The human platelet as a model for the glutamatergic neuron: platelet uptake of L-glutamate.

L-Glutamate uptake into human platelets revealed two components: a high-affinity system (KmH = 3.1 microM), which was sodium-dependent, and a low-affinity site (Km = 88 microM) displaying temperature rather than sodium dependency. These kinetic properties were similar to those found in crude synaptosomal preparations and brain slices. However, Vmax values were far higher in brain (VmaxH = 325 +/- 96, VmaxL = 3759 +/- 1116 pmol/mg wet weight per min) than in platelets (VmaxH = 14 +/- 6, VmaxL = 313 +/- 63 pmol/mg platelet protein per 10 min), indicating a denser population in brain than in platelets of qualitatively similar sites. Pharmacological analysis substantiated the resemblance of nerve endings and platelets: the specific uptake inhibitors threo-3-hydroxy-DL-aspartate and DL-aspartate-beta-hydroxamate as well as D- and L-glutamate and L-aspartate showed similar--though not identical-- IC50 values in both preparations; a spectrum of compounds devoid of inhibitory effects in synaptosomes also did not interfere with glutamate uptake in platelets. Uptake parameters were studied in a population of human volunteers to determine the variability of platelet glutamate uptake. Whole blood could be stored up to 6 h after venipuncture without any appreciable change in experimental values. Percentage of variation between 0.09 and 0.28 for three repetitive (weekly) assays in single subjects indicated that glutamate uptake measurements in human platelets are sufficiently suited for future clinical studies.