Changes of the force-velocity relation, isometric tension and relaxation rate during fatigue in intact, single fibres of Xenopus skeletal muscle.

We have studied the force-velocity relation and the relaxation speed in intact, single fibres from Xenopus during fatigue produced by repeated tetani. Slack tests were used to obtain the shortening velocity at zero load (V0) and ramp shortenings to get the force at intermediate velocities. The relaxation speed was measured as the slope during the initial linear phase of relaxation. During fatiguing stimulation isometric tension declined following a typical pattern with three phases. During the initial 10-15 tetani (phase 1) isometric tension fell to about 80% of the pre-fatigue tension (P0), while V0 showed no significant change. Thereafter V0 fell almost linearly with time, whereas isometric tension first fell very slowly (phase 2) and then rapidly (phase 3). In fatigue V0 was reduced to 46% of the control and isometric tension to 0.34 P0. The force velocity relation seemed less curved during fatigue. The relaxation speed was almost halved during phase 1 and thereafter fell more slowly to less than 10% of the control in fatigue. We suggest changes of isometric tension and shortening velocity during phase 1 and 2 to reflect altered crossbridge function due to changes of intracellular pH, inorganic phosphate and ADP concentration; the additional tension decline during phase 3 would reflect impaired Ca2+ activation of the crossbridges. The rapid slowing of relaxation during phase 1 probably involves Ca2+ saturation of parvalbumin, whereas the additional decline during phase 2 and 3 would reflect the above metabolic changes, acting either on crossbridges or active Ca2+ reuptake into the sarcoplasmic reticulum.