Investigation of fading responses induced by non-depolarising muscle relaxants in the evoked EMG of the gastrocnemius muscle of the cat.

Abstract

During partial neuromuscular blockade indirect repetitive nerve stimulation causes fade in the response of the muscle. We studied the intensity of the fade induced by intravenous administration of three steroidal muscle relaxants, and investigated the mechanism of fade by comparing with results obtained during partial blockade with animal toxins and vesamicol. In 60 cats, we measured the fade in the compound action potentials of the gastrocnemius muscle evoked by repetitive sciatic nerve stimulation at 100 Hz during partial neuromuscular blockade with rocuronium, vecuronium, pancuronium, alpha-bungarotoxin, mu-conotoxin and vesamicol, respectively. Profound fade was induced by all three non-depolarising muscle relaxants (rocuronium=vecuronium<pancuronium) and by vesamicol, in contrast to alpha-bungarotoxin and mu-conotoxin which induced no fading. Our results suggest that non-depolarising muscle relaxant-induced fade is caused by an inhibitory effect on acetylcholine release. Pancuronium seems to have a higher potency of presynaptic inhibition of acetylcholine release than both rocuronium and vecuronium.