Transport of Histidine into Synaptosomes of the Rat Central Nervous System

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Abstract
Histidine transport into synaptosomes was studied in order to characterize this aspect of histamine synthesis in neurons. Histidine transport was independent of Na+, Ca2+ and Mg2+ and dependent upon K+ and Cl. Histidine transport was energy dependent, and subcellular fractionation studies suggested it was highly localized to nerve terminals. Kinetic analysis of histidine transport in several brain regions indicated the presence of 2 uptake sites, a high-affininty site with a Km .apprx. 35 .mu.M and a low-affinity site with a Km in the millimolar range. Density of the high-affinity site, as reflected by Vmax, correlates well with density of proposed histaminergic innervation. Rate of histidine transport was not altered by prior depolarizaiton of the synaptosomes, indicating that histidine transport probably does not play a regulatory role in histamine synthesis.