Characterization of non-radiolabeled Thyroxine (T4) uptake in cryopreserved rat hepatocyte suspensions: Pharmacokinetic implications for PFOA and PFOS chemical exposure

The alteration of thyroxine (T-4) cellular uptake by an environmental chemical can serve as a contributing factor in thyroid hormone (TH) disruption. Herein, we describe a non-radiolabeled (LC-MS/MS) oil-filtration technique designed to characterize the mechanism(s) responsible for T-4 cellular uptake in cryopreserved rat hepatocyte suspensions. The environmental chemicals perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) were evaluated for their effect on T-4 hepatic uptake. At 37 degrees C, hepatic assays demonstrated saturable kinetics with increasing T-4 concentrations, while a linear uptake rate consistent with passive diffusion was detected at 4 degrees C. Carrier-mediated (37-4 degrees C) transport of T-4 was the predominant hepatic uptake process versus passive diffusion. Cyclosporin A (CsA) chemically inhibited T-4 hepatic uptake, whereas PFOA/PFOS displayed no inhibition of T-4 translocation. Increasing PFOA/PFOS concentration levels with the T-4 serum carrier-protein transthyretin (TTR) present resulted in a dose-response increase in T-4 hepatic uptake rates, correlating with increased T-4 free fraction values. Hepatic assays conducted in the presence of PFOA/PFOS and TTR displayed an enhanced first-order T-4 hepatic uptake rate consistent with carrier-mediated transport. These in vitro findings characterizing increased T-4 hepatic uptake provides mechanistic insight regarding decreased T-4 serum levels (hypothyroxinemia) previously observed within in vivo rodent studies following perfluorinated chemical exposure.