Effect of inhibitors of γ‐aminobutyrate aminotransferase on the accumulation of H‐γ‐aminobutyric acid by the retina

Summary Rat retinae pre‐incubated and incubated at 37° C in media containing amino‐oxyacetic acid (AOAA) (0·1 μm to 1 mm) accumulated more 3H‐γ‐aminobutyric acid (3H‐GABA) than control retinae incubated in the absence of AOAA. This increased accumulation of 3H‐GABA by tissue exposed to AOAA was not apparent at short incubation times (0–20 min), but became significant after incubations of 30 min, and maximal after incubation for 60 minutes. At a concentration of 10 μm, AOAA did not alter the apparent Km for 3H‐GABA uptake or Vmax for either the low or the high affinity GABA uptake systems present in retina. The potentiation of 3H‐GABA accumulation produced by AOAA appeared to parallel the inhibitory effect of this compound on 2‐oxoglutarate‐4‐aminobutyrate aminotransferase (GABA‐T). Similarly, hydrazinopropionic acid inhibited retinal GABA‐T and potentiated the accumulation of 3H‐GABA, but hydroxylamine and thiosemicarbazide which did not affect GABA‐T, were also without effect on the retinal accumulation of 3H‐GABA. In vitro incubation with AOAA did not increase the endogenous levels of GABA or other amino acids in the retina. AOAA did not significantly increase the retinal accumulation of radioactive l‐glutamate, l‐glutamine, taurine, glycine, α‐aminoisobutyrate or dopamine: the accumulation of l‐aspartate was increased by approximately 30%. The inhibition of retinal GABA‐T by AOAA was time‐dependent and was not reversed by pyridoxal‐5′‐phosphate or by repeated washing of the tissue with fresh medium. AOAA also inhibited glutamate decarboxylase (GAD) in retinae incubated in vitro. This inhibitory effect was partially reversed by pyridoxal‐5′‐phosphate. Efflux of radioactivity from the retina was strikingly reduced in the presence of AOAA at concentrations sufficient to inhibit GABA‐T by 100%. These findings suggest that AOAA potentiates the accumulation of 3H‐GABA by isolated retina, not by increasing the exchange of 3H‐GABA with the endogenous GABA pools, but by reducing the metabolism of the amino acid and hence reducing the loss of radioactivity from the tissue in the form of tritiated metabolites.