Outward transfer of dopamine precursor L-3,4-dihydroxyphenylalanine (L-dopa) by native and human P-glycoprotein in LLC-PK1 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-PK1 and LLC-GA5 C ol300 cells, a renal cell line expressing the human P-gp in the apical memb rane. In the absence of verapamil, LLC-GA5 Col300 cells accumulate less cal cein (0.5 mu M) than do LLC-PK1 cells. In LLC-PK1 cells, pretreatment with verapamil (25 mu M) for 30 min increased the rate of accumulation of calcei n 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 mu M) was found to increase the rate of accumulation of calcein by 2.5 -fold in LLC-PK1 cells and by 3.7-fold in LLC-GA5 Col300 cells. A 30-min ex posure to UIC2 (3 mu g/ml) or verapamil (25 mu M) increased L-dopa accumula tion in LLC-PK1 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 mu g/ml) or verapamil (25 mu M) 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% in crease) and a pronounced decrease (91 +/- 1 and 92 +/- 1% reduction) in the apical extrusion of L-dopa. It is concluded that LLC-PK1 cells are endowed with P-gp and that the outward transfer of L-dopa at the apical cell borde r in both LLC-PK1 and LLC-GA5 Col300 cells is in part promoted through this transporter.