Efflux of amino acid neurotransmitters from rat spinal cord slices. I. Factors influencing the spontaneous efflux of [¹⁴C]glycine and ³H-GABA

The efflux from nervous tissue of presumptive amino acid transmitters was studied by superfusing rat spinal cord slices preloaded with [¹⁴C]glycine and ³H-GABA. The rate of efflux of the amino acids was slower than that of urea and was retarded by superfusion at 4°C. Extracellular glycine exchanged with intracellular [¹⁴C]glycine, but did not affect the rates of efflux of labeled GABA, proline, l-lysine, or cycloleucine. The efflux of [¹⁴C]glycine was accelerated by extracellular GABA, unaffected by extracellular AIB or proline, and retarded by extracellular l-lysine and l-leucine. Ouabain increased the rate of efflux of [¹⁴C]glycine and ³H-GABA, but did not affect [¹⁴C]urea efflux. Cyanide stimulated the efflux of [¹⁴C]glycine but not ³H-GABA. The rates of efflux of both amino acids were increased in Ca-free medium containing EDTA, unchanged in 30 mM K medium, and decreased in low Na medium. The results indicate that there are mediated transport systems with considerable specificity which regulate the efflux of glycine and GABA across neural membranes.