The effect of temperature change upon transmitter release, facilitation and post‐tetanic potentiation

1. End‐plate potentials (e.p.p.s) and miniature end‐plate potentials (m.e.p.p.s) were intracellularly recorded from rat diaphragm phrenic nerve preparations in vitro at temperatures between 7° and 40°C. 2. The quantal content of e.p.p.s and the frequency of m.e.p.p.s showed broadly similar relationships with temperature, with maxima about 20° and above 39°C. 3. Analysis of the change in e.p.p. quantal content showed that the maximum about 20°C was accompanied by a similar maximum of p, the probability of release of quanta. The maximum above 39°C was associated with a rise in n, a presynaptic store of material needed for release. 4. The rate at which transmitter could be mobilized was linear in an Arrhenius plot with an apparent activation energy of 25 kcal deg⁻¹. 5. Facilitation and post‐tetanic potentiation (PTP) were shown to be entirely attributable to changes in p. 6. It is suggested that facilitation and PTP have a common basis and that the (temperature‐dependent) rate of Ca removal from intracellular sites at which it exerts its action is as important a determinant of the magnitude of quantal release as is the amount of Ca combining with these sites.