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 in rat papilla in vivo. To examine this process in the absence of metabolism, we performed continuous microinfusions of rat renal papillary ascending thin limbs (ATLs) and vasa recta in vivo, and microperfusions of isolated rat renal papillary descending thin limbs (DTLs) and ATLs in vitro using the nonmetabolizable, synthetic, neutral amino acid cycloleucine. Like naturally occurring amino acids, ≃25% of radiolabeled cycloleucine microinfused into ATLs in vivo was reabsorbed by a process that was not saturable or inhibitable. Also, like naturally occurring amino acids, ≃47% (relative to inulin) of radiolabeled cycloleucine microinfused into ascending vasa recta in vivo was transferred directly into ipsilateral tubular structures (probably DTLs) by a saturable and inhibitable process. In DTLs perfused in vitro, unidirectional bath-to-lumen fluxes (J(bl)) tended to exceed unidirectional lumen-tobath fluxes (J(lb)), whereas in ATLs perfused in vitro J(lb) tended to exceed J(bl), but the differences were not statistically significant. Moreover, none of the unidirectional fluxes was saturable or inhibitable, an observation compatible with apparent reabsorption from ATLs in vivo but incompatible with apparent movement from vasa recta to DTLs in vivo. These in vitro observations are like those made previously for the naturally occurring neutral amino acid L-alanine. The lack of saturation and inhibition, like the previous data on L-alanine, suggest that transepithelial movement of amino acids in thin limbs of Henle's loop may occur via a paracellular route and that regulation of amino acid movement in vivo may involve vasa recta, not DTLs. They also suggest that cycloleucine is a good nonmetabolizable surrogate for the study of neutral amino acid transport in the kidney.