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This Article Full Text Full Text (PDF) All Versions of this Article: 295/1/C249    most recent 90640.2007v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Lynch, G. S. Articles by Brooks, S. V. PubMed PubMed Citation Articles by Lynch, G. S. Articles by Brooks, S. V.

MUSCLE CELL BIOLOGY AND CELL MOTILITY

Force deficits and breakage rates after single lengthening contractions of single fast fibers from unconditioned and conditioned muscles of young and old rats

¹Basic and Clinical Myology Laboratory, Department of Physiology, The University of Melbourne, Victoria, Australia; and ²Departments of Molecular and Integrative Physiology and Biomedical Engineering, The University of Michigan, Ann Arbor, Michigan

Submitted 19 December 2007 ; accepted in final form 22 May 2008

The deficit in force generation is a measure of the magnitude of damage to sarcomeres caused by lengthening contractions of either single fibers or whole muscles. In addition, permeabilized single fibers may suffer breakages. Our goal was to understand the interaction between breakages and force deficits in "young" and "old" permeabilized single fibers from control muscles of young and old rats and "conditioned" fibers from muscles that completed a 6-wk program of in vivo lengthening contractions. Following single lengthening contractions of old-control fibers compared with young-control fibers, the twofold greater force deficits at a 10% strain support the concept of an age-related increase in the susceptibility of fibers to mechanical damage. In addition, the much higher breakage rates for old fibers at all strains tested indicate an increase with aging in the number of fibers at risk of being severely injured during any given stretch. Following the 6-wk program of lengthening contractions, young-conditioned fibers and old-conditioned fibers were not different with respect to force deficit or the frequency of breakages. A potential mechanism for the increased resistance to stretch-induced damage of old-conditioned fibers is that, through intracellular damage and subsequent degeneration and regeneration, weaker sarcomeres were replaced by stronger sarcomeres. These data indicate that, despite the association of high fiber breakage rates and large force deficits with aging, the detrimental characteristics of old fibers were improved by a conditioning program that altered both sarcomeric characteristics as well as the overall structural integrity of the fibers.

contraction-induced injury; permeabilized single fibers; aging