UPTAKE MECHANISM OF 3,5,3'-TRIIODOTHYRONINE IN MASU SALMON ONCORHYNCHUS-MASOU HEPATOCYTES

The uptake mechanisms of 3,5,3'-triiodothyronine (T-3) by isolated mas u salmon hepatocytes were investigated. [I-125]T-3 uptake into the hep atocytes and its binding to nuclei were stereospecifically inhibited b y unlabeled T-3. The uptake and binding affinity of T-3 was 2-3 and 10 times higher than those of thyroxine (T-4), respectively. Isolated nu clei of masu salmon hepatocytes were seen to have one class of high af finity T-3 binding sites (dissociation constant, 5.73+/-0.29 nM; maxim um binding capacity, 43.7+/-5.1 pmol per 5 x 10(6) nuclei). Furthermor e, it was seen that the uptake and binding mechanisms were highly temp erature dependent, showing maximum uptake at 15 degrees C and binding at 25 degrees C. T-3 binding to the nuclei was 1/10 of T-3 taken up by the hepatocytes. Incorporated thyroxine was deiodinated to T-3 in the hepatocytes, and maximum T-3 concentrations were observed at 3 h incu bation. The small amounts of 3,3',5'-T-3 were also observed in the hep atocytes, but metabolized rapidly. Our results suggest that the uptake mechanisms of thyroid hormone by masu salmon hepatocytes are similar to those demonstrated in mammals.