Ca 2+/calmodulin mediated pathways regulate the uptake of L-DOPA in mouse neuroblastoma neuro 2A cells

Abstract

The present study examined the involvement of protein kinase A (PKA), protein kinase G (PKG), protein kinase C (PKC), protein tyrosine kinase (PTK) and Ca 2+/calmodulin mediated pathways on the uptake of L-DOPA through the L-type amino acid transporter in Neuro 2A cells, an in vitro model of neuronal cells. Non-linear analysis of the saturation curve for L-DOPA revealed a K m value (in μM) of 54±2 and a V max value (in nmol mg protein/6 min) of 34±1. L-DOPA uptake was a sodium-independent process and insensitive to N-(methylamino)-isobutyric acid (MeAIB, 1 mM), but sensitive to 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BHC, IC 50=82 μM). The Ca 2+/calmodulin inhibitors calmidazolium and trifluoperazine inhibited L-DOPA (2.5 μM) uptake with IC 50 Äs of 33 and 105 μM, respectively. The inhibitory effect of BHC on the accumulation of L-DOPA was of the competitive type, whereas that of calmidazolium and trifluoperazine was of the non-competitive type. Modulators of PKA (cyclic AMP, forskolin, isobutylmethylxanthine and cholera toxin), PKG (cyclic GMP, zaprinast, LY 83583 and sodium nitroprusside), PKC (phorbol 12,13-dibutirate, phorbol 12-myristate 13-acetate and chelerythrine) and PTK (genistein and tyrphostin 25) failed to affect the accumulation of a non-saturating (2.5 μM) concentration of L-DOPA. It is concluded that L-DOPA uptake in Neuro 2A cells is promoted through the L-type amino acid transporter and appears to be under the control of Ca 2+/calmodulin mediated pathways.