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1 Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università di Firenze, e Istituto Nazionale di Fisica della Materia, Firenze,2 Dipartimento di Medicina Sperimentale, Sezione Fisiologia Umana, Via Forlanini, 6-27100 Pavia3 Dipartimento di Anatomia e Fisiologia Umana, Via Marzolo, 3-35131 Padova, Italy
Force enhancement during lengthening of an active muscle, a condition that normally occurs during locomotion in vivo, is attributed to recruitment of myosin heads that exhibit fast attachment to and detachment from actin in a cycle that does not imply ATP splitting. We investigated the kinetic and mechanical features of this cycle in Ca2+ activated single skinned fibres from human skeletal muscles containing different myosin heavy chain (MHC) isoforms, identified with single-fibre gel electrophoresis. Fibres were activated by using a new set-up that allows development of most of the tension following a temperature jump from 0–1°C to the test temperature (
12°C). In this way we could prevent the development of sarcomere non-uniformity and record sarcomere length changes with a striation follower in any phase of the mechanical protocol. We found that: (i) fibres with fast MHC isoforms develop 40–70% larger isometric forces than those with slow isoforms, as a result of both a larger fraction of force-generating myosin heads and a higher force per head; (ii) in both slow and fast fibres, force enhancement by stretch is due to recruitment of myosin head attachments, without increase in strain per head above the value generated by the isometric heads; and (iii) the extent of recruitment is larger in slow fibres than in fast fibres, so that the steady force and power output elicited by lengthening become similar, indicating that mechanical and kinetic properties of the actin–myosin interactions under stretch become independent of the MHC isoform.
(Received 21 July 2003;
accepted after revision 9 October 2003;
first published online 10 October 2003)
Corresponding author V. Lombardi: Laboratorio di Fisiologia, Viale GB Morgagni, 63-50134 Firenze, Italy. Email: vincenzo.lombardi{at}unifi.it
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