Transport systems for GABA and for other amino acids in incubated chick brain tissue during development

Abstract

The transport of six amino acids (GABA(γ-aminobutyric acid), glycine, AIB (α-aminoisobutyric acid), leucine, d-glutamate, and lysine) was studied in brain slices from chick embryos and young chicks at different developmental stages. Initial accumulation rates increased significantly during embryonic development, and, with the exception of glycine, decreased somewhat after hatching. The greatest changes were observed with GABA. Steady state accumulation levels were reached more slowly and increased less than initial accumulation rates in more immature brains, suggesting that exit rates also increase during development. The influx of the six amino acids was analyzed kinetically: double-reciprocal plots of AIB, leucine, d-glutamate, and lysine influx gave straight lines at all the ages studied. For any of these amino acids, apparent K m values remained constant with age, but V values varied significantly, suggesting that the availability or the turnover of carriers, but not their affinity for the transported molecules, changes during development. Double-reciprocal plots of GABA and glycine influx gave two-limbed curves with embryonic brains (GABA) and also with young chick brain (glycine), which seems to indicate that at least two modes of mediated entry exist for these amino acids. With GABA one of the two transport systems tends to disappear, or to become undetectable, with increasing age. Substrate specificity studies of GABA influx showed that only amino acids having the cationic group in beta or gamma position are effective inhibitors of GABA transport. The developmental pattern of the inhibitions obtained at selected GABA concentrations suggests that a high affinity and low capacity transport system for GABA tends to disappear during development. Changes in the pattern of the sodium dependence of GABA influx also indicate the existence of more than one transport system at early developmental stages. When the substrate specificity and the sodium-dependence of AIB influx were studied, similar inhibitory effects were obtained at different AIB concentrations, confirming that this amino acid is transported by only one transport system. However, a progressive increase in the overlapping between AIB and leucine transport systems during development was demonstrated by the progressively increasing inhibition of AIB influx by leucine.