Riboflavin transport by isolated perfused rabbit renal proximal tubules

Rabbit renal proximal tubular transport of riboflavin (RF) was examined by using the in vitro isolated tubule perfusion technique. We found that proxi mal tubules actively reabsorbed (J(lb)) and secreted (J(bl)) RF. At 0.1 muM RF concentration, J(bl) was significantly higher than Jlb, resulting in a net secretion. This net secretion of RF was decreased at 0.01 muM RF concen tration and increased at 1 muM RF concentration. Both J(lb) and J(bl) were inhibited by lowering temperature or by adding iodoacetate, a metabolic inh ibitor, 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, sug gesting the relevance of RF secretion to renal organic anion transport. J(b l) was also inhibited by alkaline pH (8.0) and by the calmodulin inhibitor trifluoperazine, indicating the influence of pH and Ca2+/calmodulin-depende nt pathway on RF secretion. Finally, we found that addition of chlorpromazi ne, 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.