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First published online on March 9, 2006.
 Copyright © 2006 by The Physiological Society
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jphysiol.2006.105809v1
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Ivo A. Telley
Robert Stehle
K. W. Ranatunga
Gabriele Pfitzer
Edgar Stüssi
Jachen Denoth
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Received January 23, 2006
Revised February 6, 2006
Accepted after revision March 7, 2006

Dynamic behaviour of half-sarcomeres during and after stretch in activated psoas myofibrils: sarcomere asymmetry but no 'sarcomere popping'

Ivo A. Telley1, Robert Stehle2, K. W. Ranatunga3, Gabriele Pfitzer2, Edgar Stüssi1, and Jachen Denoth1*

1 Laboratory for Biomechanics, D-MAVT, ETH Zurich
2 Institute of Physiology, University of Cologne
3 Department of Physiology, School of Medical Sciences

* To whom correspondence should be addressed. E-mail: jdenoth{at}ethz.ch.

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. 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 stabilises the uniformity of half-sarcomere lengths and sarcomere symmetry. In general, the half-sarcomere length changes (dynamics) prior and after stretch were slow and the dynamics after stretch was not readily predictable on the basis of the steady-state force-sarcomere length relationship.


Key words: Muscle contraction • Muscle mechanics • Muscle stretch




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