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Effects of rapid shortening on rate of force regeneration and myoplasmic [Ca²⁺] in intact frog skeletal muscle fibres

R. Vandenboom, D. R. Claflin and F. J. Julian

1. The effect of rapid shortening on rate of force regeneration (dF/dt_R) was examined in single, intact frog (Rana temporaria) skeletal muscle fibres (3·0 °C). Step releases leading to unloaded shortening were applied after 500 ms of stimulation, during the plateau of an isometric tetanus. Initial mean sarcomere length ranged from 2·05 to 2·35 µm; force regeneration after shortening was at 2·00 µm.

2. Values for dF/dt_R following a 25 nm half-sarcomere^-1 release were 3·17 ± 0·17 (mean ± s.e.m., n = 8) times greater than the initial rate of rise of force before release (dF/dt_I). As release size was increased from 25 to 175 nm half-sarcomere^-1, the relationship between release size and dF/dt_R decreased sharply before attaining a plateau value that was 1·34 ± 0·09 times greater than dF/dt_I. Despite wide variations in dF/dt_R, the velocity of unloaded shortening remained constant (2·92 ± 0·08 µm half-sarcomere^-1 s^-1; n = 8) for the different release amplitudes used in this study.

3. To investigate its role in the attenuation of dF/dt_R with increased shortening, the effects of rapid ramp (constant velocity) shortening on intracellular free Ca²⁺ concentration ([Ca²⁺]_i) were monitored using the Ca²⁺-sensitive fluorescent dye furaptra. Compared with an isometric contraction, rapid fibre shortening was associated with a transient increase in [Ca²⁺]_i while force regeneration after shortening was associated with a transient reduction in [Ca²⁺]_i. The greatest reductions in [Ca²⁺]_i were associated with the largest amplitude ramps.

4. Cross-bridge-mediated modifications of the Ca²⁺ affinity of troponin C (TnC) may explain the fluctuations in [Ca²⁺]_i observed during and after ramps. Associated fluctuations in TnC Ca²⁺ occupancy could play a role in the reduction of dF/dt_R with increasing release size.

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