An inhibitory effect of extracellular Ca2+ on Ca2+-dependent exocytosis.
OBJECTIVE
Neurotransmitter release is elicited by an elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)). The action potential triggers Ca(2+) influx through Ca(2+) channels which causes local changes of [Ca(2+)](i) for vesicle release. However, any direct role of extracellular Ca(2+) (besides Ca(2+) influx) on Ca(2+)-dependent exocytosis remains elusive. Here we set out to investigate this possibility on rat dorsal root ganglion (DRG) neurons and chromaffin cells, widely used models for studying vesicle exocytosis.


RESULTS
Using photolysis of caged Ca(2+) and caffeine-induced release of stored Ca(2+), we found that extracellular Ca(2+) inhibited exocytosis following moderate [Ca(2+)](i) rises (2-3 µM). The IC(50) for extracellular Ca(2+) inhibition of exocytosis (ECIE) was 1.38 mM and a physiological reduction (∼30%) of extracellular Ca(2+) concentration ([Ca(2+)](o)) significantly increased the evoked exocytosis. At the single vesicle level, quantal size and release frequency were also altered by physiological [Ca(2+)](o). The calcimimetics Mg(2+), Cd(2+), G418, and neomycin all inhibited exocytosis. The extracellular Ca(2+)-sensing receptor (CaSR) was not involved because specific drugs and knockdown of CaSR in DRG neurons did not affect ECIE.


CONCLUSIONS
As an extension of the classic Ca(2+) hypothesis of synaptic release, physiological levels of extracellular Ca(2+) play dual roles in evoked exocytosis by providing a source of Ca(2+) influx, and by directly regulating quantal size and release probability in neuronal cells.
State Key Laboratory of Biomembrane Engineering and Center for Life Sciences, Institute of Molecular Medicine, Peking University, Beijing, China.
Wei
W
Xiong
Tao
T
Liu
Yeshi
Y
Wang
Xiaowei
X
Chen
Lei
L
Sun
Ning
N
Guo
Hui
H
Zheng
Lianghong
L
Zheng
Martial
M
Ruat
Weiping
W
Han
Claire Xi
CX
Zhang
Zhuan
Z
Zhou
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D057966:Calcimimetic Agents / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D019439:Chromaffin Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D005089:Exocytosis / Q000187:drug effects; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism*; D005727:Ganglia, Spinal / Q000166:cytology; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010782:Photolysis; D051381:Rats; D017208:Rats, Wistar
D057966:Calcimimetic Agents; D002110:Caffeine; D002118:Calcium