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

Abstract

The uptake of tri-iodothyronine (T 3 ) in cultured neonatal rat cardiomyocytes 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 distinguishing 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 uptake respectively). Furthermore, T 3 uptake could be inhibited by tryptophan (20%) and tyrosine (12%), while 2-aminobicyclo[2,2,1]heptanecarboxylic acid had no effect. Co-incubation with tryptophan and oligomycin resulted in an additive inhibition of T 3 uptake (77%). We therefore conclude that (i) T 3 uptake is energy dependent, (ii) receptor-mediated endocytosis may be involved and (iii) the aromatic amino acid transport system T may play a role, while system L is not involved in T 3 transport in cardiomyocytes. Co-incubation 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% inhibition 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 nucleus, and 62% remained associated to the plasma membrane. Unidirectional uptake rates did not saturate over a free T 3 concentration range up to 3·9 µM. We have concluded that T 3 uptake in neonatal rat cardiomyocytes occurs by an energy-and temperaturedependent mechanism that may include endocytosis and amino acid transport system T, and is not sensitive to the Na + gradient. Elucidation of the molecular basis for the T 3 transporter is the subject of current investigation.

Membrane transporters for thyroid hormone

“…Initially, no saturable T4 uptake was observed in rat cardiomyocytes, but subsequent studies indicated the presence of low-affinity (Km in mM range) transporter(s) shared by T3 and T4 in primary rat cardiomyocytes as well as in the H9c2 embryonic heart cell line. T4 and T3 uptake in these cells is Na + independent and partially inhibited by Trp, suggesting that is mediated in part by a T-type amino acid transporter [32,33].…”

“…Finally, carrier-mediated transport has also been reported for T3 in rat neonatal cardiomyocytes in primary culture [31,32]. Initially, no saturable T4 uptake was observed in rat cardiomyocytes, but subsequent studies indicated the presence of low-affinity (Km in mM range) transporter(s) shared by T3 and T4 in primary rat cardiomyocytes as well as in the H9c2 embryonic heart cell line.…”