Ions and the Transport of Gamma‐aminobutyric Acid by Synaptosomes

The initial rate of uptake of [2,3‐ 3 H]gamma‐aminobutyric acid by rat brain synaptosomes was studied under incubation conditions in which GABA metabolism was minimal. The presence of both sodium and potassium in the incubation medium was essential for sustained uptake. Uptake proceeded for a short period of time in the absence of potassium and then ceased. No uptake was observed when sodium chloride was completely replaced with sucrose or choline chloride. The sodium‐dependence curve for GABA uptake was markedly sigmoid. The sigmoid character of the curve was not attributable to a lag phase in uptake at low sodium concentrations. Calcium strongly stimulated the initial rate of uptake at low sodium concentrations but had little effect at sodium concentrations above 100 m m and was not able to support uptake in the absence of sodium. The sigmoid character of thesodium‐dependence curve was completely eliminated by 20 m m calcium ion. Magnesium and phosphate had little effect on the initial rate of GABA uptake.