Kinetics of Tryptophan Influx into Brain Slices Depleted of Sodium and Loaded withl‐Histidine

Abstract

The kinetics of tryptophan influx were studied with rat brain slices preloaded with L-histidine and/or depleted of Na+ ions. The best fits of the data (velocity of influx vs. tryptophan concentration) were computed by use of a model consisting of a saturable (Michaelis-Menten type) and an unsaturable (diffusional) component with an iterative nonlinear regression anaysis. Na+ depletion of the slices reduced the maximal velocity of saturable influx. In histidine-preloaded slices, depleted or not depleted of Na+ ions, the most marked alteration again occurred in the maximal velocity, which more than doubled. Slices preloaded with histidine contained greatly elevated levels of glutamine and histidine, which may have stimulated the influx by exhange with extracellular tryptophan even in the absence of Na+ ions. The maximal velocity was higher with increasing concentration of large neutral amino acids in slices at the start of the influx measurements. The influx of tryptophan in brain cells is apparently modified by changes in the intracellular amino acid pool, which, when increased, also counteracts the effect of Na+ depletion on the tryptophan influx.