BACKGROUND Unlike intestinal absorption, renal transport of thiamin has received little attention. This study was designed to investigate the reabsorptive steps of thiamin transport in brush border membrane vesicles (BBMVs) from rat kidney proximal tubules using tritiated thiamin with a high specific activity. METHODS BBMVs prepared from the cortex kidney of rats were suspended in different media, controlling the composition of the intravesicular fluid by prolonged equilibration at 4 degrees C in preincubation buffers of desired composition. Then they were held on ice until used, when they were warmed at 25 degrees C for the uptake experiments. The amount of radioactivity taken up by the vesicles was measured radiometrically after separation with a rapid-filtration procedure. RESULTS The time course profile of thiamin uptake was Na+ independent; 53% of thiamin taken up was membrane bound. The concentration curve had a biphasic course that was nonlinear (saturable) at physiological concentrations (<1.25 micromol/L) and linear at higher ones. Thiamin uptake was stimulated several-fold by an outwardly directed H+ gradient (pHin 6:pHout 7.5), which caused a transient accumulation of thiamin inside BBMVs against a concentration gradient. The enhanced thiamin uptake was only due to the H+ gradient, which made thiamin binding virtually negligible compared with translocation, and maintained the biphasic course of the concentration curve. The saturable component, however, had kinetic constants 12-fold higher than those in the absence of gradient. Moreover, the saturable component was inhibited by nonlabeled thiamin and its structural analogues, by inhibitors of intestinal thiamin/H+, renal guanidine/H+, and Na+/H+ antiporters, while it remained unmodified by some typical organic cations transported in renal BBMVs. CONCLUSIONS The results provide strong evidence for the presence in renal BBMVs of a thiamin/H+ antiport having a 1:1 stoichiometric ratio.