Cardiac Uptake and Metabolism of 3-Iodothyronamine

Abstract: 3-Iodothyronamine (T1AM) is a naturally occurring derivative of thyroid hormone able to interact with specific G protein-coupled receptors, known as trace amine-associated receptors, and to produce significant functional effects, which include reducing body temperature, heart rate and cardiac contractility. In the present study we investigated the uptake and metabolism of exogenous T1AM in cardiac preparations. We used two experimental models: i) Isolated working rat hearts were perfused with T1AM (50 nM) for 180 minutes and analyses were performed in the recirculating buffer at different time points and in the whole heart homogenate at the end of the experiment; ii) Isolated cardiomyocytes (H9c2 cells) were exposed to T1AM, T3 or T4 (50 nM each) for 180 minutes and in parallel experiments both incubation medium and cell lysate were collected at different time points and submitted to analysis. Metabolites were assayed by high performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS-MS). We developed a method which allowed contemporary detection of T1AM and its putative catabolites, namely 3-iodothyroacetic acid (TA1), thyronamine (T0AM) and thyroacetic acid (TA0). In both models significant uptake of exogenous T1AM was observed. At steady state the total cellular or tissue T1AM concentration exceeded the extracellular concentration by over 20-fold. In H9c2 cells T1AM uptake was abolished if sodium was replaced with choline in the incubation buffer, suggesting a sodium-dependent uptake mechanism. In both preparations T1AM underwent oxidative deamination to 3-iodothyroacetic acid (TA1) and our data suggest that TA1 production occurred intracellularly and was followed by TA1 release. Pretreatment of H9c2 cells or isolated hearts with the non specific amine oxidase inhibitor iproniazid abolished TA1 production, while TA1 was still detected in the presence of either pargiline, a monoamine oxidase (MAO) inhibitor, or semicarbazide, which inhibits the so-called semicarbazide-sensitive amine oxidase (SSAO). Deiodinated derivatives (i.e. T0AM and TA0) were not detected in any model. Limited T1AM production was observed when H9c2 cells were exposed to T3, while no significant production was seen after exposure to T4. We conclude that T1AM is taken up by cardiomyocytes through a sodium-dependent mechanism and can be catabolized to TA1 by iproniazid-sensitive amine oxidases. Small but significant cardiac T1AM production can occur from T3.