SARCOMERE DYNAMICS AND CONTRACTION-INDUCED INJURY TO MAXIMALLY ACTIVATED SINGLE MUSCLE-FIBERS FROM SOLEUS MUSCLES OF RATS

1. The focal nature of contraction-induced injury to skeletal muscle f ibres may arise from heterogeneities in sarcomere length that develop during contractions. We tested the hypothesis that when a maximally ac tivated single permeabilized fibre segment is stretched and a deficit in maximum isometric force (force deficit) is produced, the regions of sarcomeres with the longest lengths prior to tile stretch contain the majority of the damaged sarcomeres when the fibre is returned to opti mum length (L-o) after the stretch. 2. Single fibre segments (n = 16) mere obtained from soleus muscles of rats. Average sarcomere length at five discrete positions along the length of each fibre was determined by lateral deflection of a diode laser spot. Diffraction patterns wer e obtained while fibres were relaxed and immediately before, during an d after a single stretch of 40% strain relative to L-o. Following the stretch, the regions of each fibre that potentially contained damaged sarcomeres were identified by an increased scatter of the first-order diffraction patterns. The damage was confirmed by light and electron m icroscopy. 3. While single fibre segments were in relaxing solution, t he mean value for all of the average sarcomere lengths sampled (n = 80 ) was 2.53 +/- 0.01 mu m (range, 2.40-2.68 mu m). During the maximum i sometric contraction before each stretch, the mean sarcomere length de creased to 2.42 +/- 0.02 mu m and the range increased to 2.12 - 3.01 m u m. 4. During the stretch of 40% strain, all regions of sarcomeres we re stretched onto the descending limb of the length-force curve, but s arcomere lengthening was non-uniform. After the stretch, when the maxi mally activated fibres were returned to L-o, the force deficit was 10 +/- 1%. Microscopic evaluation confirmed that the regions with the lon gest sarcomere lengths before the stretch contained the majority of th e damaged sarcomeres after the stretch. We conclude that when heteroge neities in sarcomere length develop in single permeabilized fibre segm ents during a maximum isometric contraction, the sarcomeres in the reg ions with the longest lengths are the most susceptible to contraction- induced injury.