Penicillamine-induced myasthenic responses in the guinea pig

To study the basis for the myasthenic illness that develops in some patients receiving D-penicillamine (D-P), we measured neuromuscular transmission in D-P-treated guinea pigs. Treated animals manifested edrophonium chloride-repairable decremental responses to 30-Hz nerve stimulation (8.5% +/- 1.5%--mean +/- SEM; N = 16) compared with the response in saline-treated control animals (-0.3% +/- 0.85%, N = 10, p less than 0.005). Posttetanic phenomena were studied in 5 treated animals; 3 revealed posttetanic exhaustion. Miniature endplate potential (MEPP) amplitudes were significantly reduced in 11 treated animals (0.612 +/- 0.016 mV) compared with 6 controls (0.713 +/- 0.024 mV, p less than 0.01). The MEPP frequency appeared reduced in treated animals. Both MEPP amplitude and MEEP frequency correlated with the decrement.