Further characterization of [3H]gamma-aminobutyric acid release from isolated neuronal growth cones: role of intracellular Ca2+ stores.
We have recently shown that growth cones isolated from neonatal rat forebrain possess uptake and release mechanisms for the neurotransmitter gamma-aminobutyric acid. About half of the K+-induced release of [3H]gamma-aminobutyric acid from isolated growth cones is dependent on extracellular Ca2+. The remaining component of the [3H]gamma-aminobutyric acid release is unaffected by removal of extracellular Ca2+ and is resistant to blockade by the voltage-sensitive Ca2+-channel blocker methoxyverapamil. In the present series of experiments we have used caffeine to assess the possible role of intracellular stores of Ca2+ in supporting that component of the K+-induced release of [3H]gamma-aminobutyric acid from isolated growth cones that is independent of extracellular Ca2+. We have chosen caffeine because of its well established effect of releasing Ca2+ from smooth endoplasmic reticulum in muscle. We found that caffeine can release [3H]gamma-aminobutyric acid from isolated growth cones. This effect persists in Ca2+-free medium, in the presence of methoxyverapamil and in the absence of Na+. Furthermore, isobutylmethylxanthine could not substitute for caffeine suggesting that the caffeine effect is not due to phosphodiesterase inhibition and the subsequent rise in intracellular cyclic nucleotides. A combination of the mitochondrial poisons, Antimycin A and sodium azide had no effect on the release of [3H]gamma-aminobutyric acid induced either by caffeine or by high K+. We conclude that caffeine causes the release of Ca2+ from a non-mitochondrial store within the growth cone and that this Ca2+ store supports that component of the K+-induced release of [3H]gamma-aminobutyric acid that is independent of extracellular Ca2+.
Lockerbie
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Gordon-Weeks
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D000818:Animals; D000968:Antimycin A / Q000494:pharmacology; D001921:Brain / Q000254:growth & development / Q000378:metabolism*; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D005711:Gallopamil / Q000494:pharmacology; D008928:Mitochondria / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D013347:Subcellular Fractions; D013997:Time Factors; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D005711:Gallopamil; D002110:Caffeine; D005680:gamma-Aminobutyric Acid; D000968:Antimycin A; D011188:Potassium; D002118:Calcium