EFFECTS OF RAPID SHORTENING ON RATE OF FORCE REGENERATION AND MYOPLASMIC [CA2-MUSCLE FIBERS(] IN INTACT FROG SKELETAL)

1. The effect of rapid shortening on rate of force regeneration (dF/dt (R)) was examined in single, intact frog (Rana temporaria) skeletal mu scle fibres (3.0 degrees C). Step releases leading to unloaded shorten ing 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 mu m; force regeneration after shortening was at 2.00 mu m. 2. Va lues 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 ra te of rise of force before release (dF/dt(I)). As release size was inc reased from 25 to 175 nm half-sarcomere(-1) the relationship between r elease 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 mu m 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 ef fects of rapid ramp (constant velocity) shortening on intracellular fr ee Ca2+ concentration ([Ca2+](i)) were monitored using the Ca2+-sensit ive fluorescent dye furaptra. Compared with an isometric contraction, rapid fibre shortening was associated with a transient increase in [Ca 2+](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-me diated modifications of the Ca2+ affinity of troponin C (TnC) may expl ain the fluctuations in [Ca2+](i) observed during and after ramps. Ass ociated fluctuations in TnC Ca2+ occupancy could play a role in the re duction of dF/dt(R) with increasing release size.