Calcium uptake in preterminal central synapses: importance of mitochondria

Energy dependent 45Ca2+ uptake in the synaptosomal preparation from guinea pig cortex has been investigated. 45Ca2+ uptake was stimulated by ATP, the absolute value of uptake being dependent on the extent of synaptosomal disruption caused by osmotic shock. A quantitative comparison of microsomal and mitochondrial ATP-dependent 45Ca2+ uptake showed that only mitochondria had a large enough capacity to account for the Ca uptake levels observed in the synaptosomal preparation. ATP-stimulated 45Ca2+ uptake in mitochondria, 'intact' and 'shocked' synaptosomes was inhibited by atractyloside, DNP, oligomycin and ruthenium red but unaffected by antimycin A and rotenone. This was interpreted as evidence that mitochondria were responsible for ATP-dependent synaptosomal Ca2+ uptake, the increase in uptake seen on osmotic lysis being due to the deocclusion of intraterminal mitochondria. Synaptosomal and mitochondrial 45Ca2+ uptake was also stimulated by the mitochondrial respiratory substrate glutamate; this uptake was sensitive to antimycin A, DNP, rotenone and ruthenium red but insensitive to atractyloside or oligomycin thus indicating it was of mitochondrial origin. No change in glutamate-dependent 45Ca2+ uptake was seen on osmotic lysis of the synaptosomes as the expected increase due to the release of occluded mitochondria was counterbalanced by the damaging effect of hypo-osmotic shock on the glutamate-stimulated 45Ca2+ uptake process.