DURING STRETCHES OF SINGLE PERMEABILIZED MUSCLE FIBERS FROM OLD COMPARED WITH YOUNG RATS, GREATER HETEROGENIETY IN SARCOMERE LENGTHS PREDICTS GREATER FORCE DEFICITS

Abstract

The focal nature of the damage when muscle fibers are stretched during activation results from heterogeneity in sarcomere length that arises during the contraction. We tested the hypothesis that muscle fibers that are more susceptible to contraction-induced injury, old vs. young, demonstrate greater heterogeneity in sarcomere length and consequently greater injury. An apparatus has been developed that measures spatially averaged sarcomere lengths (diffraction patterns) at five discrete regions along the length of a single fiber segment and the forces developed during experiments. Single permeabilized fiber segments were obtained from soleus muscles of 6 and 28 month old male Wistar rats. Maximally activated single fibers were stretched through a 40% strain at 0.5 fiber lengths/sec. Diffraction patterns were obtained on relaxed and maximally activated fibers, before and after the stretch. The two-fold greater force deficit produced in fibers of old, 20% force deficit, compared with young, 10% force deficit, rats was attributable to mechanical weaknesses associated with increased heterogeniety in sarcomere length. After the stretch, 80% of muscle fibers from young rats had only a single region that lost its diffraction pattern. In contrast, 70% of the fiber segments from muscles of old rats lost diffraction patterns in three or more regions. In electron micrographs of regions that maintained diffraction patterns 4% of the sarcomeres were damaged, whereas in regions that lost the pattern 15% were damaged. Consequently, for whole fiber segments, the number of damaged sarcomeres ranged from 2% to 7% for young rats and from 4% to 15% for old rats. For both groups, force deficits corresponded reasonably well with the number of damaged sarcomeres. Supported by NIA: AG-06157.