Supersensitivity to ACh in muscles after prolonged nerve block.

Sciatic nerves of rats and tibial nerves of rabbits were kept anaesthetized in situ for periods of 3-11 days by applying silastic cuffs containing lidocaine base or marcaine hydrochloride. To insure a more uniform release of the rapidly diffusing lidocaine base, the drug was contained in compartments at some distance from the nerve and the cuffs were covered with polystyrene. The completeness of anaesthesia and the functional state of the nerve were tested by stimulating the exposed nerves proximal and distal to the cuff and by observing the behaviour of the muscle prior to killing the animals. The ACh sensitivity was tested by electrophoretic application of ACh from micropipettes and by recording the results changes of the resting membrane potential in individual muscle fibres. The ACh sensitivity was found to be present in the extrajunctional area of all muscle fibres including those displaying miniature end-plate potentials. These was no difference between the behaviour of muscles from rats and rabbits and between the action of lidocaine base and marcaine hydrochloride. Previous reports on the absence of extrajunctional ACh sensitivity in muscles of rabbits whose nerves had been treated by lidocaine base were explained by a relatively rapid loss of the drug from the usual type of nerve cuffs (more than 70% of the drug lost in one day), permitting a premature recovery of the nerves from anaesthesia.