McGill University, Montréal, QC, Canada.
Can J Physiol Pharmacol. 2010 Oct;88(10):937-48. doi: 10.1139/y10-068.
The maximal shortening velocity of a muscle (V(max)) provides a link between its macroscopic properties and the underlying biochemical reactions and is altered in some diseases. Two methods that are widely used for determining V(max) are afterloaded and isotonic release contractions. To determine whether these two methods give equivalent results, we calculated V(max) in 9 intact single fibres from the lumbrical muscles of the frog Xenopus laevis (9.5-15.5 °C, stimulation frequency 35-70 Hz). The data were modelled using a 3-state cross-bridge model in which the states were inactive, detached, and attached. Afterloaded contractions gave lower predictions of Vmax than did isotonic release contractions in all 9 fibres (3.20 ± 0.84 versus 4.11 ± 1.08 lengths per second, respectively; means ± SD, p = 0.001) and underestimated unloaded shortening velocity measured with the slack test by an average of 29% (p = 0.001, n = 6). Excellent model predictions could be obtained by assuming that activation is inhibited by shortening. We conclude that under the experimental conditions used in this study, afterloaded and isotonic release contractions do not give equivalent results. When a change in the V(max) measured with afterloaded contractions is observed in diseased muscle, it is important to consider that this may reflect differences in either activation kinetics or cross-bridge cycling rates.
肌肉的最大缩短速度（V(max)）将其宏观性质与其潜在的生化反应联系起来，并且在某些疾病中发生改变。两种广泛用于确定 V(max)的方法是后加载和等张释放收缩。为了确定这两种方法是否给出等效的结果，我们在 9 条完整的单个纤维中计算了来自青蛙 Xenopus laevis 的蚓状肌的 V(max)（9.5-15.5°C，刺激频率 35-70 Hz）。使用 3 状态交联桥模型对数据进行建模，其中状态为非活性、分离和附着。在后加载收缩中，所有 9 条纤维的 Vmax 的预测值均低于等张释放收缩（分别为 3.20 ± 0.84 和 4.11 ± 1.08 每秒长度；平均值 ± SD，p = 0.001），并且平均低估了松弛测试测量的空载缩短速度 29%（p = 0.001，n = 6）。通过假设激活被缩短抑制，可以获得出色的模型预测。我们得出结论，在本研究中使用的实验条件下，后加载和等张释放收缩不会给出等效的结果。当在后加载收缩中测量的 V(max)发生变化时，在患病肌肉中观察到，重要的是要考虑到这可能反映了激活动力学或交联桥循环速率的差异。
