Characterization of uptake and compartmentalization of 3,5,3 '-tri-iodothyronine in cultured neonatal rat cardiomyocytes

The uptake of tri-iodothyronine (T-3) in cultured neonatal rat cardiomyocyt es was investigated and compared with the uptake of reverse T-3 (rT(3)) and thyroxine (T-4). Cellular compartmentalization of T-3 was studied by disti nguishing T-3 activity associated with the plasma membrane from that in the cytosol or incorporated in the cell nucleus. T-3 and T-4 uptake displayed similar temperature dependencies which, in magnitude, differed from that of rT(3) uptake. T-3 uptake was Na+ independent, and sensitive to oligomycin and monodansylcadaverine (42-49% and 25% inhibition of 15-min cellular upta ke respectively). Furthermore, T-3, uptake could be inhibited by tryptophan (20%) and tyrosine (12%), while 2-aminobicyclo[2,2,1]heptanecarboxylic aci d had no effect. Co-incubation with tryptophan and oligomycin resulted in a n additive inhibition of T-3 uptake (77%). We therefore conclude that (i) T -3 uptake is energy dependent, (ii) receptor-mediated endocytosis may be in volved and (iii) the aromatic amino acid transport system T may play a role , while system L is not involved in T transport in cardiomyocytes. Co-incub ation with unlabeled iodothyronines showed that 3,3'-di-iodothyronine and T -3 itself were the most effective inhibitors of T-3 uptake (30% and 36% inh ibition of 15-min cellular uptake respectively). At 15-min incubation time, 38% of the total cell-associated T-3 was present in the cytosol and nucleu s, and 62% remained associated to the plasma membrane. Unidirectional uptak e rates did not saturate over a free T-3 concentration range up to 3.9 muM. We have concluded that T-3 uptake in neonatal rat cardiomyocytes occurs by an energy- and temperature-dependent mechanism that may include endocytosi s and amino acid transport system T, and is not sensitive to the Na+ gradie nt. Elucidation of the molecular basis for the T-3 transporter is the subje ct of current investigation.