Uptake of aspartic and glutamic acid by photoreceptors in goldfish retina.

Abstract

The uptake of acidic amino acids by goldfish photoreceptors was investigated by light microscope autoradiography. Isolated retinas were incubated in media containing micromolar amounts of L-[3H]aspartate, L-[3H]glutamate and D-[3H]aspartate. Four major observations were made. Rods accumulate L-[3H]glutamate with a high-affinity transport system; they exhibit a glutamate-to-aspartate selectivity ratio of 30:1. When incubated in 1-10 .mu.M L-[3H]glutamate, rods label more densely than cones. A unit area of rod membrane transports glutamate 30 times better than a unit area of cone membrnae. Red-sensitive and green-sensitive cones show accumulation of L-[3H]aspartate, D-[3H]aspartate and L-[3H]glutamate, apparently with high affinity, but little selectivity. Because rods have poor aspartate uptake, red-sensitive and green-sensitive cones may be preferentially labeled with L-[3H]aspartate or D-[3H]aspartate. Blue-sensitive cones show no uptake of L-[3H]aspartate, D-[3H]aspartate or L-[3H]glutamate other than that attributable to low-affinity transport. Various cell types in the goldfish retina can discriminate between glutamate and aspartate, unlike acidic amino acid transport systems described in mammalian brain.