Changes in end-plate cholinesterase and axons during muscle degeneration and regeneration

Injection of bupivacaine into the rat anterior tibial muscle produced rapid necrosis with disintegration of muscle fibres and phagocytic infiltration within two days. Regenerating myotubes appeared four days after injection, and muscle structure was restored seven days after injection. One day after injection, axons and end-plate cholinesterase appeared normal in spite of the underlying muscle fibre necrosis. Intensity of cholinesterase staining was gradually reduced through the second and third day but axons remained intact. On the fourth day, newly formed end-plates delineated by cholinesterase stain were seen on myotubes. Some were innervated by thin regenerating axons. Axonal outgrowth appeared to compensate for occasional neuronal degeneration and for the greater number of end-plates on myotubes. In some areas, not all end-plates were innervated by the fourth day but were by the fifth day. On the sixth and seventh day after myotube maturation, a process of degeneration of superfluous end-plates and axons occurred, reducing their number to the number of muscle fibres. Fourteen and 28 days after injection, the innervation pattern was normal except for a slight increase in terminal innervation band width. These results indicate that injection of bupivacaine does not directly affect intramuscular nerves and that the changes observed in this experimental model are the consequence of muscle fibre degeneration and regeneration.