Neuromuscular blocking profile of the vecuronium analogue, Org‐9487, in the rat isolated hemidiaphragm preparation

1 The neuromuscular effects of the short‐acting aminosteroid muscle relaxant Org‐9487 have been studied in the in vitro rat phrenic nerve/hemidiaphragm preparation by use of twitch tension and electrophysiological recording techniques. 2 Org‐9487 (5–100 μm) produced a concentration‐dependent decrease in the amplitude of twitches (0.1 Hz) and tetanic contractions (50 Hz) evoked by motor nerve stimulation. The compound produced fade of force during both 50 Hz stimulation and train‐of‐four stimulation at 2 Hz, indicating a prejunctional component of action. 3 Anticholinesterases only partially reversed the effect of Org‐9487 on twitch responses. This was possibly because, at the concentrations required to block twitches in the rat, Org‐9487 itself was found to possess significant anticholinesterase activity. 4 Org‐9487 (3 μm) increased the rundown of endplate current amplitudes during a 2 s train of 50 Hz nerve stimulation. This was because Org‐9487 increased the quantal content of the first endplate current in the train without affecting acetylcholine release towards the latter part of the train. 5 Org‐9487 (10 μm) produced a voltage‐dependent decrease in the time constant of decay of endplate currents at 32°;C and 0.5 Hz, indicative of a block of endplate ion channels. The blocking rate constant increased with membrane hyperpolarization.