Laser diffraction studies of sarcomere dynamics during 'isometric' relaxation in isolated muscle fibres of the frog

Abstract

1. A study has been made of changes in sarcomere length and tension which occur during relaxation from isometric (;fixed ends') tetani in isolated muscle fibres of the frog. Sarcomere lengths were calculated from measurements of the separation of the zero-to-first-order intensity maxima in diffraction patterns generated by illuminating small segments of fibre with a He-Ne laser. Diffraction spectra were recorded continuously on cine-film using the method of ;streak' photography.2. Many sarcomeres in a muscle fibre are found to undergo active shortening during relaxation, at the expense of others located elsewhere which become passively extended. The time of onset of changes in sarcomere length coincides with the well known ;shoulder' on the tension record, and their amplitude is maximal at the time when isometric force approaches zero. The original pattern of sarcomere lengths is only re-established after tension has disappeared.3. The variability in the pattern of sarcomere length changes during relaxation was studied in a sample of twenty fibres, by making ;streak' recordings from successive 1 mm segments along the entire length of each fibre. This survey showed that segments which elongate are located predominantly, though not exclusively, close to the fibre ends. The fractional length of the fibre which underwent shortening was found to vary considerably in different preparations. In many ;streak' recordings the first order line fragmented into several distinct intensity maxima.4. The effects of varying the initial fibre length on the time course of the tension decay and on the accompanying changes in sarcomere length were studied. The range of fibre lengths investigated corresponded with sarcomere spacings of 1.9-3.2 mum. The rate constant, R, for the exponential (later) component of the tension decay decreased with increasing fibre length and this was accompanied by a reduction in the mean amplitude of sarcomere shortening [Formula: see text] and lengthening [Formula: see text]. However, the time interval during which sarcomeres shortened or elongated increased, and so the mean velocities of sarcomere length changes ([Formula: see text] and [Formula: see text]) also displayed an inverse dependence upon fibre length.5. The effects of altering temperature (0-20 degrees C) on the time course of the changes in sarcomere length and tension were investigated. The time to onset of the length changes, t(1), the time to maximum amplitude, t(2), and the time to full recovery, t(3), all decreased exponentially with increasing temperature, but to differing extents. Temperature co-efficients for the velocity of sarcomere length changes during the interval t(2) - t(1) and for the fast tension decay were 2.47 ([Formula: see text]), 2.54 ([Formula: see text]) and 2.45 (R). The rate of the slow tension decay (before the tension ;shoulder') also increased with temperature, with a Q(10) of 2.43.6. The complex patterns of sarcomere length changes seen during relaxation appear to be due to variations in the duration of mechanical activity in different fibre segments. A model is presented which shows that the characteristic form of the tension decay can be accounted for by the progressive emergence of local inequalities in the relative strengths of adjacent sarcomeres, as relaxation proceeds.