Interacting effects of temperature and extracellular calcium on the spontaneous release of transmitter at the frog neuromuscular junction

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Abstract
Temperature had a characteristic effect on the frequency of MEPP [miniature endplate potential] at the frog neuromuscular junction; the spontaneous release of transmitter was not affected by temperature changes below 10.degree. C whereas the system is highly temperature sensitive above 20.degree. C. A very similar result was obtained when the experiment was repeated in saline containing Ca2+ buffered at 5 .times. 10-7 M, suggesting that it is unlikely that the major action of temperature is to cause an increase in Ca2+ influx. The main effect of temperature at the presynaptic terminals may be a modification of [Ca2+]i [intracellular Ca2+ concentration] by an action on intracellular Ca2+ stores. The interacting effects of theophylline and the divalent cation ionophore A23187 on MEPP frequency suggested that intracellular Ca2+ stores, in addition to the mitochondria, may well be of importance in controlling [Ca2+]i. Changes in [Ca2+]o [extracellular] produced a modification of MEPP frequency, but the details of the response were dependent on temperature. The spontaneous release of transmitter was most sensitive to an increase in [Ca2+]o at 23.degree. C, whereas the greater effect was found at 13.degree. C when [Ca2+]o was lowered. MEPP frequency is apparently primarily determined by [Ca2+]i at the presynaptic terminals, and the presynaptic terminals are normally able to maintain [Ca2+]i almost constant in spite of increases in Ca influx associated with ionophore treatment or with a rise in [Ca2+]o. However, if the steady-state position of [Ca2+]i is previously raised by an increased efflux from intracellular stores (produced by elevated temperature or theophylline pre-treatment), increased influx causes a rise in both [Ca2+]i and in MEPP frequency.