The mechanical behavior of individual sarcomeres of myofibrils isolated from rabbit psoas muscle

Abstract

The goal of this study was to develop a system to experiment with sarcomeres mechanically isolated from skeletal muscles. Single myofibrils from rabbit psoas were transferred into a temperature-controlled (22°C or 15°C) experimental chamber, and sarcomeres were isolated using precalibrated glass microneedles that were pierced externally, adjacent to the Z-lines. The force produced during activation was measured by tracking the displacement of the microneedles, and the sarcomere and half-sarcomere changes were measured by continuously tracking the Z-lines and A-bands position during the experiments. Sarcomeres produced a stress (force/cross-sectional area) of 112.75 ± 4.96 nN/μm2 (15°C) and 128.47 ± 5.58 nN/μm2 (22°C) at lengths between 2.0 μm and 2.4 μm. The descending limb was fitted with linear regression for length between 2.4 μm and 3.5 μm, which provided an abscissa extrapolating to 3.87 μm. The force-length relation was remarkably similar to a theoretical curve based on the degree of filament overlap. During sarcomere activation, we tracked the distance between the center of the A-band and the Z-lines. At lengths below 1.6 μm, movements of A-band were not detected. A-band movements increased with length to achieve a maximum displacement of 59.40 ± 10.1 nm from the center at 2.0 μm–2.4 μm. A-band displacement decreased linearly in sarcomere lengths between 2.6 μm and 3.6 μm. A technique for monitoring force and length in single sarcomeres isolated from myofibrils represents a reliable technique to evaluate contractile mechanisms at the most basic, intact level of muscle organization, opening the possibility to clarify long-standing issues in the field of muscle contraction.