Basic amino acid transport in renal papilla: Microinfusion of Henle's loops and vasa recta

To determine whether basic amino acids, like acidic and neutral amino acids, could be reabsorbed distal to tips of Henle's loops and recycled between loops and vasa recta in the renal papilla, we continuously microinfused ascending Henle's loops and vasa recta with ¹⁴C-labeled L-lysine (L-Lys; 1.28 mM) or L-arginine (L-Arg; 1.17 mM) and ³H-labeled inulin. We also determined percent of recovered radiolabel as intact amino acid. Like acidic and neutral amino acids, relative to inulin, ∼30% of L-Lys and ∼45% of L-Arg microinfused into Henle's loops were reabsorbed. However, whereas radiolabeled L-Lys reabsorption, like reabsorption of acidic and neutral amino acids, was not readily inhibited, radiolabeled L-Arg reabsorption was reduced to ∼25% by addition of unlabeled L-Arg (50 mM) or L-homoarginine (L-Homo-Arg) (50 mM) to infusate. This observation provides greater evidence for specific, carrier-mediated reabsorption for L-Arg than for acidic or neutral amino acids. About 36% (relative to inulin) of each of these amino acids microinfused into ascending vasa recta apparently was transferred directly into ipsilateral tubular structures (probably thin descending limbs of Henle's loops). Transfer of radiolabeled L-Arg was reduced to ∼8% by the inclusion of unlabeled L-Arg (50 mM) in infusate. Transfer of unlabeled L-Lys was unaffected by inclusion of unlabeled L-Lys (50 mM) in infusate but was reduced to ∼20% by inclusion of unlabeled L-Arg (50 mM) or L-Homo-Arg (50 mM) in infusate. These data suggest that L-Lys and L-Arg share transport system into ipsilateral loops but that the system has a greater affinity for L-Arg than L-Lys. Like acidic and neutral amino acids, basic amino acids apparently can be reabsorbed distal to tips of Henle's loops and recycled in the papilla.