Westerblad H, Lännergren J
Department of Physiology II, Karolinska Institute, Stockholm, Sweden.
J Muscle Res Cell Motil. 1994 Jun;15(3):287-98. doi: 10.1007/BF00123481.
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.
我们研究了非洲爪蟾完整单纤维在重复强直刺激导致疲劳过程中的力-速度关系和松弛速度。采用松弛试验来获取零负荷下的缩短速度（V0），并通过斜坡缩短试验来得到中等速度下的力。松弛速度通过松弛初始线性阶段的斜率来测量。在疲劳刺激过程中，等长张力按照典型的三个阶段模式下降。在最初的10 - 15次强直刺激（阶段1）中，等长张力降至疲劳前张力（P0）的约80%，而V0没有显著变化。此后，V0几乎随时间呈线性下降，而等长张力首先下降非常缓慢（阶段2），然后迅速下降（阶段3）。在疲劳状态下，V0降至对照值的46%，等长张力降至0.34P0。疲劳过程中力-速度关系的曲线似乎变缓。松弛速度在阶段1几乎减半，此后下降更缓慢，在疲劳时降至对照值的不到10%。我们认为，阶段1和2中等长张力和缩短速度的变化反映了由于细胞内pH值、无机磷酸盐和ADP浓度变化导致的横桥功能改变；阶段3中额外的张力下降将反映横桥Ca2+激活受损。阶段1中松弛的迅速减慢可能涉及小清蛋白的Ca2+饱和，而阶段2和3中额外的下降将反映上述代谢变化，其作用于横桥或Ca2+主动再摄取回肌浆网。
