Active lucifer yellow secretion in renal proximal tubule: Evidence for organic anion transport system crossover

Recent studies show that organic anion secretion in renal proximal tubule i s mediated by distinct sodium-dependent and sodium-independent transport sy stems. Here we investigated the possibility that organic anions entering th e cells on one system can exit into the lumen on a transporter associated w ith the other system. In isolated rat kidneys perfused with 10 mu M lucifer yellow (LY, a fluorescent organic anion) plus 100 mu g/ml inulin, the LY-t o-inulin clearance ratio averaged 1.6 +/- 0.2, indicating net tubular secre tion. Probenecid significantly reduced both LY clearance and LY accumulatio n in kidney tissue. In intact killifish proximal tubules, confocal microsco py was used to measure steady-state LY uptake into cells and secretion into the tubular lumen. Probenecid, p-aminohippurate, and ouabain nearly abolis hed both uptake and secretion. To this point, the data indicated that LY wa s handled by the sodium-dependent and ouabain-sensitive organic anion trans port system. However, leukotriene C-4, an inhibitor of the luminal step for the sodium-independent and ouabain-insensitive organic anion system, reduc ed luminal secretion of LY by 50%. Leukotriene C-4 did not affect cellular accumulation of LY or the transport of fluorescein on the sodium-dependent system. A similar inhibition pattern was found for another fluorescent orga nic anion, a mercapturic acid derivative of monochlorobimane. Thus, both or ganic anions entered the cells on the basolateral transporter for the class ical, sodium-dependent system, but about half of the transport into the lum en was handled by the luminal carrier for the sodium-independent system, wh ich is most likely the multidrug resistance-associated protein. This is the first demonstration that xenobiotics can enter renal proximal tubule cells on the carrier associated with one organic anion transport system and exit into the tubular lumen on multiple carriers, one of which is associated wi th a second system.