Ca2+ dynamics at the frog motor nerve terminal.
Rises in free [Ca2+]i in response to various tetanic stimuli (Ca2+ transient) in frog motor nerve terminals were measured by recording fluorescence changes of Ca2+ indicators and analyzed in relation to short-term synaptic plasticity. Ca2+ transients reached a plateau after 10-20 impulses at 100 Hz and decayed in a three-exponential manner, in which the fast component was predominant. The plateau and fast component of the Ca2+ transient were elevated infralinearly with an increase in tetanus frequency. Computer simulation showed that the Ca2+ transients estimated from fluorescence changes faithfully reflect the true changes in [Ca2+]i except for the initial 20 ms. A slow Ca2+ chelator, EGTA, loaded into the nerve terminal, decreased the magnitude of both the fast and slow components of facilitation of transmitter release and the time constant of the former. A fast Ca2+ chelator, BAPTA, decreased the magnitude of fast facilitation but slightly increased its time constant. These results suggest that Ca2+ transients in the frog motor nerve terminals are primarily caused by Ca2+ entry and are dissipated by three components, in which the rate of the fast component is equivalent to that of free Ca2+ diffusion. The residual Ca2+ in the nerve terminals after stimulation accounts for the fast component of facilitation.
Suzuki
S
S
Department of Physics, School of Science, Nagoya University, Japan.
Osanai
M
M
Murase
M
M
Suzuki
N
N
Ito
K
K
Shirasaki
T
T
Narita
K
K
Ohnuma
K
K
Kuba
K
K
Kijima
H
H
D000818:Animals; D002118:Calcium / Q000493:pharmacokinetics*; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D005456:Fluorescent Dyes; D007211:Indoles; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008856:Microscopy, Fluorescence; D009046:Motor Neurons / Q000378:metabolism*; D009473:Neuronal Plasticity / Q000502:physiology; D017729:Presynaptic Terminals / Q000378:metabolism*; D011898:Ranidae; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology
D002614:Chelating Agents; D005456:Fluorescent Dyes; D007211:Indoles; D004533:Egtazic Acid; D008274:Magnesium; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; C048960:indo-1; D002118:Calcium