Changes in the time course of transmitter action produced by procaine.

1. In addition to reducing the sensitivity of the end-plate to transmitter, procaine produced changes in the time course of the end-plate potential. The time to peak potential was reduced by procaine, whether recording from fibres paralysed by tubocurarine or from fibres paralysed by high magnesium concentration. The rate of decay of potential was also markedly reduced. Similar observations were made recording spontaneous miniature end-plate potentials.2. Procaine did not produce any marked change in the electrical constants of muscle fibres.3. In the presence of procaine the accumulation of charge at the end-plate was prolonged; the rate of decay of charge was reduced. It is suggested that the prolonged end-plate potentials arise from a prolongation of the end-plate current.