HOW IS THE CYTOPLASMIC CALCIUM-CONCENTRATION CONTROLLED IN NERVE-TERMINALS

  • 1 January 1980
    • journal article
    • research article
    • Vol. 76  (5) , 459-470
Abstract
The ability of intraterminal organelles to sequester Ca and buffer the cytoplasmic free Ca2+ concentration ([Ca2+]i) was investigated in isolated mammalian presynaptic nerve terminals (synaptosomes). A combination of biochemical and morphological methods was used on rat brain preparations. When the plasmalemma of synaptosomes is disrupted by osmotic shock or saponin, Ca from the medium can be sequestered by 2 types of intraterminal organelles in the presence of ATP. Typical mitochondrial poisons (e.g., oligomycin, azide and 2,4-dinitrophenol) block the Ca uptake into 1 type of organelle (mitochondria); the 2nd type of organelle, which has a higher affinity for Ca (half-saturation .simeq. 0.35 .mu.M Ca2+) is spared by the mitochondrial poisons. When the leaky synaptosomes are incubated in media containing oxalate, and then fixed and prepared for EM, electron-dense deposits are observed in the intraterminal mitonchondria and smooth endoplasmic reticulum (SER). Mitochondrial poisons block the formation of the deposits in the mitochondria, but spare the SER. X-ray microprobe analysis demonstrates that these deposits contain Ca. Experiments with the Ca-sensitive metallochromic indicator, arsenazo III, demonstrate that the intraterminal organelles in the leaky synaptosomes can buffer Ca2+ in the medium to below 5 .times. 10-7 M. With small (physiological) Ca loads, the Ca2+ is effectively buffered (to < 5 .times. 10-7 M) even in the presence of mitochondrial poisons. SER in presynaptic terminals may play an important role in helping to buffer the Ca that normally enters during neuronal activity.