3,5,3′-Triiodothyronine deprivation affects phenotype and intracellular [Ca2+]i of human cardiomyocytes in culture 2001 Abstract Objective: A decrease in plasma T3 concentration is a frequent finding in patients with heart failure. However, the role of this ‘low T3 syndrome’ on disease evolution has never been clarified. As phenotypic and functional cardiomyocyte impairments are alterations that correlate with the failing myocardium, we studied the long-term effects of T3 deprivation on human cardiomyocyte structure and calcium handling. Methods: Atrial cardiomyocytes and myocardial tissue were cultured with or without 3 nM T3. Microscopical examination of structural features was followed by analysis of α-sarcomeric actinin and sarcoplasmic reticulum calcium ATP-ase (SERCA-2) content. Calcium handling was studied by [Ca2+]i imaging. Results: When stimulated with cyclopiazonic acid, a SERCA-2 inhibitor, T3-deprived cardiomyocytes showed significantly faster (P=0.03) and more transient (P=0.04) increases in [Ca2+]i than T3-supplemented cells. Moreover, in the T3-free cultures a significantly lower number of cells (P=0.003) responded to caffeine, a typical activator of sarcoplasmic reticulum Ca2+-release channel. T3-deprived cardiomyocytes also presented altered morphology with larger dimensions than T3-supplemented cells (P<0.0001). Additionally, in T3-deprived samples α-sarcomeric actinin and SERCA-2 protein levels were reduced to 65.6±3% (P<0.0001) and 74.1±4% (P=0.005), respectively, when compared with the T3-supplemented group. Conclusions: Our data show that human cardiomyocyte calcium handling and phenotype are strongly influenced by T3 suggesting important implications of the ‘low T3 syndrome’ on the progression of heart failure.