Calcium buffering in presynaptic nerve terminals. Free calcium levels measured with arsenazo III

The particulate fraction from osmotically shocked synaptosomes (‘synaptosomal membranes’) sequesters Ca when incubated with ATP-containing solutions. This net accumulation of Ca can reduce the free Ca²⁺] of the bathing medium to sub-micromolar levels (measured with arsenazo III). Two distinct types of Ca sequestration site are responsible for the Ca²⁺ buffering. One site, presumed to be smooth endoplasmic reticulum, operates at low Ca²⁺] (less than 1 μM), and has a relatively small capacity. Ca sequestration at this site is prevented by the Ca²⁺ ionophore, A-23187, but not by mitochondrial poisons. The second (mitochondrial) site, in contrast, is blocked by the mitochondrial uncoupler, carbonyl cyanide p-trifluoromethoxyphenylhydrazone, and oligomycin. Since the intraterminal organelles can buffer Ca²⁺] to about 0.3–0.5 μM, this may be an upper limit to the normal resting level of Ca²⁺]_i in nerve terminals. In the steady state, total cell Ca and Ca²⁺]_i will be governed principally by Ca transport mechanisms in the plasmalemma; the intracellular organelle transport systems then operate in equilibrium with this Ca²⁺]. During activity, however, Ca rapidly enters the terminals and Ca²⁺]_i rises. The intracellular buffering mechanisms then come into play and help to return Ca²⁺]_i toward the resting level; the non-mitochondrial Ca sequestration mechanism probably plays the major role in this Ca buffering.