Matsumoto Mari S, Hamada Taku, Watanabe Daiki
Graduate School of Sport and Health Sciences, Osaka University of Health and Sport Sciences, 1-1 Asashirodai, Kumatori-cho, Sennan-gun, Osaka, 590-0496, Japan.
J Muscle Res Cell Motil. 2025 Jul 28. doi: 10.1007/s10974-025-09702-1.
Strength-duration (S-D) relationship reflects muscle excitability which partially determines force production. Isolated muscle contains part of nerve and neuromuscular junction, suggesting that muscle excitation initiates not only muscle membrane but also nerve terminals and hence neuromuscular transmission (NMT). This study aimed to examine the effects of inhibition of neuromuscular transmission on S-D relationship in slow- and fast-twitch muscles of mice. Isolated slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles were electrically stimulated, and minimal field strength required for force generation [threshold field strength (Th)] was determined at various duration. Th significantly increased in the presence of NMT inhibitors including pancuronium (Pan) and succinylcholine in both SOL and EDL muscles. Moreover, Th in the presence of NMT inhibitors was higher in SOL compared to EDL. To clarify the mechanism of the different Th between SOL and EDL, contribution of Na channel was investigated. In the presence of 1 μM Pan, partial inhibition of Na channel by tetrodotoxin eliminated the difference in Th between EDL and SOL. These results suggest that (1) NMT significantly enhances muscle excitability, (2) threshold of action potential in fast-twitch muscle is greater compared with slow-twitch muscle if no existence of NMT, and (3) Na channel likely contribute to the difference between slow- and fast-twitch muscles.
强度-时间（S-D）关系反映了肌肉兴奋性，而肌肉兴奋性部分决定了力量的产生。分离的肌肉包含部分神经和神经肌肉接头，这表明肌肉兴奋不仅起始于肌膜，还起始于神经末梢，进而起始于神经肌肉传递（NMT）。本研究旨在检测抑制神经肌肉传递对小鼠慢肌和快肌中S-D关系的影响。对分离的慢肌比目鱼肌（SOL）和快肌趾长伸肌（EDL）进行电刺激，并在不同持续时间下测定产生力量所需的最小场强[阈场强（Th）]。在存在包括泮库溴铵（Pan）和琥珀酰胆碱在内的NMT抑制剂时，SOL和EDL肌肉中的Th均显著增加。此外，与EDL相比，存在NMT抑制剂时SOL中的Th更高。为了阐明SOL和EDL之间Th差异的机制，研究了钠通道的作用。在存在1μM Pan的情况下，河豚毒素对钠通道的部分抑制消除了EDL和SOL之间Th的差异。这些结果表明：（1）NMT显著增强肌肉兴奋性；（2）如果不存在NMT，快肌中的动作电位阈值比慢肌中的更高；（3）钠通道可能是慢肌和快肌之间差异的原因。
