Neuromuscular junctions and alpha-bungarotoxin-binding sites in denervated and contralateral cat skeletal muscles.

1. The distributions of cholinesterase (ChE) activity and acetylcholine (ACh) receptors were studied in normal and denervated cat hind-limb fast-twitch skeletal muscles and in muscles contralateral to denervated muscles. 2. On normal muscle fibres almost all receptors were confined to the immediately post-junctional membrane, although a perijunctional gradient of increased ACh receptor density was found on both fast- and slow-twitch fibres. After denervation, the extrajunctional ACh receptor density increased greatly and remained high for at least 10 months. Ectopic regions staining for ChE activity and having a high density of ACh receptors appeared in denervated muscle. The number of junctional ACh receptors decreased slowly after denervation, with a half-time of about 140 days. 3. Fast-twitch muscles contralateral to denervated muscles also showed changes, including an increase in junctional size and a small but significant increase in extra-junctional ACh receptor density.