L-arginine uptake and release by cultured avian retinal cells: Differential cellular localization in relation to nitric oxide synthase

The availability of L-arginine is of pivotal importance for the synthesis o f nitric oxide, a signaling molecule in the CNS. Here we show the presence of a high-affinity L-arginine uptake system (K-m of 4.4 +/- 0.5 mu M and a V-max of 26.0 +/- 0.9 fmol/well/min) in cultured chick retinal cells. Diffe rent compounds, such as N-G-monomethyl-L-arginine and L-lysine, were able t o inhibit the uptake that was also inhibited 60-70% in the absence of sodiu m and/or calcium ions. No trans stimulation was observed when cells were pr eloaded with L-lysine. The data indicate that the L-arginine uptake in cult ured retinal cells is partially mediated by the y(+) system, but has a grea t contribution of the B-0,B-+ system. Autoradiographic studies revealed tha t the uptake is predominant in glial cells and can also be detected in neur ons, 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-[H-3]Arginine is released from purified glial cultures incu bated with high concentrations of potassium in the extracellular medium. Mo reover, 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 synt hesis of nitric oxide.