Direct evidence that pancuronium and gallamine enhance the release of norepinephrine from the atrial sympathetic nerve by inhibiting prejunctional muscarinic receptors.

The effect of different non-depolarizing muscle relaxants (gallamine, pancuronium, vecuronium, D-tubocurarine) on [3H]norepinephrine release in response to electrical stimulation was studied in isolated guinea-pig atrium. High pressure liquid chromatography combined with electrochemical and radiochemical detection revealed that the released radioactivity was mainly in the form of [3H]norepinephrine release. Oxotremorine, a pure muscarinic agonist, reduced the release of tritium. Gallamine and pancuronium, like atropine, prevented the inhibitory effect of oxotremorine. D-Tubocurarine and vecuronium had no such effect. These findings indicate that gallamine and pancuronium exert a presynaptic antimuscarinic, atropine-like effect, by inhibiting muscarinic receptors located on the axon terminals of sympathetic neurons thereby enhancing norepinephrine release. It is suggested that this phenomenon might play some role in tachycardia observed during surgical anaesthesia when gallamine or pancuronium have been administered.