Prejunctional action of neostigmine on mouse neuromuscular preparations

We have studied the effects of neostigmine on the mouse diaphragm and triangularis sterni isolated nerve-muscle preparations. Mechanical responses of the muscle, end-plate potentials and miniature end-plate potentials, and extracellularly recorded nerve ending currents were recorded. In the mouse diaphragm nerve-muscle preparations, neostigmine 1 mumol litre-1 continued to produce some antagonism of tubocurarine-induced block after cholinesterase had been inactivated completely by diisopropyl fluorophosphate 22 mumol litre-1. In the mouse triangularis sterni preparation, neostigmine 0.1-1 mumol litre-1 increased the quantal content of the end-plate potential in a concentration-dependent manner. This effect appeared to be sufficient to account for the cholinesterase-independent antagonistic action to tubocurarine under the conditions of the experiments. Neostigmine 1-100 mumol litre-1 depressed the amplitude of the K+ currents of the perineural waveforms in a concentration-dependent manner, and this may account for its ability to increase the quantal content of the end-plate potential. Although inhibition of acetyl-cholinesterase is the main mechanism of action of neostigmine, the drug also exerts an additional direct action on motor nerve endings to block the delayed rectifier K+ channels and enhance transmitter release. This effect occurred at clinically relevant concentrations of neostigmine. Physostigmine and pyridostigmine did not possess this additional action.