Role of sodium-calcium exchange in regulation of intracellular calcium in nerve terminals

Ca efflux from rat brain presynaptic nerve terminals (synaptosomes) was examined after loading the terminals with 45Ca during a brief depolarization, usually in media containing 20 μM Ca labeled with 45Ca, to assure a small (physiological) load. Efflux of 45Ca was very slow in the absence of external Na and Ca and was greatly accelerated by Na and/or Ca. The dependence of 45Ca efflux on external Na was sigmoid, with a Hill coefficient of ∼ 4.5; this implies that more than two external Na ions are required to activate the efflux of one Ca ion. The external Na (Na0)-dependent Ca efflux was inhibited by 1 mM external La, by low temperature, and by raising external K. With small Ca loads, the mitochondrial uncoupler, carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP), had negligible effect on either Ca uptake or efflux; with large loads, however, FCCP reduced the depolarization-stimulated Ca uptake and increased the Na0-dependent Ca efflux. These effects may be attributed to reduction of mitochondrial Ca sequestration. Mitochondria do not appear to sequester much Ca when the loads are smaller. Estimations of Ca efflux indicate that ∼ 20% of a small 45Ca load may be extruded via Na+-Ca2+ exchange within 1 s; this corresponds to a net Ca efflux of ∼ 110 pmol Ca x mg protein-1 x s-1. This rate of extrusion is equivalent to the net Ca gain when the terminals fire at a frequency of ∼ 18/s. The data on the Ca efflux into Na- and Ca-free media indicate that the ATP-fueled Ca pump can only extrude ∼ 10-20 pmol Ca x mg protein-1 x s-1. Thus the results imply that Na+-Ca2+ exchange plays an important role in helping to extrude the Ca that enters during activity