ISSN:
1471-4159
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Medicine
Notes:
Abstract: 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. KC1 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 n M 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. Addition of 30 mM KCl to 4-ami-nopyridine-pretreated synaptosomes caused a large transient spike in [Ca2+]c and further release of glutamate. 4-Aminopyridine failed to increase [Ca2+]c in low-Na+ media or after addition of low concentrations of veratridine. It is proposed that an unstable membrane potential in the presence of 4-aminopyridine is amplified by repetitive firing of Na+ channels and that this leads to random opening of transient Ca2+ channels in the synaptosomal population. In contrast, KCl would induce a synchronous activation of transient Ca2+ channels followed by a plateau of low residual channel conductance. By mimicking repetitive stimulation in vivo, 4-aminopyridine may more closely model physiological excitation than does an elevated KCl level. Our results are difficult to reconcile with a major role for Na+/Ca2+ exchange in the elevation of [Ca2+]c and contradict proposals that 4-aminopyridine induces release of transmitter from isolated nerve terminals by a mechanism not linked to Na+-channel firing orCa2+ entry
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1471-4159.1989.tb09232.x
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