Effects of phospholipase of Enhydrina schistosa venom on nerve, motor end-plate and muscle of the mouse.

Phospholipase A2 from Enhydrina schistosa (sea snake) venom was administered intravenously or intramuscularly to mice. The effects on the contractile responses and neuromuscular transmission were studied in soleus (a slow-twitch muscle) and extensor digitorum longus (e.d.l.--a fast-twitch muscle). These muscles were also studied by morphological methods which included light and electron microscopy and enzyme histochemistry. There was a rapid and irreversible reduction of resting membrane potentials of muscle fibres of soleus which then failed to contract in response to direct or indirect stimulation. The e.d.l. was less severely affected. Spontaneous and evoked release of transmitter remained detectable at almost all end-plates tested and there was no apparent reduction in the sensitivity of the post-synaptic membrane. Most muscle fibres of soleus but only about 50% of those in e.d.l. became necrotic, the histochemical fibre types 1 and 2A being affected. Electron microscopy showed that in the acute phase (up to 24 h) there were lesions of the plasmalemma and severe disorganization of the internal structure of muscle fibres. In contrast, motor nerve terminals remained intact as did myelinated axons in the intramuscular nerves. There was a transient increase in the number of omega-shaped profiles in the axolemma and of coated vesicles in nerve terminals at 1-3 h, but by 24 h motor nerve terminals were normal in appearance. In regenerated muscle, post-synaptic folds at motor end-plates were reduced in depth and number.