title: Mechanisms by which human blood platelets accumulate glycine, gaba and amino acid precursors of putative neurotransmitters.
abstract: 1. 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.2. Platelets were incubated in Ca(++)-free Krebs solution at 37 degrees C with radio-active amino acids for various times and then the platelets were analysed for the radioactive substance and its metabolites.3. L-Phenylalanine, L-DOPA, L-tryptophan and L-tyrosine were rapidly accumulated, the equilibrium tissue/medium concentration ratio (C(i)/C(0)) being greater than 10:1 when the concentration of amino acid in the medium was 10(-7)M or lower. Glycine and gamma-aminobutyric acid (GABA) accumulation was less, C(i)/C(0) being lower than 3:1 when C(0) was 10(-7)M.4. Uptake of L-phenylalanine, L-DOPA and L-tryptophan were all decreased or abolished by incubation at 4 degrees C, or with metabolic inhibitors or by disruption of the platelet membrane prior to incubation, while L-tyrosine accumulation was not affected.5. 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.6. 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.7. Results are compared to reports of accumulation of these amino acids by the central nervous system.
mesh_terms: D000596:Amino Acids / Q000378:metabolism*; D000613:Aminobutyrates / Q000378:metabolism*; D001792:Blood Platelets / Q000378:metabolism*; D002247:Carbon Isotopes; D002490:Central Nervous System; D004295:Dihydroxyphenylalanine / Q000378:metabolism; D005260:Female; D005998:Glycine / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D010649:Phenylalanine / Q000378:metabolism; D011485:Protein Binding; D011955:Receptors, Drug; D009435:Synaptic Transmission; D014364:Tryptophan / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism
chemical_list: D000596:Amino Acids; D000613:Aminobutyrates; D002247:Carbon Isotopes; D011955:Receptors, Drug; D014443:Tyrosine; D010649:Phenylalanine; D004295:Dihydroxyphenylalanine; D014364:Tryptophan; D005998:Glycine