Na + Dependence of Tyrosine Transport Across the Synaptosomal Membrane Reflects Changes in the Morphology of Synaptosomes

The Na + dependence of tyrosine uptake into rat brain synaptosomes and synaptosomal plasma membrane vesicles (SPMV) was examined in the present study. At low tyrosine concentrations, the isoosmotic substitution of Na + by sucrose in the incubation medium led to an increase of tyrosine uptake in synaptosomes and to a decrease in SPMV. The removal of extracellular Ca 2+ and Mg 2+ and addition of isoosmotic sucrose completely prevented the augmented tyrosine uptake in Na + ‐free incubated synaptosomes. Morphological differences were found at the electron‐microscopic level when synaptosomes were incubated in Na + ‐free and Na + ‐containing media. The internal volume measured for synaptosomes incubated in a Na + ‐free medium was almost half of that obtained in a Na + ‐containing medium, in good agreement with the observations made with the electron microscope. Also, the omission of Ca 2+ and Mg 2+ resulted in a specific swelling of only the synaptosomes incubated in Na + ‐free medium. When synaptosomes and SPMV were preloaded with several neutral amino acids, the tyrosine uptake rate was greatly increased, indicating fully operational exchange mechanisms for these amino acids. We propose that the enhancement of high‐affinity synaptosomal tyrosine uptake observed in Na + ‐free medium is a consequence of a specific shrinkage of the synaptosomes and a parallel increase of the exchange rate with endogenous neutral amino acids.