Biophys J, December 1998, p. 2971-2983, Vol. 75, No. 6

Phase Transition in Force during Ramp Stretches of Skeletal Muscle

 ^*Department of Biochemistry and Biophysics, and the Cardiovascular Research Institute, University of California, San Francisco, California 94143;  ^#UCSF/UCB Bioengineering Graduate Group, University of California, San Francisco, California 94143, and Berkeley, California 94720; and  ^§Department of Integrative Biology, University of California, Berkeley, California 94720 USA

Active glycerinated rabbit psoas fibers were stretched at constant velocity (0.1-3.0 lengths/s) under sarcomere length control. As observed by previous investigators, force rose in two phases: an initial rapid increase over a small stretch (phase I), and a slower, more modest rise over the remainder of the stretch (phase II). The transition between the two phases occurred at a critical stretch (L_C) of 7.7 ± 0.1 nm/half-sarcomere that is independent of velocity. The force at critical stretch (P_C) increased with velocity up to 1 length/s, then was constant at 3.26 ± 0.06 times isometric force. The decay of the force response to a small step stretch was much faster during stretch than in isometric fibers. The addition of 3 mM vanadate reduced isometric tension to 0.08 ± 0.01 times control isometric tension (P₀), but only reduced P_C to 0.82 ± 0.06 times P₀, demonstrating that prepowerstroke states contribute to force rise during stretch. The data can be explained by a model in which actin-attached cross-bridges in a prepowerstroke state are stretched into regions of high force and detach very rapidly when stretched beyond this region. The prepowerstroke state acts as a mechanical rectifier, producing large forces during stretch but small forces during shortening.

Biophys J, December 1998, p. 2971-2983, Vol. 75, No. 6
© 1998 by the Biophysical Society   0006-3495/98/12/2971/13  $2.00

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