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How is the cytoplasmic calcium concentration controlled in nerve terminals?
Journal De Physiologie
|September 1, 1980
Summary
Presynaptic nerve terminals use intraterminal organelles to buffer calcium. The smooth endoplasmic reticulum (SER) plays a key role in buffering cytoplasmic calcium, even when mitochondria are inhibited.
Area of Science:
- Neuroscience
- Cell Biology
- Biochemistry
Background:
- Cytoplasmic free Ca2+ concentration ([Ca2+]i) regulation is crucial for neuronal function.
- Intraterminal organelles within presynaptic nerve terminals (synaptosomes) are known to sequester calcium.
- Understanding the specific roles of these organelles in calcium buffering is essential.
Purpose of the Study:
- To investigate the calcium-sequestering and buffering capabilities of intraterminal organelles in mammalian presynaptic nerve terminals.
- To differentiate the contributions of mitochondria and smooth endoplasmic reticulum (SER) to calcium homeostasis.
- To determine the role of SER in buffering physiological calcium loads.
Main Methods:
- Isolated mammalian presynaptic nerve terminals (synaptosomes) were used.
- Biochemical methods included the use of mitochondrial poisons (oligomycin, azide, 2,4-dinitrophenol) and a Ca2+-sensitive indicator (arsenazo III).
- Morphological methods involved electron microscopy with X-ray microprobe analysis after incubation with oxalate.
Main Results:
- Disrupted synaptosomes sequestered Ca2+ into two types of organelles in an ATP-dependent manner.
- Mitochondrial poisons inhibited Ca2+ uptake into mitochondria but spared a second organelle with higher Ca2+ affinity.
- Electron microscopy revealed Ca2+ deposits in mitochondria and SER; mitochondrial poisons blocked mitochondrial deposits but not SER deposits.
- X-ray microprobe analysis confirmed the presence of Ca2+ in these deposits.
- Intraterminal organelles buffered Ca2+ to below 5 X 10(-7) M, with SER effectively buffering physiological Ca2+ loads even when mitochondria were inhibited.
Conclusions:
- Mammalian presynaptic nerve terminals possess distinct intraterminal organelles for calcium sequestration.
- The smooth endoplasmic reticulum (SER) demonstrates a high affinity for Ca2+ and plays a significant role in buffering cytoplasmic calcium.
- SER contributes importantly to calcium buffering during neuronal activity, particularly under physiological calcium loads.