5-Methyltetrahydrofolate Urinary Excretion: Modeling by Cultured Human Kidney Cells

Abstract

Urinary folate excretion appears to be controlled by the degree of reabsorption in the renal proximal tubule.¹ We have studied the membrane binding and intracellular transport of folate by cultured human proximal tubule (HPT) cells and have suggested that these cells serve as good models for renal folate reabsorption.² However, these studies were conducted with cells grown on plastic, which exposes only the apical side to the media. To produce a better model of the transcellular transport of folate, HPT cells have been grown on microporous filter inserts in which the confluent monolayers effectively separate the apical (AP) and basolateral (BL) chambers, allowing for separate manipulation of their media contents.³ Initial experiments on the transport of 5-methyl-tetrahydrofolate (5M) in these inserts produced surprising results. A large amount of 5M was transported to the BL chamber and this was not suppressed by excess amounts of 5M, suggesting leakage through a paracellular pathway. Such a pathway should be minimized by the existence of functional tight junctions, which are normally present in confluent cultures of HPT cells.^3,4 The presence of tight junctions is typically determined by measuring the transepithelial electrical resistance (TER) across the monolayer on porous membrane supports.⁵ The present studies were designed to assess the tight junction status during the folate transport experiments in order to overcome the “disappointing” results.