The dopamine precursorl-dihydroxyphenylalanine is transported by the amino acid transporters rBAT and LAT2 in renal cortex

Abstract

The intrarenal autocrine-paracrine dopamine (DA) system is critical for Na+homeostasis. l-Dihydroxyphenylalanine (l-DOPA) uptake from the glomerular filtrate and plasma provides the substrate for DA generation by the renal proximal tubule. The transporter(s) responsible for proximal tubule l-DOPA uptake has not been characterized. Renal cortical poly-A+RNA injected into Xenopus laevis oocytes induced l-DOPA uptake in a time- and dose-dependent fashion with biphasic Kms in the millimolar and micromolar range and independent of inward Na+, K+, or H+gradients, suggesting the presence of low- and high-affinity l-DOPA carriers. Complementary RNA from two amino acid transporters yielded l-DOPA uptake significantly above water-injected controls the rBAT/b0,+AT dimer (rBAT) and the LAT2/4F2 dimer (LAT2). In contradistinction to renal cortical poly-A+, l-DOPA kinetics of rBAT and LAT2 showed classic Michaelis-Menton kinetics with Kms in the micromolar and millimolar range, respectively. Sequence-specific antisense oligonucleotides to rBAT or LAT2 (AS) caused inhibition of rBAT and LAT2 cRNA-induced l-DOPA transport and cortical poly-A+-induced arginine and phenylalanine transport. However, the same ASs only partially blocked poly-A+-induced l-DOPA transport. In cultured kidney cells, silencing inhibitory RNA (siRNA) to rBAT significantly inhibited l-DOPA uptake. We conclude that rBAT and LAT2 can mediate apical and basolateral l-DOPA uptake into the proximal tubule, respectively. Additional l-DOPA transport mechanisms exist in the renal cortex that remain to be identified.