Efflux from isolated rat retina of amino acids formed from different substrates

Abstract

The efflux of GABA, glutamine, glutamate and aspartate was studied in isolated superfused rat retinae which had been incubated with radioactive glucose, acetate or GABA. Apart from the glutamate derived from GABA, amino acid efflux was always biphasic in nature and possibly represented the outflow from two separate tissue compartments. The corresponding rate constants for the two phases were calculated from the efflux curves. These showed that the outputs of all of the amino acids occurred at different rates with each of the three precursors, suggesting that different amino acid pools were being labelled by the different substrates. With exogenous GABA as the precursor. but not with glucose or acetate, aspartate and glutamine were particularly firmly bound by the retinae. With GABA also, amino acid release took place equally well in the presence of AOAA (0.1 mM). implying that outflow was not affected by the continuing metabolism of GABA within the tissue. Addition of cold GABA (0.4 mM) to the superfusion medium enhanced the release of radioactive GABA and glutamine derived from each of the substrates, probably by homoexchange and heteroex-change respectively. Glutamate efflux was always suppressed under these conditions, while aspartate release was not modified. Once again, there were considerable differences in the magnitudes of these responses depending on the radioactive substrate used to label the tissue, further suggesting the separate identity of the pools of amino acids involved.