not distributed uniformly among the sectors, and (2) during the stretch, sectors at long Ls prior to the stretch elongated more than those at short lengths. The findings support the hypothesis that during stretches of maximally-activated skeletal muscles, sarcomeres at longer lengths are more susceptible to damage by excessive strain.
Key Words:
contraction-induced injury, laser diffraction, lengthening contraction, sarcomere length, sarcomere length non-uniformity
