Synergism between different transport systems stimulates the uptake of neutral amino acids by isolated brain microvessels.

The polar long-chain amino acids glutamine and methionine can be transported across the endothelial cells of brain microvessels either by an L-system which operates by a facilitated diffusion, exchanging mechanism, or by a concentrating, energy-dependent A-system. The presence of glutamine and/or of methionine can induce a synergism between the two transport systems which results, by a transstimulation mechanism, in a net increased uptake of neutral hydrophobic aminoacids. The methionine analog S-methylthiocysteine, which is the mixed disulfide resulting from the combination of methanethiol with cysteine, behaves similarly to methionine in stimulating the uptake of neutral hydrophobic amino acids. The same transstimulating effect can even be obtained in collagenase-treated, A-system-deprived microvessels by inducing the direct formation of S-methylthiocysteine within the cytoplasmic compartment of the endothelial cells.