Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCA197

Poster Communications

The muscle relaxant methocarbamol blocks the muscular voltage-gated Na+ channel Nav1.4 and affects force characteristics of isolated mouse soleus muscles

J. Y. Zhang1, P. Otto1, S. Kröger2, H. Brinkmeier1

1. Pathophysiology, Universitymedicine Greifswald, Karlsburg, Germany. 2. Biomedical Center, Ludwig-Maximilians-University, Munich, Germany.

The muscle relaxant methocarbamol is widely used to treat skeletal muscle spasms and pain syndromes. Besides its central actions, there are reports about peripheral effects of methocarbamol, e.g. a prolongation of the refractory period of muscles. The current study was designed to clarify peripheral effects of methocarbamol and to detect possible molecular targets in skeletal muscle. The refractory period is mainly determined by voltage-gated Na+ channels. Thus, we investigated effects of methocarbamol on Nav1.4 channels and studied its action on electrically stimulated mouse muscles. Voltage-gated Na+-currents were recorded from HEK 293 cells stably expressing the human Nav1.4. I/V curves, steady-state inactivation curves and recovery of the channels from inactivation were studied in the presence and absence of 2 mM methocarbamol. Muscle force and fatigue were studied on isolated soleus muscles from 100 d old mice. Muscles were stimulated to produce twitches and tetani. Then, in a second run twitches and tetani were recorded the presence and absence of methocarbamol. Subsequently, a protocol of sustained repetitive stimulation was applied. Transient Na+ inward currents could be stably recoded from HEK 293 cells expressing the Nav1.4 (voltage pulses from -85 mV to -10 mV). On application of methocarbamol the Na+ current amplitudes were inhibited on average by 37 ± 8 % (n=16). The effect occurred within 10 s and was reversible. While I/V curves were only marginally affected by methocarbamol, inactivation curves were shifted on average by - 8.4 mV (from -71.6 mV to -80.0 mV, n=16). Methocarbamol slowed recovery from inactivation of Nav1.4 channels. The mean recovery time constant was 3.2 ms in external solution. In the presence of methocarbamol a second slow recovery phase with a time constant of 55 ± 20 ms (n=12) appeared. Methocarbamol did not affect twitches, but tetanic force at 120 Hz. Normalized tetanic force was reduced to 0.88 ± 0.01 by methocarbamol (p < 0.001). In agreement with the mentioned findings, the tetanus/twitch ratio was lowered by methocarbamol by 15%. The fatigue process was accelerated. The time to reach 50% of initial force was reduced from 180 ± 29 s (control) to 149 ± 37 s (2 mM methocarbamol, p < 0.05). The muscle relaxant methocarbamol has marked effects on force characteristics of isolated mouse muscles. It reduced tetanic force and accelerated muscle fatigue during long-term stimulation. These observations agree with the effect of methocarbamol on Nav1.4 channels, responsible for the refractory period. The findings are also in line with the observation that recovery from inactivation of Nav1.4 channels was slowed by methocarbamol. The study shows evidence that Nav1.4 channels are molecular targets of methocarbamol, explaining some of its peripheral effects.

Where applicable, experiments conform with Society ethical requirements