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

Abstract

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.

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

“…Nonuniformity in half-sarcomere lengths induced at the beginning of activation would be increased as the thick filaments (A-band) would be pulled toward one end of the sarcomere. In fact, half-sarcomere nonuniformities have been observed previously in myofibrils (44,45) and isolated sarcomeres (32). Assuming that the nonuniformity will increase with stretch (5,45), it could lead to levels of force significantly different from that produced during isometric contractions.…”

“…In fact, half-sarcomere nonuniformities have been observed previously in myofibrils (44,45) and isolated sarcomeres (32). Assuming that the nonuniformity will increase with stretch (5,45), it could lead to levels of force significantly different from that produced during isometric contractions. When we tested preparations with at least two sarcomeres in series, we observed a strong relation between the magnitude of A-band displacements and the residual force enhancement.…”

“…Recent studies with isolated myofibrils, preparations in which individual sarcomeres (22,23,36,37,41,43) and half-sarcomeres in series (44,45) can be evaluated, have shown intriguing results. Whereas a few studies suggest that force enhancement could be linked to sarcomeres and half-SL nonuniformity, as half-sarcomeres change their length continuously during activation and relaxation (36,44,45), some others show that sarcomeres may be stable (22,37), and that the increase in force after stretch are caused by contractile and/or passive structures.…”

“…Whereas a few studies suggest that force enhancement could be linked to sarcomeres and half-SL nonuniformity, as half-sarcomeres change their length continuously during activation and relaxation (36,44,45), some others show that sarcomeres may be stable (22,37), and that the increase in force after stretch are caused by contractile and/or passive structures. Furthermore, studies that attribute the residual force enhancement to sarcomeric structures present experimental data and interpretations that are highly variable.…”

Is titin a ‘winding filament’? A new twist on muscle contraction

“…However, the development of half-sarcomere non-uniformity during isometric force development appears to be a feature of vertebrate sarcomeres as well [60,74]. In rabbit psoas muscle, A-band displacements of approximately 60 nm were observed at optimum length and maximum isometric force.…”