Riboflavin transport by isolated perfused rabbit renal proximal tubules

1 Medical and Research Services, Sepulveda Veterans Affairs Medical Center, Sepulveda 91343; 2 Department of Medicine, School of Medicine, University of California at Los Angeles, Los Angeles 90024; 3 Medical Research Service, Long Beach Veterans Affairs Medical Center, Long Beach 90822; and 4 Departments of Medicine and Physiology/Biophysics, School of Medicine, University of California at Irvine, Irvine, California 92717 Rabbit renal proximal tubular transport of riboflavin (RF) was examined by using the in vitro isolated tubule perfusion technique. We found that proximal tubules actively reabsorbed ( J lb ) and secreted ( J bl ) RF. At 0.1 µM RF concentration, J bl was significantly higher than J lb , resulting in a net secretion. This net secretion of RF was decreased at 0.01 µM RF concentration and increased at 1 µM RF concentration. Both J lb and J bl were inhibited by lowering temperature or by adding iodoacetate, a metabolic inhibitor, and lumichrome, an RF analog, suggesting the involvement of carrier-mediated transport mechanisms. J bl was inhibited by probenecid, an anion transport inhibitor, and by para -aminohippuric acid, an organic anion, suggesting the relevance of RF secretion to renal organic anion transport. J bl was also inhibited by alkaline pH (8.0) and by the calmodulin inhibitor trifluoperazine, indicating the influence of pH and Ca 2+ /calmodulin-dependent pathway on RF secretion. Finally, we found that addition of chlorpromazine, a phenothiazine derivative, inhibited both J lb and J bl , raising the concern about the nutritional status in patients receiving such a type of medication. carrier-mediated transport; chlorpromazine