Dynamic behaviour of half‐sarcomeres during and after stretch in activated rabbit psoas myofibrils: sarcomere asymmetry but no ‘sarcomere popping’

We examined length changes of individual half‐sarcomeres during and after stretch in actively contracting, single rabbit psoas myofibrils containing 10–30 sarcomeres. The myofibrils were fluorescently immunostained so that both Z‐lines and M‐bands of sarcomeres could be monitored by video microscopy simultaneously with the force measurement. Half‐sarcomere lengths were determined by processing of video images and tracking the fluorescent Z‐line and M‐band signals. Upon Ca 2+ activation, during the rise in force, active half‐sarcomeres predominantly shorten but to different extents so that an active myofibril consists of half‐sarcomeres of different lengths and thus asymmetric sarcomeres, i.e. shifted A‐bands, indicating different amounts of filament overlap in the two halves. When force reached a plateau, the myofibril was stretched by 15–20% resting length ( L 0 ) at a velocity of ∼0.2 L 0 s −1 . The myofibril force response to a ramp stretch is similar to that reported from muscle fibres. Despite the ∼2.5‐fold increase in force due to the stretch, the variability in half‐sarcomere length remained almost constant during the stretch and A‐band shifts did not progress further, independent of whether half‐sarcomeres shortened or lengthened during the initial Ca 2+ activation. Moreover, albeit half‐sarcomeres lengthened to different extents during a stretch, rapid elongation of individual sarcomeres beyond filament overlap (‘popping’) was not observed. Thus, in contrast to predictions of the ‘popping sarcomere’ hypothesis, a stretch rather stabilizes the uniformity of half‐sarcomere lengths and sarcomere symmetry. In general, the half‐sarcomere length changes (dynamics) before and after stretch were slow and the dynamics after stretch were not readily predictable on the basis of the steady‐state force–sarcomere length relation.