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Am J Physiol Cell Physiol 294: C74-C78, 2008. First published October 10, 2007; doi:10.1152/ajpcell.00218.2007
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MUSCLE CELL BIOLOGY AND CELL MOTILITY

The origin of passive force enhancement in skeletal muscle

V. Joumaa,1 D. E. Rassier,2 T. R. Leonard,1 and W. Herzog1

1Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada; and 2Department of Kinesiology and Physical Education, McGill University, Montreal, Quebec, Canada

Submitted 25 May 2007 ; accepted in final form 3 October 2007

The aim of the present study was to test whether titin is a calcium-dependent spring and whether it is the source of the passive force enhancement observed in muscle and single fiber preparations. We measured passive force enhancement in troponin C (TnC)-depleted myofibrils in which active force production was completely eliminated. The TnC-depleted construct allowed for the investigation of the effect of calcium concentration on passive force, without the confounding effects of actin-myosin cross-bridge formation and active force production. Passive forces in TnC-depleted myofibrils (n = 6) were 35.0 ± 2.9 nN/ µm2 when stretched to an average sarcomere length of 3.4 µm in a solution with low calcium concentration (pCa 8.0). Passive forces in the same myofibrils increased by 25% to 30% when stretches were performed in a solution with high calcium concentration (pCa 3.5). Since it is well accepted that titin is the primary source for passive force in rabbit psoas myofibrils and since the increase in passive force in TnC-depleted myofibrils was abolished after trypsin treatment, our results suggest that increasing calcium concentration is associated with increased titin stiffness. However, this calcium-induced titin stiffness accounted for only ~25% of the passive force enhancement observed in intact myofibrils. Therefore, ~75% of the normally occurring passive force enhancement remains unexplained. The findings of the present study suggest that passive force enhancement is partly caused by a calcium-induced increase in titin stiffness but also requires cross-bridge formation and/or active force production for full manifestation.

myofibrils; residual force enhancement; titin; stiffness; calcium


Address for reprint requests and other correspondence: W. Herzog, Faculty of Kinesiology, Univ. of Calgary, Calgary, AB, Canada T2N 1N4 (e-mail: walter{at}kin.ucalgary.ca)






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