Amino acids can be reabsorbed distal to tips of Henle's loops and may be recycled between loops and vasa recta in rat papilla. Transport specificity was examined during continuous microinfusions of ascending Henle's loops and vasa recta with radiolabeled amino acids. Percent of recovered radiolabel as intact amino acid was also determined. Previous data indicated that, relative to simultaneously microinfused inulin, 30-40% of radiolabeled L- and D-Ala, L-Glu, L-Glu(NH2), and Gly, but no taurine (Tau) or mannitol, microinfused into Henle's loops was reabsorbed. In the present study, reabsorption was shown to involve intact L- and D-Ala, D-Glu, and L-Ser. L-Phe (50 mM) in infusate had no effect on reabsorption of L-Ala (2.5 mM) or L-Glu(NH2) (42.6-mu-M), and D-Asp (50 mM) had no effect on reabsorption of L-Glu (1.5 mM). Thus reabsorption from Henle's loops is not stereospecific, not different for neutral and acidic amino acids, and not inhibited by competitive inhibitors of proximal tubule amino acid transport, but it was not completely nonspecific and not a simple leak. Previous vasa recta microinfusions suggested that Ala could move directly from vasa recta to tubules. These studies were extended with simultaneous collections from ipsilateral and contralateral kidneys. Relative to simultaneously microinfused inulin, 40-50% of radiolabeled L- and D-Ala, L-Glu, and L-Glu(NH2) and 30% of L-Ser microinfused into ascending vasa recta appeared intact in urine from ipsilateral kidney, whereas only 1-3% appeared in urine from contralateral kidney. Fifty percent of infused D-Glu was excreted intact by each kidney; 70% of infused Tau was excreted intact by ipsilateral kidney, and 22% was excreted by contralateral kidney. L-Phe (50 mM) in infusate inhibited appearance of L-Ala (2.5 mM) and D-Ala (10 mM) but not L-Glu(NH2) (42.6-mu-M) in ipsilateral urine. D-Asp (50 mM) inhibited appearance of L-Glu (1.5 mM), and beta-Ala (50 mM) inhibited appearance of Tau (78-mu-M) in ipsilateral urine. Thus some amino acids can move directly from vasa recta into tubules (probably descending thin limbs of Henle's loops) by a process showing significant specificity.
