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This Article Free via Open Access: OA OA Full Text Full Text (PDF) OA All Versions of this Article: 573/1/173    most recent jphysiol.2006.105809v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Telley, I. A. Articles by Denoth, J. Search for Related Content PubMed PubMed Citation Articles by Telley, I. A. Articles by Denoth, J. Related Collections Skeletal Muscle and Exercise

¹ Laboratory for Biomechanics, ETH Zürich, 8093 Zürich, Switzerland
² Institute of Vegetative Physiology, University of Cologne, 50931 Cologne, Germany
³ Department of Physiology, University of Bristol, Bristol BS8 1TD, UK

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 Ca²⁺ 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 (L₀) at a velocity of 0.2 L₀ 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 Ca²⁺ 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.

(Received 23 January 2006; accepted after revision 7 March 2006; first published online 9 March 2006)
Corresponding author J. Denoth: Laboratory for Biomechanics, ETH Zürich, ETH Hönggerberg, HCI E 357.1, CH-8093 Zürich, Switzerland. Email: jdenoth{at}ethz.ch

Re-use of this article is permitted in accordance with the Creative Commons Deed, Atribution 2.5, which does not permit commercial exploitation.

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