Changes in the Expression of Genes Regulating Calcium Homeostasis in Rat Myocardium Induced by Voluntary Wheel Training: The Role of Thyroid Hormones

Thyroid hormones increase cardiac contractility due to their effect on the Ca 2+ homeostasis in cardiomyocytes. Type 2 deiodinase (D2), which provides local synthesis of T 3 from T 4 , is essential for the adaptation of skeletal muscles to aerobic exercise; however, its role in the myocardium has not been explored. The aim of this study was to assess the effect of wheel training on the expression of genes that regulate the heart contractility and to explore the relationship of their mRNA content in the myocardium with indicators of systemic and local T 3 production. Male Wistar rats were housed in cages with running wheels for eight weeks; the samples of their serum were used to determine hormones by ELISA and muscle tissue to determine mRNA content by reverse transcription quantitative PCR. The training was accompanied by a moderate hypertrophy of the soleus muscle and an increase in its citrate synthase mRNA content. Ventricle mass and the expression levels of αMHC, βMHC, β 1 -adrenoreceptors, Cav 1.2, Serca2, RyR2, and PLB mRNA in trained rats did not differ from respective values in the control groups of rats. Blood content of free T 3 and the expression of D2 mRNA in the left ventricle increased as a result of training; a negative correlation between these indicators was found. The levels of Serca2, Cav 1.2, and RyR2 mRNA positively correlated with the D2 mRNA content in the trained but not in control rats; however, these parameters did not correlate with the free T 3 level in the blood. The observed correlations suggest that myocardium adaptation to aerobic exercise is governed by the effect of locally synthesized T 3 rather than by an increase in its systemic production.