THE EFFECTS OF 5 PHOSPHOLIPASE-A(2) FROM THE VENOM OF KING-BROWN SNAKE, PSEUDECHIS-AUSTRALIS, ON NERVE AND MUSCLE

The effects on vertebrate neuromuscular function of five homologous phospholipases A(2) (PLA(2)) (Pa-3, Pa-8, Pa-9C, Pa-10F and Pa-12B) from the venom of the Australian king brown snake, Pseudechis australis, were determined. These isoenzymes (0.2-1.6 mu M) reduced, with different potencies, responses of chick biventer cervicis preparations to nerve stimulation and to exogenously applied acetylcholine, carbachol and KCl in a time- and concentration-dependent way but with different potencies. They also blocked twitches of mouse hemidiaphragm preparations evoked by nerve and by direct muscle stimulation. Pa-8 was the most active and Pa-9C was the least potent. There was a strong correlation between the enzymatic activity and the effect of toxins on the responses of mouse hemidiaphragm to direct muscle stimulation, but weak correlation between the effects on indirect responses and enzymatic activity. Intracellular recording from endplate regions of mouse triangularis sterni nerve-muscle preparations showed that Pa-10F and Pa-12B at 0.2 mu M significantly reduced quantal content after 10 min. Pa-8 (0.2 mu M) reduced the amplitude of endplate potentials by about 25% and abolished miniature endplate potentials within 15 min. Pa-3 (0.2 mu M) and Pa-9C (0.8 mu M) also significantly reduced quantal content by about 30% of control after 30 min. Among these toxins, Pa-3 and Pa-8 at 0.2 mu M depolarised mouse muscle fibres after 30 min. Extracellular recording of action potentials at motor nerve terminals of mouse triangularis sterni preparations indicated that these isoenzymes reduced the waveforms associated with both Na+ and K+ conductances. Since no facilitatory effect on the release process has been observed, the apparent blockade of K+ conductance by some of these toxins may not be a selective action on K+ channels, but may be secondary to membrane depolarisation. An in vivo study with Pa-8 and Pa-10F demonstrated myotoxic effects. Light microscopic examination showed a degeneration of mouse and rat skeletal muscle fibres caused by Pa-8 and Pa-10F. For the in vivo study, rats received 80 mu g/kg of the toxins s.c. and mice were injected i.m. with the toxins (40 mu g/kg). Myotoxicity appears to be the predominant effect of these five toxins.