Contractile Characteristics of Sarcomeres Arranged in Series or Mechanically Isolated from Myofibrils

Abstract

The mechanisms of contraction are intrinsically connected to -sarcomere mechanics during muscle activation and relaxation. This chapter presents two sets of experiments performed with (1) myofibrils, in which individual sarcomeres in series can be evaluated during contractions, and (2) mechanically isolated sarcomeres. When activated at optimal length (approximately 2.0-2.4 μm), myofibrils and sarcomeres produce similar forces. However, their dependence on length differs: sarcomeres in series in a myofibril are able to produce similar forces at distinct lengths, while isolated sarcomeres show a force-length relation that resembles that obtained in original studies performed with single muscle fibers. Although force in isolated sarcomeres is rapidly stabilized during activation, significant movements of A-band are present when the contraction is produced at optimal lengths, which leads to different dynamics in the two half-sarcomeres. A-band movements decrease linearly with increasing lengths between 2.6 and 3.6 μm. Myofibrils and sarcomeres represent reliable techniques to evaluate contractile mechanisms at the most basic level of muscle organization. However, they present different mechanical characteristics that must be taken into account when scientists evaluate mechanisms of contraction.

Force produced by isolated sarcomeres and half-sarcomeres after an imposed stretch

“…Although the levels of nonuniformities and A-band displacements are larger during the experiment shown in the left, it is also present in the experiment shown in the right. muscle fibers (10,12,17,46) and myofibrils (41) and well described by the predictions of overlap between myosin and actin filaments (32,40). The forces produced by single sarcomeres in the current study are in the range of those observed with myofibrils (23,35,36), single fibers (6), and single sarcomeres (32).…”

“…1). The centroids of the peaks were detected by a minimum average risk (42), as used previously in experiments with isolated sarcomeres (32,40) and myofibrils (31,35,36).…”

“…System for activation of the sarcomeres and myofibrils. All experiments performed in this study used a technique for solution exchange originally developed for working with myofibrils (34) and modified slightly for working with single sarcomeres (32,40). Double-barreled pipettes produced by a vertical puller were polished to reach a final inner diameter of ϳ600 m; each of the two channels with an inner diameter of ϳ300 m. The solution was dragged out of the experimental chamber by an adjustable pump, providing a flow rate in the experimental chamber of ϳ5 l/s.…”

“…Recently, we have developed a preparation to study single, mechanically isolated sarcomeres (32,40), the only preparation that allows mechanical measurements without the influence of SL nonuniformities. Single sarcomeres can be activated repeatedly and reliably (32,40), and the half-sarcomeres can be tracked during the experiments.…”

“…Single sarcomeres can be activated repeatedly and reliably (32,40), and the half-sarcomeres can be tracked during the experiments. In the current study, we expanded the single sarcomere technique to test preparations with predetermined numbers of sarcomeres (n ϭ 1, 2. .…”