Effects of D-penicillamine on neuromuscular transmission in rats

Treatment of patients with D-penicillamine (D-P) has been associated with a syndrome similar to myasthenia gravis (MG). To explore this association, we examined the effects of D-P on neuromuscular transmission in rat muscle. In the first experiment, bath-applied D-P had no significant effect on either miniature endplate potential (MEPP) amplitude or action potential (AP) amplitude. Endplate potential (EPP) amplitude and spontaneous MEPP frequency decreased significantly at concentrations approximately 40 times the maximum human therapeutic level. In the second experiment, rats receiving D-P by daily injections for 33 to 37 days did not differ from controls in any of the measured electrophysiologic characteristics. Electron microscopy of muscle endplates from rats treated with D-P showed no evidence of degeneration or simplification. In all cases, thymus histology by light microscopy was normal, and no antireceptor antibodies were found. Thus, D-P has a mild direct presynaptic effect on neuromuscular transmission at high concentrations, but this effect is too small to account for the weakness seen in the myasthenia-like syndrome in humans.

Short-term effects of prednisolone on neuromuscular transmission in normal rats and those with experimental autoimmune myasthenia gravis

Electrophysiological investigations of the effects of bath-applied prednisolone at the neuromuscular junction were performed in muscles from normal rats and rats with experimental autoimmune myasthenia gravis (EAMG). In muscles from both groups, prednisolone reversible and significantly depressed the amplitudes of minature end-plate potentials (MEPPs), end-plate potentials (EPPs) and indirectly elicited action potentials (APs) without affecting resting membrane potentials. Prednisolone also caused a significant reduction in EPP rise time to peak and half-decay time while markedly increasing MEPP frequency and AP rise time to peak and duration. These effects were shown to be dose-dependent. The percentage decrease in amplitude after prednisolone perfusion was similar for EPPs and MEPPs, indicating that the depressive effect of prednisolone at the junction is postsynaptic. In all of the parameters studied, the percentage effect of prednisolone was the same in EAMG and normal preparations. No stimulus-linked repetitive EPPs or APs were observed after prednisolone. It is concluded that prednisolone has a depressive effect on neuromuscular transmission, but that this occurs only at high concentrations of the drug which are not achieved during the treatment of myasthenia gravis.

Single-fiber electromyography in myasthenia gravis

One-hundred-sixty single-fiber EMG studies of the extensor digitorum communis muscle were performed on 127 patients with myasthenia gravis; 131 demonstrated defective neuromuscular transmission. Jitter determinations in the biceps, deltoid or frontalis muscles increased the diagnostic yield significantly. Evoked-potential EMG studies were abnormal in less than 50 percent of patients in whom they were performed. The most sensitive criterion of abnormality was the percentage of fibers with increased jitter; the sensitivity of the test was enhanced, however, if the mean jitter of the tested muscle was also used as a criterion of abnormality. Since increased jitter may also be seen in primary muscle and nerve disease, these disorders must be excluded by other means before diagnosing myasthenia gravis on the basis of the single-fiber studies.

Neonatal experimental autoimmune myasthenia gravis

Neonatal rats born of and nursed by mothers immunized with Torpedo acetylcholine receptor protein developed a defect of neuromuscular transmission as indicated by reduced miniature endplate potential amplitudes. It is likely that antibodies to the Torpedo receptor protein were passively transferred to the neonates in the milk. With the exception of the route of transfer, this neonatal form of experimental autoimmune myasthenia gravis appears to be similar to its human counterpart, and thus can serve as an experimental model.

Experimental autoimmune myasthenia gravis in rats. Modification by thymectomy and prednisolone

In rats immunized with purified acetylcholine receptor protein (AChRP) from Torpedo electroplax, a defect of neuromuscular transmission physiologically identical to that seen in myasthenia gravis developed. The most sensitive index of the neuromuscular blockage was miniature end-plate potential (MEPP) amplitude. As early as 24 hours after inoculation with AChRP, the thymus showed reactive changes that are probably nonspecific. Removal of the thymus before or within three days after immunization delayed, but did not prevent, development of reduced MEPP amplitude. Prednisolone given within 35 days after immunization produced reversal of MEPP reduction within 24 hours, but had no such immediate effect when given 15 days later. It is probable that prednisolone acted by reducing the immunologic responsiveness of the animals during the developing phase of the defect of neuromuscular transmission.

Myotonia congenita with painful muscle contractions

We present a family with dominantly inherited myotonia congenita and painful, electrically silent muscle contractions after exertion. In two family members, painful muscle contractions occurred coincident with hypothyroidism, and improved after thyroid replacement therapy. It is probable that this family represents a disease that is similar to but distinct from the dominant form of myotonia congenita.