Contractile Properties of Half-Sarcomeres Mechanically Isolated from Skeletal Muscle Myofibrils

Abstract

The goal of this study was to develop a system to experiment with half-sarcomeres mechanically isolated from skeletal muscle. Single myofibrils from rabbit psoas were transferred into a temperature-controlled (10°C) bath. Half-sarcomeres were isolated using two pre-calibrated glass microneedles; the first pierced externally adjacent to the Z-line, and the second internally, adjacent to the M-line. The force produced during activation of the half-sarcomere was measured by tracking the displacement of the microneedles. The half-sarcomere length (HSL) was obtained by interpolating the displacement of the needles from the initial to the final distances measured from the Z-line to the center of the sarcomere. Half-sarcomeres (n=12) produced a stress of 24.0 ±3.7 nN/um2 at HSL between 1.0 and 1.4µm. The result was comparable to that observed in isolated sarcomeres (25.5 ±3.1 nN/um2, at SL between 1.8 and 3.0µm). Preliminary trials in which we imposed stretches (ranging from 12 to 33% of the initial HSL/SL, at a speeds of 1.35-3.15 μm·s−1·HSL−1) (n = 6 half-sarcomeres; n = 13 single sarcomeres) during activation show an increase in force during and after stretch, which was long lasting (i.e. residual force enhancement). Despite a high variability, half-sarcomeres showed a tendency to produce a smaller level of residual force enhancement when compared to single sarcomeres. Our results show that half-sarcomeres generate levels of isometric force that are similar to those observed in sarcomeres and myofibrils. However, half-sarcomeres produced a lower level of stretch-induced force enhancement when compared to sarcomeres.