The effect of tetanic and post-tetanic potentiation on facilitation of transmitter release at the frog neuromuscular junction.

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions blocked with high Mg and/or low Ca.2. Estimates of f(t), the facilitation contributed by each impulse, were obtained during and following repetitive stimulation from the incremental change in e.p.p. amplitudes following step changes in the stimulation rate during the conditioning and testing trains.3. Estimates of f(t) increased during the conditioning stimulation and returned to control in the post-tetanic period. This increase in f(t) was proportional to the magnitude of tetanic or post-tetanic potentiation (PTP) present.4. These results are described by: [Formula: see text] where h(t)/h(c) is the e.p.p. amplitude at time t expressed in terms of the control, P(t) is potentiation, F(t) is facilitation and 1 is the base level of transmitter release. Thus, potentiation has a multiplicative (gain) effect on facilitation and the base level of transmitter release.5. PTP was present immediately following the conditioning train. However, if depression occurred during the conditioning train, PTP developed after a delay.6. It is suggested that facilitation and potentiation represent increases in two independent factors which act jointly to increase the probability of transmitter release.