The uptake of l-glutamate by the retina

Abstract

The accumulation of l-[ 14C]glutamate by the isolated rat retina has been studied. When retinae were incubated at 37 °C in a medium containing l-[ 14C]glutamate, tissue/medium ratios of about 40:1 were achieved after 60 min. The labelled l-glutamate was rapidly metabolised and after 10 min about 50% of the radioactivity in the tissue amino acids was present as glutamine, aspartate, and 4-aminobutyrate (GABA). The process responsible for l-glutamate uptake showed many of the properties of an active uptake system: it was temperature sensitive, sodium dependent, inhibited by metabolic inhibitors and showed saturation kinetics. The saturable uptake process could be resolved into two components; a ‘high’ affinity process (apparentK m= 21 μM,V max= 35nmoles/min/g tissue) and a ‘low’ affinity process (K m = 630 μM,V max= 881nmoles/min/g tissue). The ‘high’ affinity and ‘low’ affinity uptake processes for l-glutamate appeared to have identical properties in the retina. The uptake of l-glutamate was not specific and was inhibited by other acidic amino acids including d-glutamate but not by neutral or basic or amino acids. The retinal uptake of l-glutamate is not likely to be due to a homoexchange phenomenon because the retina was capable of achieving a large net uptake of glutamate and the efflux of l-[ 14C]glutamate from the tissue was not increased by the addition of non-radioactive l-glutamate to the incubation medium. Autoradiographic studies indicated that the sites for glutamate uptake are largely in the neuroglial Mu¨ller cells.