Renal folate absorption and the kidney folate binding protein. II. Microinfusion studies.

Surface proximal convoluted tubules (PCT) in rats were microinfused in situ with [3H]folic acid to study the role of folate binding protein (FBP) in the kidney brush-border membrane for renal conservation and transport of folate [3H]folic acid absorption was linearly related to tubular length of PCT and occurred largely in this segment of the tubule. Unlabeled folate derivatives inhibited [3H]folic acid absorption, the extent of which was dependent on the type of unlabeled folate used and its concentration. At equivalent concentrations, inhibition was most effective with unlabeled folic acid, slightly lower than with 5-methyltetrahydrofolate and least effective with methotrexate. Comparisons between [3H]folic acid absorption before and after infusion of a saturating dose of unlabeled folic acid or repetitive injections of [3H]folic acid into the same tubular site revealed continuous and rapid regeneration of unsaturated folic acid uptake sites with an apparent half-life of 28.75 +/- 8.75 s. Determination of [3H] retained in the tubule at various periods after microinfusion of [3H]folic acid revealed slow cellular disappearance with an apparent half-life of 47.3 +/- 5.4 min. It is proposed that the brush-border FBP functions as a receptor of infused folic acid and that following the binding of the ligand the folic acid/FBP complex undergoes a rapid change that results in the internalization of folic acid and regeneration of unsaturated binding sites at the membrane surface. Internalized folic acid is slowly released into renal capillaries.