Unloaded shortening after a quick release of a contracting, single fibre from crayfish slow muscle

Summary

The time course of shortening at zero load was studied by the slack test method during tetanic contractions in isolated, single, slow muscle fibres of the crayfish. In 28 of 32 shortenings (from 14 different fibres) a biphasic shortening was seen, which consisted of an initial high-velocity phase lasting 3.3–20.8 ms and a following slow-velocity phase lasting for the entire time examined (up to 89.2 ms). Provided that the shortening occurred uniformly along the fibre length, velocity in the initial phase, V₁, of the biphasic shortening was 14.4±3.4 (mean±SD,n=10) μm s⁻¹ per half sarcomere at Lo, the slack length, at 20°C, while that in the second phase, V₂, was 7.4±1.4 μm s⁻¹ per half sarcomere. Lowering temperature decreased both V₁ and V₂ with Q₁₀=1.4 for V₁ and 2.0 for V₂. Lowering the external Ca concentration from 15 mM, the standard, to 2 mM resulted in a tetanic tension below one-third of that at 15 mM Ca and decreased both V₁ (t test; p<0.01) and V₂ (p<0.001). Prestretching the fibre to 1.5 Lo had no significant effect on V₂ (p<0.3) but increased V₁ (p<0.001). The distance shortened during the initial high-velocity phase, LV₁, was 4.0±1.8% Lo (mean±D,n=10) at 20°C or about 0.14 μm per half sarcomere on average. LV₁ was independent of the tetanic tension level when it was changed by lowering the external Ca concentration or temperature in the same fibre. Prestretching the fibre to 1.5 Lo, at which the sum of the active and the resting tension was lower than Po at Lo in two of three fibres, increased LV₁ significantly (p<0.001). The independency of LV₁ from the tension level indicates that the initial high-velocity phase was not from shortening of some inert components in the fibre. One possibility is that the initial high-velocity phase was brought about by an acceleration of shortening by a compressive force, the origin of which has been discussed. The slow-velocity phase seemed to result from the crossbridge turnover with little exogeneous stress on myofilaments. Four different fibres exhibited an unloaded shortening with a constant velocity during the entire time examined (29.9–61.8 ms). This type of shortening had a velocity between the usual V₁ and V₂ values, suggesting that a compressive force accelerated the shortening during the entire time.