IN VITRO Ca2+ DEPENDENT PROTEOLYSIS IN BRAIN TISSUES: A POSSIBLE SOURCE OF ERROR IN TRANSMITTER METABOLISM STUDIES

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Abstract
—The effects of Ca2+ ions on the metabolism of [3H]serotonin and [3H]‐labelled catecholamines have been examined in hippocampal slices or synaptosomes. The formation of [3H]‐5 hydroxyindoles ([3H]serotonin + [3H]‐5 hydroxyindoleacetic acid) from [3H]tryptophan and that of [3H]‐labelled catecholamines from [3H]tyrosine were increased when Ca2+ was omitted from the incubating medium. However, the total synthesis of 5‐HT from tryptophan and that of catecholamines from tyrosine did not seem to be significantly changed. Altered formation of tritiated amines were due to changes in the specific activities of respective precursor amino acids. This reflected altered sizes of the free amino acid pools caused by Ca2+‐dependent in vitro proteolysis. This must be taken into consideration when studying in vitro Ca2+ dependency of neutrotransmitter metabolism.