[The impairment of excitation-contraction (E-C) coupling in myasthenia gravis]

We have developed a new novel method to assess the function of excitation-contraction (E-C) coupling in patients with myasthenia gravis (MG). In our procedure, masseteric compound muscle action potential (CMAP) and mandibular movement-related potentials (MRP) were recorded simultaneously after stimulating the trigeminal motor nerve with a needle electrode. The E-C coupling time (ECCT) was calculated by the latency difference between CMAP and MRP. Bite force was measured using a pressure-sensitive sheet. Our serial studies demonstrate that masseteric E-C coupling is impaired in some MG patients, and functional recovery of E-C coupling contributes at least in part to the increase in bite force after treatment. We also reveal that presence of anti-RyR antibodies is associated with significantly prolonged masseteric ECCT compared to absence of the antibodies in MG, and tacrolimus (FK506) induces ECCT shortening accompanying clinical improvement within 2 weeks. These results indicate the contribution of anti-ryanodine receptor (RyR) antibody to E-C coupling impairment, and the early effect of tacrolimus as a pharmacological enhancement of RyR function to improve E-C coupling in MG. In further studies, the present method may be applied to assess the post-tetanic potentiation of E-C coupling in human skeletal muscles.