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Department of Zoology, University of Liverpool, P.O. Box 147, Liverpool L69 3BX

1. Temperature has a characteristic effect on the frequency of m.e.p.p.s at the frog neuromuscular junction; the spontaneous release of transmitter is not affected by temperature changes below 10° C whereas the system is highly temperature-sensitive above 20° C.

2. A very similar result is obtained when the experiment is repeated in saline containing Ca²⁺ buffered at 5 x 10^-7 M, suggesting that it is unlikely that the major action of temperature is to cause an increase in Ca²⁺ influx.

3. It is suggested that the main effect of temperature at the presynaptic terminals is a modification of [Ca²⁺]_i by an action on intracellular Ca²⁺ stores.

4. The interacting effects of theophylline and the divalent cation ionophore A23187 on m.e.p.p. frequency suggest that intracellular Ca²⁺ stores, in addition to the mitochondria, may well be of importance in controlling [Ca²⁺]_i.

5. Changes in [Ca²⁺]_o produce a modification of m.e.p.p. frequency, but the details of the response are dependent on temperature. The spontaneous release of transmitter is most sensitive to an increase in [Ca²⁺]_o at 23° C, whereas the greater effect is found at 13° C when [Ca²⁺]_o is lowered.

6. It is suggested (i) that m.e.p.p. frequency is primarily determined by [Ca²⁺]_i at the presynaptic terminals, (ii) that 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 m.e.p.p. frequency.

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