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RELATIONSHIP BETWEEN Ca 2+ ‐DEPENDENT AND INDEPENDENT RELEASE OF [ 3 H]GABA EVOKED BY HIGH K + , VERATRIDINE OR ELECTRICAL STIMULATION FROM RAT CORTICAL SLICES
Author(s) -
Szerb John C.
Publication year - 1979
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1979.tb11100.x
Subject(s) - veratridine , tetrodotoxin , stimulation , depolarization , chemistry , calcium , biophysics , sodium , endocrinology , biology , sodium channel , organic chemistry
— It has been reported that the release of GABA by high K + is Ca 2+ ‐dependent while release induced by veratridine or electrical stimulation has been frequently found to be independent of Ca 2+ . To see the source of Ca 2+ ‐dependent and independent release of GABA, cortical slices which had accumulated [ 3 H]GABA were exposed to 50 m m ‐K + or 50 μ m ‐veratridine for 48min. In the presence of Ca 2+ the 2 agents released approx the same amount of [ 3 H]GABA but tetrodotoxin (TTX) abolished release induced only by veratridine, while omission of Ca 2+ reduced release induced only by 50m m ‐K + . Pre‐exposure of the slices for 48min to 50m m ‐K + in the presence of Ca 2+ reduced the second release by 50m m ‐K + by 77% and that by veratridine by 74%, suggesting that in the presence of Ca 2+ the 2 depolarizing agents release [ 3 H]GABA from the same pool. Pre‐exposure to 50m m ‐K + in the absence of Ca 2+ reduced the second release by 50m m ‐K + or by veratridine only by 37 and 27% respectively, indicating that most of the reduction in release was the result of a depletion of releasable [ 3 H]GABA stores. The second exposure to 50m m ‐K + in the absence of Ca 2+ reduced the evoked release further, while exposure to veratridine in the absence of Ca 2+ , after depletion of the stores, enhanced release 2.7 times. Electrical stimulation (64 Hz, 2 ms, 40 mA, alternating polarity) during 24min in the presence of Ca” caused an initial 5‐fold increase in efflux, which declined subsequently. In the absence of Ca 2+ , instead of a rapid increase, a slow but smaller increase in the efflux of [ 3 H]GABA was found. TTX almost completely abolished the electrically evoked increase in release. Pre‐treatment with 50m m ‐K + reduced the electrically evoked release by 94% but electrical stimulation in the absence of Ca 2+ after depletion of releasable stores doubled this release. Results suggest that in the presence of Ca 2+ , high K + , veratridine and electrical stimulation release [ 3 H]GABA from the same Ca 2+ ‐dependent store, but in the absence of Ca 2+ veratridine and electrical stimulation enhance the release from a Ca 2+ ‐independent store, probably as a result of an increased influx of Na + .