The transport of [125I]thyroxine (T4) and [125I]triiodothyronine (T3) into liver was investigated with a tissue sampling-portal vein injection technique in the anesthetized rat. The method allows the investigation of the effects of plasma proteins in human serum on the unidirectional influx of T4 or T3 into liver cells. The percent extraction of unidirectional clearance of T3 and T4 was 77±2% and 43±2%, respectively, after portal injection of a bolus of Ringer's solution. Cell membrane transport of T4 or T3 was nonsaturable because 50-μM concentrations of unlabeled hormone had no effect on transport. The addition of bovine albumin in concentrations of 1, 5, or 10 g/100 ml bound >98% of T4 or T3 in vitro, but had no significant effect on T3 or T4 transport in vivo. Conversely, 10% rabbit antisera specific for T3 or T4, completely abolished the intracellular distribution of thyroid hormone into liver. In the presence of rat serum, which contains albumin and thyroid hormone binding pre-albumin (TBPA), 18 and 81% of total plasma T4 and T3, respectively, were available for transport in vivo. The fraction of hormone available for transport in the presence of normal human serum, which contains albumin, TBPA, and thyroid hormone binding globulin (TBG) was 11% for T4 and 72% for T3. The fraction of hormone transported into liver after injection of serum obtained from pregnant or birth control pilltreated volunteers was 4% for T4 (but this was not significantly different from zero) and 54% for T3.
William M. Pardridge, Lawrence J. Mietus