5-methyltetrahydrofolate urinary excretion: modeling by cultured human kidney cells.

Initial attempts to model urinary folate reabsorption using cultures of HPT cells on porous filter inserts produced disappointing results in that large amounts of 5M were transported across the epithelial monolayer in a nonspecific manner. Since the impermeable molecule inulin was also transported, there apparently existed a significant leakage pathway in the way that the cultured cells were used for transport studies. 5M was bound to the AP membrane and taken up into the HPT cell by specific processes, while inulin was excluded, suggesting that the HPT cells were nevertheless operating functionally. TER values from cultured HPT cells plateaued at a high level when cells became confluent, suggesting an epithelial layer with functional tight junctions. However, when growth media were removed and replaced with transport buffers, there was an immediate loss of TER that fully recovered if the transport buffers were preincubated for 60 min. Under these conditions, transport studies showed the expected results--no movement of inulin through the cell layer and much reduced transfer of folate through the paracellular pathway. These results suggest that a transient opening of tight junctions occurs when growth media are replaced with biological buffers (or with fresh growth media), but that recovery of tight junction function occurs with time.