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J Physiol Vol 271, Issue 3 pp 641-672
Copyright © 1977 by The Physiological Society
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The effect of calcium ions and temperature on the binomial parameters that control acetylcholine release by a nerve impulse at amphibian neuromuscular synapses

 M. R. Bennett, C. Fisher, T. Florin, M. Quine and J. Robinson

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 °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 °C in a [Ca]o of 0·6 mM or 0·35 mM, n increased with a Q10 of 2·5.

3. When [Ca]o was increased in the range from 0·25 to 1·0 mM at 18 °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 °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 Q10 of over 4 as did the time constant of decline of the histograms in the temperature range from 7 to 18 °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.




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