STUDIES ON THE PHARMACOLOGY OF THE INWARD TRANSPORT OF L-DOPA IN RAT RENAL TUBULES

1 The accumulation of L-DOPA in suspensions of renal tubules obtained from male Wistar rats occurred through non-saturable and saturable mec hanisms. The kinetics of the saturable component of L-DOPA uptake in r enal tubules was as follows: V-max = 46.3 +/- 2.8 nmol mg(-1) protein h(-1) and K-m = 114.4 (95% confidence limits; 83.8, 156.2) mu M (n = 5 ). The diffusion constant (in nmol(-1)) of the non-saturable component for the accumulation of L-DOPA was 1.3 (1.1, 1.6; n = 8). 2 The effec t of 2,4-dinitrophenol was a marked reduction in the tubular uptake of L-DOPA, with an IC50 value of 12.1 (4.0, 36.9) mu M. Cocaine produced a slight (P = 0.08) decrease (22% reduction at 50 mu M) in the tubula r uptake of L-DOPA. Corticosterone produced a considerable inhibitory effect on the uptake of L-DOPA with an IC50 value of 11.0 (3.6, 33.5) mu M. The maximal inhibitory effect of probenecid was a 24% decrease i n the uptake of L-DOPA; however, the more selective organic anion tran sport inhibitor 4,4'-diisothiocyantostilbene-2,2'-disulphonic acid (DI DS) was, found not to affect the tubular uptake of L-DOPA. The organic cation transport inhibitor, cyanine 863, was found to produce a marke d decrease in the uptake of L-DOPA (IC50 = 2.02 [1.07, 3.79]). The cya nine derivatives 1,1'-diethyl-2,2'-cyanine (decynium 22) and 1,1'-diet hyl-2,4'-cyanine (decynium 24) also potently inhibited L-DOPA uptake w ith IC50 values (in mu M) of 0.63 (0.39, 1.01) and 0.10 (0.08, 0.13), respectively, both compounds were found to be more potent than cyanine 863. 3 The inhibitory effect of decynium 24 (0.2 mu M) on the tubular uptake of L-DOPA was dependent on the pH of the incubation medium; at pH = 6.5 the accumulation of L-DOPA was reduced up to 37 +/- 2% of co ntrol values, whereas at pH = 7.4 and pH = 8.2 the accumulation of L-D OPA was reduced by 56 +/- 1% and 60 +/- 6%, respectively. Cyanine 863 (2 mu M), decynium 22 (1 mu M) and decynium 24 (0.2 mu M) were found t o decrease the V-max values for the saturable component of L-DOPA upta ke without changes in K-m values. 4 It is concluded that the tubular u ptake of L-DOPA might be promoted through a mechanism which is depende nt on the activity of the organic cation transport system.