The effect of calcium ions and temperature on the binomial parameters that control acetylcholine release by a nerve impulse at amphibian neuromuscular synapses

Abstract

1. A study has been made of the effects of changing the external calcium concentration, [Ca](o), and the temperature on both the number of quanta available for release by the nerve impulse (n) as well as the increase in release probability of a quantum p(t) during the release period (from 0 to T) following a nerve impulse at synapses in amphibian striated muscle.2. When [Ca](o) was increased in the low range from 0.25 to 0.4 mM at 18 degrees C, the average quantal content of the e.p.p. (m) increased as the fourth power of [Ca](o) and this was primarily due to a third power dependence of n on [Ca](o); the dissociation constants and power dependence of n on calcium determined in the [Ca](o) range from 0.25 to 1.0 mM were successfully used to predict the changes in size of the e.p.p. in the very high [Ca](o) range from 1 to 10 mM. When the temperature was increased from 7 to 18 degrees C in a [Ca](o) of 0.6 mM or 0.35 mM, n increased with a Q(10) of 2.5.3. When [Ca](o) was increased in the range from 0.25 to 1.0 mM at 18 degrees C, the probability that a quantum initially available for release is released during the release period (p(T)) was very sensitive to [Ca](o), increasing as the third power of [Ca](o) and with a dissociation constant of 0.13 mM. When the temperature was increased from 7 to 18 degrees C in a [Ca](o) of 0.6 mM or 0.35 mM, p(T) decreased.4. The histograms of latencies of individual quanta following a nerve impulse was very temperature dependent: the time to peak of the histograms (i.e. the interval in which most quanta fell) had a Q(10) of over 4 as did the time constant of decline of the histograms in the temperature range from 7 to 18 degrees C.5. The average number of quanta released up to time t during the release period following a nerve impulse, namely np(t), was well described by a stochastic process in which p(t) was determined by two reactions; one of these reactions released available quanta from the nerve terminal whilst the other made some of the available quanta unavailable for release by the nerve impulse.