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 cardiomyocyte Ca2+ homeostasis. Deiodinase type 2 (D2), which provides local synthesis of T-3 from T-4, is essential for the adaptation of skeletal muscles to aerobic exercise, but in the myocardium its role has not been explored. The aim of the work was to assess the effect of wheel training on the expression of genes regulating the heart contractility and to study 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 8 weeks, then serum and muscle tissue samples were obtained to determine hormones and mRNA content by ELISA and quantitative PCR, respectively. 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 alpha MHC, beta MHC, beta(1)-adrenoreceptors, Cav1.2, Serca2, RyR2, and PLB mRNA were not altered as compared with control. Serum content of free T-3 and the content of D2 mRNA in the left ventricle increased as a result of training; a negative correlation was found between these indicators. In trained but not in control rats, the levels of Serca2, Cav1.2, and RyR2 mRNA positively correlated with the D2 mRNA content; however, these parameters did not correlate with the blood free T-3 level. The observed correlations suggest that myocardium adaptation to aerobic exercise is governed by the influence of locally synthesized T-3 rather than by an increase in its systemic production.