A dual effect of repetitive stimulation on post‐tetanic potentiation of transmitter release at the frog neuromuscular junction.

End plate potentials were recorded with a surface electrode from frog neuromuscular junctions blocked with high Mg and low Ca, to study post tetanic potentiation (potentiation). Potentiation is found to decay exponentially over most of its time course. The time constant tau characterizing this exponential decay is a function of the previous history (frequency and duration) of stimulation. For example, tau increased from about 20 sec following a few impulses, to over 70 sec following more than 1000 impulses. A new method is presented to obtain estimates of the rise of potentiation (uncontaminated by facilitation or an intermediate facilitatory process) during repetitive stimulation. It is found that potentiation is present following short trains of impulses, and continually increases in magnitude with the duration of the conditioning stimulation. Potentiation was at a maximum immediately following the conditioning trains. The relationship between P(T), the magnitude of potentiation immediately following repetitive stimulation, and tau, the time constant for the decay of this potentiation is given by tau = A exp (P(T)/B), where A = 19.8 ± 5.1 sec (mean ±S.D. of an observation) and B increases from 2.2 ± 2.1 to 5.7 ± 2.7 as the stimulation rate increases from 5 to 30/sec. The value of A in the above equation can be considered to represent the minimal time constant for the decay of potentiation, that is, the time constant for decay after a single impulse. Evidence is presented for a facilitatory process with a time constant of decay of about 3 sec which is intermediate in duration between facilitation and potentiation. It is suggested that repetitive stimulation had a dual effect on potentiation; each impulse adds an increment of potentiation, and increases tau, the time constant for the decay of potentiation.