Mechanisms by which human blood platelets accumulate glycine, gaba and amino acid precursors of putative neurotransmitters

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Abstract
Summary: . We have examined the accumulation by human blood platelets of amino acids that are believed to be involved in neurohumoral transmission in the central nervous system. . Platelets were incubated in Ca++‐free Krebs solution at 37° C with radioactive amino acids for various times and then the platelets were analysed for the radioactive substance and its metabolites. . l‐Phenylalanine, l‐DOPA, l‐tryptophan and l‐tyrosine were rapidly accumulated, the equilibrium tissue/medium concentration ratio (Ci/Co) being greater than 10: 1 when the concentration of amino acid in the medium was 10−7m or lower. Glycine and γ‐aminobutyric acid (GABA) accumulation was less, Ci/Co being lower than 3: 1 when Co was 10−7m. . Uptake of l‐phenylalanine, l‐DOPA and l‐tryptophan were all decreased or abolished by incubation at 4° C, or with metabolic inhibitors or by disruption of the platelet membrane prior to incubation, while l‐tyrosine accumulation was not affected. . It is considered that l‐phenylalanine, l‐DOPA and l‐tryptophan are accumulated by saturable, energy‐dependent processes; that glycine and GABA diffuse into the platelet, and l‐tyrosine accumulates as a result of diffusion and intracellular binding. . None of the amino acids examined showed any significant metabolism during a 20 min incubation. However, some evidence for tyrosine binding to soluble protein was obtained. . Results are compared to reports of accumulation of these amino acids by the central nervous system.