The action of polymyxin B at the frog neuromuscular juntion

Nicholas N. Durant, Jeremy J. Lambert

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Abstract

The effects of polymyxin B at the neuromuscular junction of the frog were studied by conventional electrophysiological and voltage clamp techniques. At a concentration of 2.5 μg/ml polymyxin B produced neuromuscular blockade in 10 to 15 min and endplate potentials (e.p.ps) could be recorded. Resting membrane potential was unaffected. The neuromuscular block was characterized by a depressed e.p.p. quantal content (28 ± 7), which was similar to that determined from endplates exposed to 13 mm magnesium (23 ± 3), and a low e.p.p. quantal size, which was similar to that determined from endplates exposed to 3 μm (+)‐tubocurarine. Polymyxin B (0.25 to 0.75 μg/ml) decreased mean miniature e.p.p. amplitude with little effect on frequency. At a concentration of 5 μg/ml polymyxin B markedly shortened the duration of endplate currents (e.p.cs) and abolished the relationship between holding potential and the time to half‐decay at negative potentials greater than −;60 mV. This action is consistent with block of open acetylcholine activated ionic channels. 4‐Aminopyridine (20 μm) antagonized the depressed e.p.p. quantal content produced by polymyxin B but did not alter the shortened e.p.c. duration. It is concluded that polymyxin B decreases quantal release and produces some degree of postjunctional receptor blockade and a marked and persistent blockade of acetylcholine activated channels. The latter action may explain the difficulty of reversal of polymyxin B‐induced neuromuscular blockade and its non‐competitive nature. 1981 British Pharmacological Society

Original languageEnglish
Pages (from-to)41-47
Number of pages7
JournalBritish Journal of Pharmacology
Volume72
Issue number1
DOIs
Publication statusPublished - 1 Jan 1981

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