This study was undertaken to investigate the mechanism by which the toxin from the bee venom, apamin, might exert beneficial effects in patients suffering from myotonic dystrophy. The effects of apamin were compared with those produced by another potassium channel blocker, 4-aminopyridine, on rat hemidiaphragm preparations stimulated at a 100 Hz frequency via the phrenic nerve. Apamin and 4-aminopyridine increased nerve-evoked tetanic fade without changing the maximal tetanic tension. The inhibitory effect of apamin was mimicked by acetylcholine. In contrast with apamin, 4-aminopyridine increased the amplitude of muscle contractions induced by nerve stimulation at 0.2 Hz frequency. All these compounds were devoid of effect when diaphragm muscle fibres were stimulated directly in the presence of the neuromuscular blocker, D-tubocurarine. The muscarinic M(2) receptor antagonist, methoctramine, prevented the inhibitory effects of both apamin and acetylcholine. Blockade of presynaptic facilitatory muscarinic M(1) and nicotinic receptors respectively with pirenzepine and hexamethonium increased apamin-induced tetanic fade. Data suggest that apamin inhibits neuromuscular transmission by a mechanism independent of the blockade of Ca(2+)-activated K(+) channels, which might involve the activation of inhibitory muscarinic M(2) receptors on motor nerve terminals. Such a mechanism may be the origin of the beneficial effect of apamin controlling muscle excitability in patients suffering from myotonic diseases.