ATP-dependent regulation of cytoplasmic free calcium in nerve terminals

ATP-dependent Ca uptake was studied in hyperpermeable (saponin treated) rat brain isolated nerve terminals (synaptosomes). The Ca uptake was measured at short incubation times (1-30 s) in the absence and presence of mitochondrial poisons, at various free Ca2+ concentrations (0.03-30 microM). Saponin treatment made the plasma membranes leaky without affecting the ATP-dependent Ca uptake by intracellular organelles. When the free Ca2+ concentration in the incubation medium was varied up to approximately 5 microM free Ca2+, mitochondrial blockers had no effect on the ATP-dependent Ca2+ uptake in the saponin-treated synaptosomes. At higher free Ca2+ concentrations, the blockers inhibited a portion of the ATP-dependent Ca uptake. This indicates that, in the dynamic physiological range of free Ca2+, the nonmitochondrial Ca uptake system (presumably the smooth endoplasmic reticulum, SER) is a more important Ca buffering system than the mitochondrial system. The SER sequesters Ca half maximally at free Ca2+ congruent to 0.4 microM and has a maximal Ca storage capacity of approximately 2 nmol/mg protein. The initial rate of SER Ca uptake is 0.1 nmol X mg protein-1 X s-1. This rate is too slow to account for the very rapid reduction of free Ca2+ that is required to terminate transmitter release immediately after presynaptic depolarization. Nevertheless, Ca sequestration in SER may play an important role in regulating longer term processes such as facilitation and post-tetanic potentiation.