Kinetics of cerebral uptake processes in vitro of L-glutamine, branched-chain L-amino acids, and L-phenylalanine: effects of ouabain.

Estimates have been made of the amounts and rates of uptake of radioactive branched-chain L-amino acids, L-phenylalanine, and L-glutamine into incubated rat brain cortex slices. Estimates have also been made of the binding of these amino acids to brain cell fragments. It is shown that such binding, as well as the process of passive diffusion, is not affected by the presence of ouabain (0.2 mM), which suppresses the energy-dependent concentrative uptakes of the amino acids investigated. The maximum specific binding of L-glutamine is about three times that of the other amino acids and amounts to about 11% of the total uptake of the amino acid by rat brain cortex slices in 12 min from a medium containing 0.25 mM-glutamine. The sodium-ion concentration of the medium appears not to play a significant role in determining the rate of L-glutamine uptake in brain slices except at relatively low concentrations (< 20 mequiv./1). The presence of Na+, however, is essential for the attainment of a tissue-to-medium concentration ratio greater than 2.0 for L-glutamine. At relatively low concentrations (0.25 mM) the rapidity of uptake of L-glutamine into a suspension of nerve terminals exceeds that into brain cortex slices. The uptakes of L-glutamine (Km's = 0.66 mM and 2.25 mM) and of the branched chain L-amino acids (Km's approx. 0.3 mM and 2 mM) by rat brain cortex slices are characterized by a double affinity system, but that of L-phenylalanine has only one affinity system (Km = 0.23 mM). The Km's have been calculated after subtracting the ouabain-insensitive passive uptakes of the amino acids from the total observed uptakes.