Uptake, Metabolism, and Release of N‐[3H]‐Acetylaspartylglutamate by the Avian Retina

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Abstract
N‐Acetylaspartylglutamate (NAAG) is a nervous system‐specific dipeptide that is released from retinal neurons on depolarization. In the present study, extracellular metabolism, uptake, and release of [3H]NAAG were examined in the chick retina. After in vitro incubation with NAAG radiolabeled in the glutamate moiety, [3H]‐glutamate and [3H]NAAG increased in retinal cells through time‐ and temperature‐dependent processes, which were reduced in the absence of extracellular sodium. Coincubation of cells with [3H]NAAG and aspartylglutamate or phosphate resulted in the decreased extracellular appearance of [3H]glutamate, produced by hydrolysis of radiolabeled NAAG, and a consequent increased availability of [3H]‐NAAG for transport into the retinal cells. When this tissue was incubated with radiolabeled NAAG, glutamate, glutamine, or aspartate under similar conditions, only [3H]‐NAAG served as a significant source for the appearance of intracellular [3H]NAAG. These data support the conclusion that [3H]NAAG can be transported into retinal cells, whereas [3H]glutamate transport is the predominant process after release of this amino acid from NAAG by extracellular peptidase activities. After uptake, [3H]NAAG entered a cellular pool, from which the peptide was secreted under depolarizing conditions and in a calcium‐dependent manner.

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