1. The pathology of mammalian skeletal muscle following a single subcutaneous injection of a purified toxin from the venom of the Australian tiger snake, Notechis scutatus scutatus, has been investigated.
2. The toxin was injected into the antero-lateral aspect of one hind limb of rats and the effects of the injection on the histology, histo-chemistry and physiology of the extensor digitorum longus muscles were studied.
3. The muscles underwent degenerative necrosis, with oedema and the infiltration of lymphocytes, polymorphs and macrophages within 12–24 h after the injection.
4. Three days after injection, the oedema had subsided and the necrotic fibres had been completely destroyed by phagocytes. Small uninucleate cells, with basophilic cytoplasm and vesicular nuclei were present at this stage; on the basis of these criteria they were identified as regenerating myoblasts.
5. By 5 days the myoblasts had fused to form myotubes, but differentiation of the myotubes into histochemically distinct muscle fibre types did not commence until around 7 days after the injection.
6. Regeneration and differentiation was virtually complete by 21 days after injection.
7. Between 3 and 5 days, many of the fibres were sensitive to acetylcholine, and muscle fibre action potentials were resistant to tetrodotoxin. Miniature end-plate potentials were of low amplitude and frequency; they may have been absent from many fibres.
8. By 7–10 days, the proportion of fibres with tetrodotoxin-resistant action potentials was declining, and acetylcholine sensitivity was less marked; miniature end-plate potentials, though of normal amplitude, were of reduced frequency. The fibres were virtually normal by 14–21 days.
9. It is considered likely that these physiological properties were recorded from regenerating muscle fibres that reached maturity by 28 days; the possibility that they were recorded from functionally denervated fibres is discussed.
10. The rapid rate of regeneration and differentiation of the toxin-damaged muscle was sustained only if the peripheral nerve supply was left intact.
11. Preliminary results suggested that mitochondria-rich fibres were preferentially damaged by the toxin, and that the toxin is less active in vitro than in vivo. These problems are currently being investigated.
12. It is concluded that the toxin has a direct myotoxic effect on muscles; the relationship of this effect to the previously described neurotoxic effect is also currently under investigation.