Influx of thyroid hormones into rat liver in vivo. Differential availability of thyroxine and triiodothyronine bound by plasma proteins.

The transport of [(125)I]thyroxine (T(4)) and [(125)I]triiodothyronine (T(3)) 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 T(4) or T(3) into liver cells. The percent extraction of unidirectional clearance of T(3) and T(4) was 77+/-2% and 43+/-2%, respectively, after portal injection of a bolus of Ringer's solution. Cell membrane transport of T(4) or T(3) was nonsaturable because 50-muM 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 T(4) or T(3) in vitro, but had no significant effect on T(3) or T(4) transport in vivo. Conversely, 10% rabbit antisera specific for T(3) or T(4), 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 T(4) and T(3), 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 T(4) and 72% for T(3). The fraction of hormone transported into liver after injection of serum obtained from pregnant or birth control pilltreated volunteers was 4% for T(4) (but this was not significantly different from zero) and 54% for T(3). THESE DATA SUGGEST: (a) The mechanism by which T(4) and T(3) traverse the liver cell membrane is probably free diffusion. (b) Albumin-bound T(4) or T(3) is freely cleared by liver, approximately 50% of TBG-bound T(3) is transported, but little, if any, of TBPA-bound T(4) or TBG-bound T(4) is cleared by liver cells. (c) Although the albumin-bound fraction of T(4) greatly exceeds the free (dialyzable) moiety, the two fractions are both inversely related to the existing TBA or TBG level; therefore, in vitro measurements of free T(4) would be expected to accurately reflect what is available for transport in vivo. Conversely, TBG-bound T(3) is readily transported in vivo; therefore, it is proposed that in vitro measurements of free T(3) do not reliably predict the fraction of T(3) available for transport into liver in vivo.