Amino acid transport: microinfusion and micropuncture of Henle's loops and vasa recta

Amino acids appear to be reabsorbed distal to the tips of loops of Henle and may be recycled between loops and vasa recta in rat papilla. These possibilities were examined further by microinfusion and micropuncture of loops of Henle and vasa recta. To obtain information on the specificity of amino acid transport distal to the tips of the loops, ascending limbs were continuously microinfused with radioactively labeled L- and D-alanine, L-glutamate, taurine, and mannitol. About 30% of the L- and D-alanine and L-glutamate but none of the taurine or mannitol appeared to be reabsorbed. These results suggest that an acidic and a neutral amino acid are reabsorbed to a similar extent, that reabsorption is not stereospecific, but that it does not occur indiscriminately for all amino acids or for all molecules of similar size. Continuous microinfusion of ascending vasa recta with radioactively labeled L- and D-alanine suggests that amino acids may be able to move from vasa recta into tubules (apparently loops of Henle) without first entering the systemic circulation. However, micropuncture measurements of concentrations of endogenous amino acids in thin ascending limbs and adjacent descending vasa recta do not demonstrate a gradient for passive movement out of loops.