Klug G A, Botterman B R, Stull J T
J Biol Chem. 1982 May 10;257(9):4688-90.
Phosphorylation of the P-light chain of myosin from skeletal muscle by myosin light chain kinase is dependent upon calmodulin and Ca2+. Investigations were performed to determine if the rapid Ca2+ transients that occur during low frequency repetitive stimulation are sufficient to activate myosin light chain kinase with significant P-light chain phosphorylation. In addition, P-light chain phosphorylation was correlated with potentiation of isomeric twitch tension (staircase phenomenon). Stimulation of rat gastrocnemius muscle at 5 Hz in situ results in a time-dependent phosphorylation of the P-light chain of myosin. Initially, there was a rapid rate of phosphorylation within the first 10 muscle twitches (0.19 to 0.40 mol of phosphate/mol of P-light chain) followed by a slower rate of phosphorylation. These data indicate that myosin light chain kinase can be activated during repetitive stimulation at a low frequency in the range that occurs in vivo, despite the fact that the muscle is in the relaxed state during most of the period between each stimulation. Potentiation of isometric twitch tension was found to be temporally correlated to light chain phosphorylation at 5 Hz. It is postulated that the transient changes in intracellular Ca2+ concentration associated with low frequency stimulation are sufficient to activate myosin light chain kinase, and, furthermore, the magnitude of the potentiation of isometric twitch tension may be related to the extent of phosphorylation of myosin during a stimulus train.
肌球蛋白轻链激酶对骨骼肌肌球蛋白P轻链的磷酸化作用依赖于钙调蛋白和Ca2+。开展了多项研究，以确定在低频重复刺激过程中出现的快速Ca2+瞬变是否足以激活肌球蛋白轻链激酶并导致显著的P轻链磷酸化。此外，P轻链磷酸化与等长收缩张力增强（阶梯现象）相关。在体原位以5 Hz频率刺激大鼠腓肠肌，可导致肌球蛋白P轻链随时间发生磷酸化。起初，在前10次肌肉收缩期间磷酸化速率较快（0.19至0.40摩尔磷酸盐/摩尔P轻链），随后磷酸化速率减慢。这些数据表明，尽管在每次刺激之间的大部分时间肌肉处于松弛状态，但在体内出现的低频重复刺激过程中，肌球蛋白轻链激酶仍可被激活。发现在5 Hz频率下等长收缩张力增强与轻链磷酸化在时间上相关。据推测，与低频刺激相关的细胞内Ca2+浓度的瞬变足以激活肌球蛋白轻链激酶，此外，等长收缩张力增强的幅度可能与一串刺激期间肌球蛋白的磷酸化程度有关。
