Mechanical Properties of Individual Sarcomeres Isolated From Skeletal Muscles

Abstract

Mechanical properties of skeletal muscles have been investigated with muscle cells and myofibrils, preparations in which large sarcomere length non-uniformities are observed. The purpose of this study was to investigate the dependence of force on length of isolated sarcomeres. Myofibrils were dissected from rabbit psoas muscles and one sarcomere was selected for experimentation. Two pre-calibrated micro-needles (stiffness: 200 - 377 nm/μm) controlled by micromanipulators were used to capture the sarcomere, a few nanometers externally adjacent to each Z-line. One micro-needle was attached to a motor that is used for inducing fine, computer-controlled length changes. The sarcomere was set at a length between 1.48 and 3.48μm, and was activated using an automated perfusion system. The force produced by the sarcomeres was determined by the deflection of the micro-needles (force = K1d1 + K2d2, where K = stiffness, d = displacement, 1 and 2 = micro-needles 1 and 2, respectively). During activation, sarcomeres shortened by 0.34 ± 0.01μm (mean ± SEM). The amount of shortening showed a weak dependence of initial length (r2=0.15). The forces produced by sarcomeres contracting between 2.26 and 2.43μm, the plateau of the theoretical force-length (FL) relation, was 123.07 ± 8.16 nN (mean ± SEM), comparable to previous studies with myofibrils. Forces along the ascending limb (from 1.27 to 2.26 μm) followed the predictions of the theoretical FL relation, but forces along the descending limb (between 2.43 and 3.91μm) were higher than those predicted by the theoretical FL relation, especially at sarcomeres beyond 3.0μm; a result that needs further examination. The single sarcomere technique represents a reliable method to evaluate mechanical properties of striated muscles, and the FL relation may be investigated without confounding effects arising from sarcomere non-uniformity and instability.