Characterization of thyroid hormone uptake in heart

Abstract

Transport of T3 and T4 across the plasma membrane is the first step in the sequence of intracellular thyroid hormone action. It is generally accepted that this is mediated by specific carrier proteins. The knowledge about these proteins in liver is abundant, but information about thyroid hormone uptake into the heart is scarce. The aim of this thesis was to characterize the cell physiological properties of T3 and T4 uptake in heart and to obtain information on the molecular structure(s) of the transport protein(s). Experiments with neonatal rat cardiomyocytes showed that T3 and T4 uptake is mediated by an energy- and temperature-dependent mechanism, which is not dependent on the Na+ gradient. Furthermore, T3 uptake is reduced by tryptophan, a ligand for the amino acid system T, suggesting the involvement of an accessory transport mechanism. T3 and T4 uptake have also been explored in the embryonic rat heart derived cell line H9c2(2-1). In these cells, they were found to display similar characteristics as those of the neonatal cardiomyocytes. Interestingly, uptake of T3 and T4 also exhibited mutual inhibition; unlabeled T3 inhibited the uptake of T4, and vice versa. Based on these properties of T3 and T4 uptake and their mutual inhibition, we suggest that heart cells share a common mechanism for the uptake of T3 and T4. This is different from the situation in the liver, where two separate carriers for T3 and T4 have been described. The potential clinical importance of this tissue-selectivity is the development of thyromimetic agents that improve cardiac contractility and decrease plasma cholesterol without increasing the beating frequency of the heart. A number of thyroid hormone metabolites and analogues were tested for their effects on T3 uptake in neonatal rat cardiomyocytes. Only 3,3’-diiodothyronine (3,3’-T2) and diiodothyropropionic acid (DITPA) were found to interfere with plasma membrane uptake of T3, while triiodothyroacetic acid (Triac) was taken up in significant amounts by the cardiomyocytes but probably by a mechanism different from that of T3. Because expression cloning in Xenopus laevis oocytes of mRNA, isolated from neonatal or adult rat hearts, did not result in a stimulation of T3 uptake, we were not able to identify the molecular structure of the T3 transporter in heart. On the other hand, uptake studies in oocytes injected with the cRNA encoding the fatty acid translocase (FAT) demonstrated that FAT induces thyroid hormone uptake. Furthermore, T3 uptake by FAT did not depend on the Na+ gradient, which is in accordance with the cell physiological studies described in this thesis. These observations suggest that fatty acid transporters may play a role in thyroid hormone uptake in heart. In summary, uptake of T3 and T4 in the heart is mediated by a common mechanism which is energy- and temperature dependent, but not Na+-dependent. The molecular basis of this mechanism needs to be further examined as well as the physiological role of transporters like FAT.