Outward transfer of dopamine precursor L-3,4-dihydroxyphenylalanine (L-dopa) by native and human P-glycoprotein in LLC-PK(1) and LLC-GA5 col300 renal cells.

The role of P-glycoprotein (P-gp) in the basal-to-apical uptake and flux of L-3,4-dihydroxyphenylalanine (L-dopa) was studied in LLC-PK(1) and LLC-GA5 Col300 cells, a renal cell line expressing the human P-gp in the apical membrane. In the absence of verapamil, LLC-GA5 Col300 cells accumulate less calcein (0.5 microM) than do LLC-PK(1) cells. In LLC-PK(1) cells, pretreatment with verapamil (25 microM) for 30 min increased the rate of accumulation of calcein by 5-fold, whereas in LLC-GA5 Col300 cells, no significant change in the rate of accumulation of calcein was observed. Exposure for 3 h to verapamil (25 microM) was found to increase the rate of accumulation of calcein by 2.5-fold in LLC-PK(1) cells and by 3. 7-fold in LLC-GA5 Col300 cells. A 30-min exposure to UIC2 (3 microg/ml) or verapamil (25 microM) increased L-dopa accumulation in LLC-PK(1) cells by 27 +/- 4 and 88 +/- 14% and reduced L-dopa apical extrusion by 29 +/- 4 and 23 +/- 1%, respectively. The exposure of LLC-GA5 Col300 cells to UIC2 (3 microg/ml) or verapamil (25 microM) for 30 min produced no significant changes in cell accumulation and apical extrusion of L-dopa. A more prolonged exposure (3 h) to UIC2 or verapamil resulted in a marked increase in L-dopa accumulation in the cell (105 +/- 13 and 146 +/- 24% increase) and a pronounced decrease (91 +/- 1 and 92 +/- 1% reduction) in the apical extrusion of L-dopa. It is concluded that LLC-PK(1) cells are endowed with P-gp and that the outward transfer of L-dopa at the apical cell border in both LLC-PK(1) and LLC-GA5 Col300 cells is in part promoted through this transporter.