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The regulation of cytosolic Ca concentration ([Ca]i) was studied with the fluorescent Ca indicator, quin2, in pinched-off presynaptic nerve endings (synaptosomes) isolated from rat brain. The resting [Ca]i is 0.1-0.2 microM, in solutions containing 1-2 mM-Ca. [Ca]i increases by only 100-150 nM when the external Ca concentration is increased from 0.02 to 2 mM. The mitochondrial inhibitors valinomycin and fluoro-carbonyl cyanide phenylhydrazone (FCCP) increase [Ca]i by 100-200 nM. This increase is not correlated with the resting level of [Ca]i prior to the addition of inhibitors, but it is dependent on the presence of external Ca. It seems likely that the effect of these inhibitors on [Ca]i is a secondary consequence of metabolic inhibition. [Ca]i increases by about 2-fold when the external Na concentration is lowered from 145 to 5 mM, and returns to its initial level when external Na is restored. This recovery occurs also in the presence of FCCP. These results suggest that Na/Ca exchange, but not mitochondrial Ca uptake, plays a role in regulating [Ca]i and in allowing the nerve terminals to recover from Ca loading.

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