NATURE AND KINETIC CHARACTERISTICS OF L-DOPA UPTAKE IN RAT RENAL PROXIMAL TUBULES

The present study was performed with the aim to determine the kinetics and the caracteristics of cellular uptake of L,-3,4-dihydroxyphenylalanine (L-DOPA) in rat renal proximal tubules. Incubation of renal tubules at 4 degrees C in the presence of increasing concentrations of L-DOPA results in a linear and concentration-dependent accumulation of the substrate. In experiments carried out at 37 degrees C, the accumulation of L-DOPA in renal tubules was found to be greater than that occurring at 4 degrees C and showed a trend for saturation. The saturable component of L-DOPA uptake was derived from the total amount of L-DOPA accumulated in renal tubules at 37 degrees C subtracted with the values obtained in experiments conducted at 4 degrees C. The V-max and K-m values for the saturable component of L-DOPA uptake in renal tubules were, respectively, 241 +/- 32 fmol mu g protein(-1)min(-1) and 567 +/- 63 mu M Cyanine 863 (5 and 10 mu M) was found to decrease the tubular uptake of L-DOPA, whereas probenecid (50 mu M) did not change the rate of uptake of L-DOPA into renal tubules. The V-max and K-m values for the saturable component of L-DOPA uptake in renal tubules incubated in the presence of 10 mu M cyanine 863 were, respectively, 97 +/- 22 fmol mu g protein(-1)min(-1) and 160 +/- 22 mu M. It is suggested that the anionic L-DOPA may behave as an amphoteric substance, both hydroxyl groups in the aromatic ring determining the binding of the molecule to the organic cation transporter.