Motor dysfunction in spontaneous thymoma rats, Buffalo/Mna

The muscle responsiveness of rats of Buffalo/Mna strain with spontaneous thymoma (Mna rats) was investigated in vivo electrophysiologically and pharmacologically in order to determine the principal cause of the muscle weakness and fatigability. Hindlimb muscles of these rats exhibited a lesser contractility and a greater fatigability during repetitive nerve stimulation than those of rats of another strain, ACI. Nevertheless, there was no difference in the time course of amplitude of evoked electromyograms between Mna and ACI rats. According to the test of drug sensitivity, Mna rats were much less sensitive to curare than ACI rats. Sensitivity to 4-aminopyridine was normal. In conclusion, Mna rats showed an increased fatigability and weakness of the muscle. The principal cause seems to be involved in contractile mechanisms subsequent to muscle cell membrane. The reduced sensitivity to curare suggests a possibility of some change at the site of neuro-muscular junction.