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¹ Department of Endocrinology and Metabolism and
² Department of Anatomy and Embryology, Academic Medical Center, University of Amsterdam, PO Box 22660, 1100DD, Amsterdam, The Netherlands

(Requests for offprints should be addressed to O Bakker at Department of Endocrinology and Metabolism, F5-171, Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands; Email: o.bakker{at}amc.uva.nl)

Nuclear thyroid hormone (T₃) receptors (TR) play a critical role in mediating the effects of T₃ on development, differentiation and normal physiology of many organs. The heart is a major target organ of T₃, and recent studies in knockout mice demonstrated distinct effects of the different TR isoforms on cardiac function, but the specific actions of TR isoforms and their specific localization in the heart remain unclear. We therefore studied the expression of TR1, TR2 and TRß1 isoforms in the mouse heart at different stages of development, using monoclonal antibodies against TR1, TR2 and TRß1. In order to identify distinct components of the embryonic heart, in situ hybridization for cardiac-specific markers was used with the expression pattern of sarcoplasmic reticulum calcium-ATPase 2a as a marker of myocardial structures, while the pattern of expression of connexin40 was used to indicate the developing chamber myocardium and peripheral ventricular conduction system. Here we show that in the ventricles of the adult heart the TRß1 isoform is confined to the cells that form the peripheral ventricular conduction system. TR1, on the other hand, is present in working myocardium as well as in the peripheral ventricular conduction system. In the atria and in the proximal conduction system (sinoatrial node, atrio-ventricular node), TR1 and TRß1 isoforms are co-expressed. We also found the heterogeneous expression of the TR1, TR2 and TRß1 isoforms in the developing mouse heart, which, in the case of the TRß1 isoform, gradually revealed a dynamic expression pattern. It was present in all cardiomyocytes at the early stages of cardiogenesis, but from embryonic day 11.5 and into adulthood, TRß1 demonstrated a gradual confinement to the peripheral ventricular conduction system (PVCS), suggesting a specific role of this isoform in the formation of PVCS. Detailed knowledge of the distribution of TR1 and TRß1 in the heart is of importance for understanding not only their mechanism of action in the heart but also the design and (clinical) use of TR isoform-specific agonists and antagonists.

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