Effects of Branched‐Chain L‐Amino Acids, L‐Phenylalanine, and L‐Methionine on the Transport of L‐Glutamine in Rat Brain Cortex In Vitro. Influence of Cations

Abstract

Uptake of L-glutamine (2 mM) by rat brain cortex slices against a concentration gradient is markedly inhibited (40%) by branched-chain L-amino acids (1 mM), L-phenylalanine (1 mM) or L-methionine (1 mM); that of L-asparagine (2 mM) is much less affected by these amino acids. Other amino acids investigated have little or no effect on cerebral L-glutamine uptake. The suppressions of L-glutamine uptake by the inhibitory amino acids are apparently blocked by high [K+], which itself has little or no effect on glutamine uptake. This abolition of suppression is partly explained by high [K+] retention of endogenous glutamine; in the absence of Ca2+ such retention disappears. The inhibitory amino acids (1 mM) also enhance the release of endogenous glutamine, exogenous glutamine with which slices were loaded, or glutamine synthesized in the slices from exogenous glutamate. The enhanced release of endogenous glutamine is diminished by high [K+]. The suppression of glutamine uptake by the branched-chain amino acids is independent of the concentration of glutamine at low concentrations (0.25-0.5 mM), indicating non-competition, but is reduced with high concentration of glutamine. The inhibition by L-phenylalanine is noncompetitive. L-Glutamine (2 mM) exerts no inhibition of the cerebral uptakes of the branched-chain L-amino acids or L-phenylalanine (0.25-2 mM). The inhibitory amino acids are as active in suppressing L-glutamine uptake with immature rat brain slices as with adult, although the uptake, against a gradient, of L-glutamine in the infant rat brain is about 1/2 that in the adult. They are also just as inhibitory on the concentrative uptake of L-glutamine by a crude synaptosomal preparation derived from rat brain cortex. Such a nerve ending preparation takes up L-glutamine (0.25 mM), against a gradient, at about 9-fold the rate at which it is take up by cortex slices (for equal amounts of protein), and the uptake is markedly suppressed by high [K+] in contrast to the effects of high [K+] with slices. The possible physiological and pathological consequences of the suppresion of glutamine uptake are discussed.