Nongenomic signaling pathways triggered by thyroid hormones and their metabolite 3‐iodothyronamine on the cardiovascular system

Thyroid hormones play a wide range of important physiological activities in almost all organism. As changes in these hormones levels—observed in hypothyroidism and hyperthyroidism—promote serious derangements of the cardiovascular system, it is important to know their mechanisms of action. Although the classic genomic actions which are dependent on interaction with nuclear receptors to modulate cardiac myocytes genes expression, there is growing evidence about T₃ and T₄‐triggered nongenomic pathways, resulted from their binding to plasma membrane, cytoplasm, or mitocondrial receptors that leads to a rapidly regulation of cardiac functions. Interestingly both actions converge to amplify thyroid hormone effects on cardiovascular system. T₃ and T₄ nongenomic actions modify inotropic and chronotropic effects, cardiac action potential duration, cardiac growth, and myocyte shape by protein translation through protein kinases‐dependent signaling cascades, which include PKA, PKC, PI3K, and MAPK, and changes on ion channels and pumps activity. In respect to the decreased systemic vascular resistance seen in hyperthyroidism, T₃ appears to activate NOS or ATP‐sensitive K⁺ channels. In addition, a novel biologically active T₄‐derived metabolite has been described, 3‐iodothyronamine, T₁AM, which also acts through membrane receptors to mediate nongenomic cardiac effects. This metabolite influences the physiological manifestations of thyroid hormone actions by inducing opposite effects from those stimulated by T₃ and T₄, such as negative inotropic and chronotropic effects. Therefore, beyond genomic and nongenomic effects of thyroid hormones, it is crucial for there to be an equilibrium between T₃ or T₄ and T₁AM levels for maintaining cardiac homeostasis. J. Cell. Physiol. 226: 21–28, 2010. © 2010 Wiley‐Liss, Inc.