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

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 degrees C whereas the system is highly temperature-sensitive above 20 degrees C.2. A very similar result is obtained when the experiment is repeated in saline containing Ca(2+) buffered at 5 x 10(-7)M, suggesting that it is unlikely that the major action of temperature is to cause an increase in Ca(2+) influx.3. It is suggested that the main effect of temperature at the presynaptic terminals is a modification of Ca(2+) by an action on intracellular Ca(2+) stores.4. The interacting effects of theophylline and the divalent cation ionophore A23187 on m.e.p.p. frequency suggest that intracellular Ca(2+) stores, in addition to the mitochondria, may well be of importance in controlling Ca(2+).5. Changes in Ca(2+) 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 Ca(2+) at 23 degrees C, whereas the greater effect is found at 13 degrees C when Ca(2+) is lowered.6. It is suggested (i) that m.e.p.p. frequency is primarily determined by Ca(2+) at the presynaptic terminals, (ii) that the presynaptic terminals are normally able to maintain Ca(2+) almost constant in spite of increases in Ca influx associated with ionophore treatment or with a rise in Ca(2+). However, if the steady-state position of Ca(2+) 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 Ca(2+) and in m.e.p.p. frequency.