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

Abstract

The effects of changing the external Ca concentration, [Ca]o, and the temperature on the number of quanta available for release by the nerve impulse (n), and the increase in release probability of a quantum p(t) during the release period (from 0-T) following a nerve impulse at synapses in amphibian [Bufo marinus] striated muscle, were studied. When [Ca]o was increased in the low range from 0.25-0.4 mM at 18.degree. C, the average quantal content of the epp [end plate potential] (.hivin.m) increased as the 4th power of [Ca]o and this was primarily due to a 3rd power dependence of n on [Ca]o; the dissociation constants and power dependence of n on Ca determined in the [Ca]o range from 0.25-1.0 mM were successfully used to predict the changes in size of the epp in the very high [Ca]o range from 1-10 mM. When temperature was increased from 7 to 18.degree. C in a [Ca]o of 0.6 mM or 0.35 mM, n increased with a Q10 of 2.5. When [Ca]o was increased in the range from 0.25-1.0 mM at 18.degree. 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 3rd power of [Ca]o and with a dissociation constant of 0.13 mM. When temperature was increased from 7 to 18.degree. C in a [Ca]o of 0.6 mM or 0.35 mM, p(T) decreased. The histograms of latencies of individual quanta following a nerve impulse was temperature dependent: the time to peak of the histograms (i.e., the interval in which most quanta fell) had a Q10 of > 4 as did the time constant of decline of the histograms in the temperature range from 7.degree.-18.degree. C. 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 2 reactions; 1 reaction released available quanta from the nerve terminal while the other made some of the available quanta unavailable for release by the nerve impulse.