AJP - Cell Physiology, Vol 252, Issue 6 C588-C594, Copyright © 1987 by American Physiological Society
ATP-dependent regulation of cytoplasmic free calcium in nerve terminals
H. Rasgado-Flores and M. P. Blaustein
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.
