P‐glycoprotein and multidrug resistance‐associated protein (MRP) reverse the intestinal transport of L‐DOPA, a LAT2 substrate

Abstract

Recently, the Na+-independent and pH-sensitive hetero amino acid exchanger LAT2 was shown to be a L-DOPA transporter in renal tissues, whose activation results in trans-stimulation of L-DOPA outward transfer (Faseb J 18, 1489–1498, 2004). The mechanism of transepithelial L-DOPA transport was studied in polarized Caco-2 cells, an in vitro model of human intestinal epithelium. [14C]-L-DOPA was efficiently transported at both poles of the cell. However, the flux in the apical-to-basolateral direction was twice that in basolateral-to-apical direction, suggesting an asymmetric distribution of L-DOPA transporter units in the apical and basolateral cell borders. The efflux of [14C]-L-DOPA was a trans-stimulable process, markedly increased by the presence of L-DOPA or L-leucine in the apical and basolateral cell side. [14C]-L-DOPA outward was also a pH-sensitive process as lowering extracellular pH promoted L-DOPA-stimulated [14C]-L-DOPA outflow, particularly at the apical cell side. Involvement of P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP) in [14C]-L-DOPA efflux was also investigated. The results obtained show that verapamil (100 μM) and MK-571 (50 μM), inhibitors of respectively P-gp and MRP, significantly inhibited (by ~ 50%) the spontaneous [14C]-L-DOPA outward transfer. These findings suggest that L-DOPA is transported across Caco-2 cells by the pH-sensitive and trans-stimulable L-type amino acid transporter 2 (LAT2). L-DOPA is also a substrate for P-gp and MRP. Supported by grant POCTI/SAU-OBS/57916/2004.