Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, C9
The effect of reducing the activating calcium on the curvature of the force-velocity relationship of permeabilized rat soleus fibres.
S. F. Gilliver1, H. Degens1, D. A. Jones1
1. Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, Manchester, United Kingdom.
Figure 1. Effect of reducing pCa on the force-velocity relationship of permeabilized rat soleus fibres. A, data fitted to the Huxley model, closed symbols pCa 4.5, open symbols pCa 5.6; B, curves normalised to isometric force (Po) and maximum shortening velocity (Vmax). Data shown for a typical fibre.
One important factor determining the power of a muscle is the curvature of the force-velocity relationship and this is of particular interest since it varies in fibres that have the same myosin heavy chain composition (Gilliver et al., 2009) and changes with fatigue (Jones et al., 2006). The determinants of curvature are poorly understood but in the Huxley (1957) model of cross-bridge interaction, curvature is given by (f + g1)/g2, where f is the rate constant for attachment, g1 that of detachment where the cross-bridge is developing force and g2 the rate of detachment of cross bridges in compression. (f + g1) approximates to the rate constant for tension redevelopment after a rapid release and re-stretch (ktr, Brenner, 1988) which is known to be sensitive to the activating calcium concentration. The rate constant g2 sets the maximum velocity of unloaded shortening which is also reported to be sensitive to the activating calcium concentration (Moss, 1982). It seems likely, therefore that curvature will be affected by low calcium but whether it will increase or decrease remains an open question. We have investigated the effect of varying the activating calcium on the curvature of the force-velocity relationship in permeabilized rat soleus type 1 fibres (N = 22). Isotonic force-velocity relationships at 15°C were established as described by Gilliver et al. (2009). Data were fitted to the Huxley model using a non-linear multiple regression routine (Solver, Excel). Compared to pCa 4.5 there was a 22 ± 8% (Mean ± S.D.) drop in isometric force at pCa 5.6, a 48 ± 6% decrease in maximum shortening velocity but a 38 ± 15% increase in (f + g1)/g2, indicating a flatter relationship and a relative preservation of power (Fig. 1). All differences were significant (P < 0.001, Student’s paired t tests). Modelling analysis indicated that g2 decreased in low calcium as did (f + g1) but the latter to a lesser extent which may be because only f is affected. An accumulation of cross-bridges in the low force state would be apparent as a decrease in f while also providing a resistance slowing the maximum velocity of unloaded shortening and the rate constant g2. The combination of these effects may account for the change in curvature.
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