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+)](i) 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+)](i).5. Changes in [Ca(2+)](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 [Ca(2+)](o) at 23 degrees C, whereas the greater effect is found at 13 degrees C when [Ca(2+)](o) is lowered.6. It is suggested (i) that m.e.p.p. frequency is primarily determined by [Ca(2+)](i) at the presynaptic terminals, (ii) that the presynaptic terminals are normally able to maintain [Ca(2+)](i) almost constant in spite of increases in Ca influx associated with ionophore treatment or with a rise in [Ca(2+)](o). However, if the steady-state position of [Ca(2+)](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 [Ca(2+)](i) and in m.e.p.p. frequency.
D000818:Animals; D001001:Anura; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology* / Q000502:physiology; D005110:Extracellular Space / Q000502:physiology; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects; D009469:Neuromuscular Junction / Q000187:drug effects / Q000502:physiology*; D018377:Neurotransmitter Agents / Q000378:metabolism*; D011896:Rana temporaria; D009435:Synaptic Transmission / Q000187:drug effects; D013696:Temperature; D013806:Theophylline / Q000494:pharmacology
D018377:Neurotransmitter Agents; D000001:Calcimycin; D013806:Theophylline; D002118:Calcium