Repetitive action potentials in isolated nerve terminals in the presence of 4-aminopyridine: effects on cytosolic free Ca2+ and glutamate release.
The mechanisms by which an elevated KCl level and the K+-channel inhibitor 4-aminopyridine induce release of transmitter glutamate from guinea-pig cerebral cortical synaptosomes are contrasted. KCl at 30 mM caused an initial spike in the cytosolic free Ca2+ concentration ([Ca2+]c), followed by a partial recovery to a plateau 112 +/- 13 nM above the polarized control. The Ca2+-dependent release of endogenous glutamate, determined by continuous fluorimetry, was largely complete by 3 min, by which time 1.70 +/- 0.35 nmol/mg was released. [Ca2+]c elevation and glutamate release were both insensitive to tetrodotoxin. KCl-induced elevation in [Ca2+]c could be observed in both low-Na+ medium and in the presence of low concentrations of veratridine. 4-Aminopyridine at 1 mM increased [Ca2+]c by 143 +/- 18 nM to a plateau similar to that following 30 mM KCl. The initial rate of increase in [Ca2+]c following 4-aminopyridine administration was slower than that following 30 mM KCl, and a transient spike was less apparent. Consistent with this, the 4-aminopyridine-induced net uptake of 45Ca2+ is much lower than that following an elevated KCl level. 4-Aminopyridine induced the Ca2+-dependent release of glutamate, although with somewhat slower kinetics than that for KCl. The measured release was 0.81 nmol of glutamate/mg in the first 3 min of 4-aminopyridine action. In contrast to KCl, glutamate release and the increase in [Ca2+]c with 4-aminopyridine were almost entirely blocked by tetrodotoxin, a result indicating repetitive firing of Na+ channels. Basal [Ca2+]c and glutamate release from polarized synaptosomes were also significantly lowered by tetrodotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)
Tibbs
G R
GR
Department of Biochemistry, Imperial College of Science and Technology, London, England.
Barrie
A P
AP
Van Mieghem
F J
FJ
McMahon
H T
HT
Nicholls
D G
DG
D015761:4-Aminopyridine / Q000494:pharmacology*; D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002540:Cerebral Cortex / Q000378:metabolism / Q000502:physiology*; D003600:Cytosol / Q000378:metabolism; D005971:Glutamates / Q000378:metabolism*; D006168:Guinea Pigs; D007700:Kinetics; D011189:Potassium Chloride / Q000494:pharmacology; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D014701:Veratridine / Q000494:pharmacology
D005971:Glutamates; D011189:Potassium Chloride; D014701:Veratridine; D015761:4-Aminopyridine; D002118:Calcium