Folate Transport Pathways Regulate Urinary Excretion of 5-Methyltetrahydrofolate in Isolated Perfused Rat Kidney

Abstract

The reabsorption of 5-methyltetrahydrofolic acid (5-CH3-H4PteGlu) by the renal proximal tubule has an important role in the maintenance of plasma folate concentrations. However, the mechanism by which this vitamin traverses the renal epithelium remains to be determined. Studies in cultured cells have suggested that the folate receptor in association with a probenecid-sensitive anion carrier may be involved in the transmembrane transport of the vitamin. Because 5-CH3-H4PteGlu is reabsorbed and metabolized in the isolated perfused rat kidney (IPRK) in a smaller manner to in vivo models, the IPRK was used to evaluate pathways involved in folate reabsorption. Reabsorption of 5-CH3-H4PteGlu could not be saturated in the isolated perfused rat kidney, even at concentrations up to 2 mumol/L. Folic acid (PteGlu) was used as a competitive inhibitor of FR-dependent reabsorption of 5-CH3-H4PteGlu. When 5-CH3-H4PteGlu was maintained at 1 nmol/L (a concentration at which receptor-mediated transport should be maximal), PteGlu (up to 100 nmol/L) had no effect on reabsorption. The addition of probenecid (1 mmol/L) did not affect the reabsorption of 5-CH3-H4PteGlu but inhibited the fractional excretion of the anion para-aminohippurate. Probenecid also inhibited the urinary excretion of 5-CH3-H4PteGlu metabolites, indicating that reabsorbed 5-CH3-H4PteGlu was metabolized to products that were subsequently secreted into the urine by anion exchange pathways. The physiological importance of a folate receptor-mediated reabsorption of 5-CH3-H4PteGlu appears to be minor in the isolated perfused rat kidney, whereas nonspecific pathways appear to play a major role in the renal folate reabsorption.

Folate deficiency reduces the GPI-anchored folate-binding protein in rat renal tubules

“…This protein has a high affinity for N 5 -methyltetrahydrofolate and can ''sequester'' this folate as the glomerular filtrate passes into the proximal tubules. After internalization by endocytosis, the folate dissociates from the GPI-FBP in the acidified endosome, exits the basolateral surfaces of the cell into the interstitial fluid, and passes into the plasma through the peritubular capillaries and may also be secreted back into the tubules (32). There is evidence that folate reabsorption may be passive diffusion through a highcapacity, low-affinity nonsaturable system (probably the RFC) when folate in the glomerular filtrate is high and via a saturable system (probably the GPI-FBP) when the folate concentration in the filtrate is low (as would occur in folate deficiency) (32).…”

The kidney in vitamin B12and folate homeostasis: characterization of receptors for tubular uptake of vitamins and carrier proteins

“…Several other mechanisms have been implicated in renal tubular folate transport, including dual-component transport systems (96) or even nonspecific pathways (97). Folate uptake has been studied in vitro using a number of different cell systems (4,54,67).…”

Characterization of in Vivo Disulfide-Reduction Mediated Drug Release in Mouse Kidneys

“…Interestingly, our result shows that 4.5 h after bolus infusion of folate–FRET, a significant amount of disulfide reduced folate–BODIPY fragment can be seen in cytosolic regions of some PT cells (Figure A). One possibility is that the cytosolic accumulation of reduced folate–BODIPY fragment was a result of a nonspecific uptake . With a much less efficient rate compared to a receptor mediated uptake, the non-carrier-mediated transport would presumably have little impact on renal toxicity, as the drug release, mediated by a reducible disulfide bond, would have occurred elsewhere long before drug concentration in the cytosol could reach a toxic level.…”

“…One possibility is that the cytosolic accumulation of reduced folate−BODIPY fragment was a result of a nonspecific uptake. 42 With a much less efficient rate compared to a receptor mediated uptake, the non-carrier-mediated transport would presumably have little impact on renal toxicity, as the drug release, mediated by a reducible disulfide bond, would have occurred elsewhere long before drug concentration in the cytosol could reach a toxic level. In fact, our data show that even though the released rhodamine accumulated within vesicular compartments in the PT cells, rhodamine was never detected in the cytosolic region.…”