Characteristics of active transport of thyroid hormone into rat hepatocytes

Thyroid hormone uptake into primary cultured rat hepatocytes was studied using 1-min incubations with radio-iodine-labelled iodothyronines. (1) Uptake of thyroxine indicates two saturable sites with apparent Km values of 1.2 nM and 1.0 microM, and non-saturable uptake. Similar kinetics of triiodothyronine uptake have been observed. (2) The high-affinity systems of both hormones are energy-dependent (i.e., inhibited by KCN and oligomycin). It is postulated that these systems represent active transport of thyroid hormone into the cell. (3) Analysis of mutual inhibition by the substrates for the triiodothyronine and thyroxine transport systems indicates that triiodothyronine and thyroxine cross the cell membrane via separate transport systems. (4) Preincubation with ouabain resulted in a decrease in uptake of both triiodothyronine and thyroxine, suggesting that a sodium gradient is essential for this transport.