The uptake of 3H-γ-aminobutyric acid by the retina

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Abstract
1 The accumulation of 3H-γ-aminobutyric acid (GABA) by the isolated rat retina has been measured. 2 When retinae were incubated at 37° C in a medium containing 3H-GABA, tissue:medium ratios of about 25:1 were attained after a 30 min incubation. 3 After incubations of 40 min at 37° C, almost all (98%) the radioactivity in the tissue was present as unchanged 3H-GABA. 4 The process responsible for 3H-GABA uptake showed many of the properties of an active uptake system: it was temperature-sensitive, required the presence of sodium ions in the external medium, was inhibited by anoxia, dinitrophenol and ouabain, and showed saturation kinetics. 5 The estimated Km value of GABA was 4·0 × 10−5M, and Vmax was 0·167 (μmoles/min)/g retina. 6 The uptake of 3H-GABA was not affected by the presence of large molar excesses of glycine, l-glutamate, l-aspartate, l-alanine, l-proline, or l-histidine, but was inhibited by dl-γ-amino-β-hydroxybutyrate, β-guanidinopropionate, and l-2,4-diaminobutyrate. 7 The retina was capable of achieving a large net uptake of GABA, indicating that the accumulation of 3H-GABA by the tissue was not due only to an exchange process with the endogenous GABA pool. 8 The uptake of 3H-GABA occurred only in tissue from the central nervous system. Thus, retina and cerebral cortex rapidly accumulated radioactivity, but slices of cornea, posterior wall of the eye, and liver achieved tissue: medium ratios of approximately one. 9 There was a rapid efflux of radioactivity from retinae placed in fresh medium and after 60 min, 90% of the radioactivity was lost from the tissue. The radioactivity released into the medium was present largely as 3H-acidic and neutral metabolites. When the metabolism of GABA was inhibited by the presence of amino-oxyacetic acid in the medium, only about 10% of the radioactivity was lost from the tissue during a similar 60 min incubation, and the radioactivity released was present largely as unchanged 3H-GABA. 10 It is suggested that the GABA uptake process may represent a possible mechanism for the inactivation of GABA if this amino acid is released at inhibitory synapses in the retina.