Cycloleucine fluxes during rat vasa recta and loop microinfusions in vivo and loop microperfusions in vitro

Amino acids are apparently recycled between loops of Henle and vasa recta i n rat papilla in vivo. To examine this process in the absence of metabolism , we performed continuous microinfusions of rat renal papillary ascending t hin limbs (ATLs) and vasa recta in vivo, and microperfusions of isolated ra t renal papillary descending thin limbs (DTLs) and ATLs in vitro using the nonmetabolizable, synthetic, neutral amino acid cyclo-leucine. Like natural ly occurring amino acids, congruent to 25% of radiolabeled cycloleucine mic roinfused into ATLs in vivo was reabsorbed by a process that was not satura ble or inhibitable. Also, like naturally occurring amino acids, congruent t o 47% (relative to inulin) of radiolabeled cycloleucine microinfused into a scending vasa recta in vivo was transferred directly into ipsilateral tubul ar structures (probably DTLs) by a saturable and inhibitable process. In DT Ls perfused in vitro, unidirectional bath-to-lumen fluxes (J(bl)) tended to exceed unidirectional lumen-to-bath fluxes (J(lb)), whereas in ATLs perfus ed in vitro J(lb) tended to exceed J(bl), but the differences were not stat istically significant. Moreover, none of the unidirectional fluxes was satu rable or inhibitable, an observation compatible with apparent reabsorption from ATLs in vivo but incompatible with apparent movement from vasa recta t o DTLs in vivo. These in vitro observations are like those made previously for the naturally occurring neutral amino acid L-alanine. The lack of satur ation and inhibition, like the previous data on L-alanine, suggest that tra nsepithelial movement of amino acids in thin limbs of Henle's loop may occu r via a paracellular route and that regulation of amino acid movement in vi vo may involve vasa recta, not DTLs. They also suggest that cycloleucine is a good nonmetabolizable surrogate for the study of neutral amino acid tran sport in the kidney.