Taurine uptake processes in the isolated rabbit retina and the effects of light

As part of a study on the mechanisms involved in regulating photoreceptor taurine levels, we have examined the characteristics of [3H]-taurine uptake by the isolated rabbit retina. The effects of light on taurine accumulation have also been studied. Rabbit retinas quickly accumulated [3H]-taurine and tissue: medium ratios of 50:1 were attained after 60 min incubation at 37 degrees C. Under these conditions no metabolites of [3H]-taurine were detected in the tissue. The efflux of [3H]-taurine from the retina was extremely slow, less than 5% of the accumulated radioactivity being released in a 30 min incubation in fresh medium. Thus, in subsequent experiments the accumulation of radioactivity was taken as a measure of [3H]-taurine uptake. Non-linear regression analysis of kinetic data revealed that taurine was accumulated by separate high- and low-affinity transport processes, the kinetic parameters being Kmh = 93 +/- 12 microM; Vmh = 72 +/- 7; KmL = 8.8 +/- 5 mM; VML = 274 +/- 79 nmol min-1 g-1 wet weight respectively. The properties of the high- and low-affinity taurine uptake processes were very similar. Both were temperature sensitive, particularly between 25 and 37 degrees C and sodium- and chloride-dependent, and were inhibited by metabolic inhibitors. The substrate specificities of the high- and low-affinity uptake processes were also similar, both processes being inhibited by beta-alanine, guanidinoethylsulphonate (GES) and GABA, but not by alpha-alanine or glycine. Hypotaurine selectively inhibited the high-affinity uptake process for [3H]-taurine. Exposure of retinas to continuous light did not affect either the high- or the low-affinity uptake of [3H]-taurine compared with dark-adapted controls. However, flickering light (0.5-30 Hz, 25% duty cycle) reduced the high-affinity accumulation of [3H]-taurine by as much as 50%. The reduction in [3H]-taurine may be due to a localized decrease in uptake (or possibly an increased release) by the photoreceptors because the same reduction was found when synaptic transmission in the retina was blocked by exposure to medium containing high Mg/low Ca. High Mg/low Ca did not itself affect taurine accumulation.