Differential taurine uptake in central and peripheral regions of goldfish retina

The transport system of taurine was investigated in fragments of goldfish retina prepared from the total tissue and from concentric regions: center and periphery. A high-affinity, saturable, sodium-dependent system was demonstrated in the three types of fragments. The Km for one-site analysis was similar in the two regions and the total retina. The analysis for two sites revealed a significant higher Km for the high-affinity site in fragments from the central region. The maximal uptake rate was higher in the central zone than in the total retina or the periphery. The Hill slopes obtained from saturation experiments of fragments of total retina, center, and periphery were similar to one other and near to 1. The slope of the time course uptake was intermediate for total retina and higher in the center than in the periphery. Hypotaurine and beta-alanine were found to inhibit taurine uptake, but GABA was a weak inhibitor. The values of Ki for hypotaurine by one- and two-site analysis were lower in the central region. The disruption of photoreceptors by shaking did not modify significantly the uptake of the amino acid. Remotion of endogenous taurine by dialysis of central and peripheral fragments increased the uptake in the periphery, but not in the center. The differences observed among the three samples revealed less affinity and high capacity for taurine uptake in the center, plus a higher sensitivity of inhibition. In addition, the peripheral zone had a greater affinity for taurine, and the maximal velocity of the entrance seems to be inhibited by the higher concentration of the amino acid in this zone. These observations may reflect differences between proliferating and non-proliferating regions of the retina (i.e., periphery and center).