RESUMO
The availability of L-arginine is of pivotal importance for the synthesis of nitric oxide, a signaling molecule in the CNS. Here we show the presence of a high-affinity L-arginine uptake system (Km of 4.4 +/- 0.5 microM and a Vmax of 26.0 +/- 0.9 fmol/well/min) in cultured chick retinal cells. Different compounds, such as N(G)-mono-methyl-L-arginine and L-lysine, were able to inhibit the uptake that was also inhibited 60-70% in the absence of sodium and/or calcium ions. No trans stimulation was observed when cells were preloaded with L-lysine. The data indicate that the L-arginine uptake in cultured retinal cells is partially mediated by the y+ system, but has a great contribution of the B(0,+) system. Autoradiographic studies revealed that the uptake is predominant in glial cells and can also be detected in neurons, whereas immunocytochemistry of nitric oxide synthase and L-citrulline showed that the enzyme is present in neurons and photoreceptors, but not in glial cells. L-[3H]Arginine is released from purified glial cultures incubated with high concentrations of potassium in the extracellular medium. Moreover, the amino acid released from preloaded glial cells was taken up by purified neuronal cultures. These results indicate that L-arginine released from glial cells is taken up by neurons and used as substrate for the synthesis of nitric oxide.
