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

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) microns s-1 per half sarcomere at Lo, the slack length, at 20 degrees C, while that in the second phase, V2, was 7.4 +/- 1.4 microns 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 +/- SD, n = 10) at 20 degrees C or about 0.14 microns 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.