Regulatory pathways and uptake of L-DOPA by capillary cerebral endothelialcells, astrocytes, and neuronal cells

We examined the nature and regulation of the inward L-3,4-dihydroxyphenylal anine (L-DOPA) transporter in rat capillary cerebral endothelial (RBE4) cel ls, type 1 astrocytes (DI TNC1), and Neuro-2a neuroblastoma cells. In all t hree cell types, the inward transfer of L-DOPA was largely promoted through the 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid-sensitive and sodium- independent L-type amino acid transporter. Only in DI TNC1 cells was the ef fect of maneuvers that increase intracellular cAMP levels accompanied by in creases in L-DOPA uptake. Also, only in DI TNC1 cells was the effect of the guanylyl cyclase inhibitor LY-83583 accompanied by a 65% increase in L-DOP A accumulation, whereas the nitric oxide donor sodium nitroprusside produce d a 25% decrease in L-DOPA accumulation. In all three cell types, the Ca2+/ calmodulin inhibitors calmidazolium and trifluoperazine inhibited L-DOPA up take in a noncompetitive manner. Thapsigargin (1 and 3 muM) and A-23187 (1 and 3 muM) failed to alter L-DOPA accumulation in RBE4 and Neuro-2a cells b ut markedly increased L-DOPA uptake in DI TNC1 cells. We concluded that L-D OPA in RBE4, DI TNC1, and Neuro-2a cells is transported through the L-type amino acid transporter and appears to be under the control of Ca2+/calmodul in- mediated pathways. Astrocytes, however, are endowed with other processe s that appear to regulate the accumulation of L-DOPA, responding positively to increases in intracellular Ca2+ and cAMP and to decreases in cGMP.