Center for Muscle Metabolism and Biophysics, Faculty of Applied Health Sciences, Brock University, St. Catharines, ON L2S 3A1, Canada.
J Muscle Res Cell Motil. 2012 Oct;33(5):359-68. doi: 10.1007/s10974-012-9325-6. Epub 2012 Oct 1.
The activity dependent potentiation of peak isometric force associated with phosphorylation of the myosin regulatory light chain (RLC) is generally restricted to low activation frequencies. The purpose of this study was to determine if muscle shortening speed influenced the stimulus frequency domain over which concentric force potentiation was observed. To this end, mouse extensor digitorum longus (EDL) muscles (in vitro, 25 °C) were activated at a range of test frequencies (10, 25, 45, 70 or 100 Hz) during shortening ramps at 0.10, 0.30 or 0.50 of the maximal velocity of shortening (V(max)). This procedure was performed before and after a standard conditioning stimulus (CS) that elevated RLC phosphorylation from 0.08 ± 0.01 (rest) to 0.55 ± 0.01 (stimulated) moles phosphate per mol RLC, respectively (n = 9-11) (P < 0.01). When data from all test frequencies were collapsed, the CS potentiated mean concentric force at 0.10, 0.30 and 0.50 V(max) to 1.02 ± 0.03, 1.37 ± 0.03 and 1.59 ± 0.05 of unpotentiated, pre-CS values, respectively (n = 8, P < 0.05). In addition, increasing shortening speed also increased the activation frequency at which concentric force potentiation was maximal, i.e. from 10 Hz at 0.10 V(max) to 10-25 and 25-45 Hz at 0.30 and 0.50 V(max), respectively. These results indicate that both the magnitude of and activation frequency dependence for concentric force potentiation of mouse EDL muscle is shortening speed dependent. Thus, muscle shortening speed may be a critical factor determining the functional utility of the myosin RLC phosphorylation mechanism.
与肌球蛋白调节轻链（RLC）磷酸化相关的峰值等长力的活动依赖性增强通常仅限于低激活频率。本研究的目的是确定肌肉缩短速度是否会影响观察到的向心力增强的刺激频率范围。为此，在 0.10、0.30 或 0.50 的最大缩短速度（V（max））下，以 0.10、0.30 或 0.50 的速度进行缩短斜坡时，以 10、25、45、70 或 100 Hz 的一系列测试频率（10、25、45、70 或 100 Hz）激活体外的小鼠伸趾长肌（EDL）肌肉（25°C）。在进行此程序之前和之后，使用标准条件刺激（CS）进行操作，该刺激使 RLC 磷酸化分别从 0.08 ± 0.01（休息）增加到 0.55 ± 0.01（刺激）摩尔每摩尔 RLC 的磷酸化（n = 9-11）（P < 0.01）。当所有测试频率的数据合并时，CS 在 0.10、0.30 和 0.50 V（max）处增强了平均向心力，分别达到未增强的 0.10、0.30 和 0.50 V（max）的 1.02 ± 0.03、1.37 ± 0.03 和 1.59 ± 0.05 的 0.03 （n = 8，P < 0.05）。此外，增加缩短速度还增加了向心力增强达到最大值的激活频率，即在 0.10 V（max）时从 10 Hz 增加到 0.30 和 0.50 V（max）时的 10-25 和 25-45 Hz。这些结果表明，小鼠 EDL 肌肉的向心力增强的幅度和激活频率依赖性都依赖于缩短速度。因此，肌肉缩短速度可能是决定肌球蛋白 RLC 磷酸化机制功能实用性的关键因素。
