Transport and metabolism of l-glutamic acid by abdominal ganglia of the hawk moth, Manduca sexta

Abstract

1. The transport of exogenous L-glutamate was characterized in an insect ganglion with an in vitro preparation consisting of whole ganglia in short-term organ culture. The characteristics studied include the ionic dependence and specificity of transport, as well as its rate of operation. 2. Rates for the Na +-dependent and Na +-independent components were measured, revealing lowaffinity carriers; the apparent K Ms are 0.48 and 1.27 mM, respectively. 3. Accumulation of exogenous glutamate is saturable, sensitive to metabolic inhibitors, and specific for certain dicarboxylic acids, thus exhibiting features of a carrier-mediated process. Transport is partially dependent on Na + in the medium, and it is reduced in the presence of the competitor amino acids, L-and D-aspartate. 4. Exogenous glutamate is metabolized by abdominal ganglia; the major amino acid metabolites were measured in extracts of the ganglia and in the culture medium. Major amino acid metabolites derived from glutamate and recovered in the ganglia are glutamine, proline, GABA, and small amounts of aspartate and alanine. 5. Proline and glutamine are spontaneously released by the ganglia. A portion of this material is newly synthesized from exogenous glutamate, and its release follows a different time course for proline and glutamine. 6. Metabolism of exogenous glutamate may be a biochemical aspect of the insect blood-brain barrier.