Alterations in cardiac contractility and gene expression during low-T3 syndrome: prevention with T3

Abstract

The low-T3 syndrome is a metabolic response resulting in a decreased serum triiodothyronine (T3) concentration that has uncertain effects on thyroid hormone-responsive gene expression and function. We measured cardiac myocyte gene expression and cardiac contractility in young adult female rats using chronic calorie deprivation as a model of the low-T3 syndrome. Sarcoplasmic reticulum calcium adenosinetriphosphatase (SERCA2) and myosin heavy chain (MHC) isoform mRNA content were measured after 28 days on a 50% calorie-restricted diet (low T3) with or without T3 treatment (6 μg ⋅ kg body wt−1 ⋅ day−1). The low-T3 animals had decreased maximal rates of contraction (−13%; P < 0.05) and relaxation (−18%; P < 0.05) compared with the control and the T3-treated groups. There was a 21% ( P < 0.05) increase in left ventricular (LV) relaxation time in the low-T3 animals vs. both control and T3-treated groups. The LV content of the SERCA2 mRNA was decreased significantly (37%) in the low-T3 rats and was increased ( P < 0.05) with T3 treatment vs. controls. The α-MHC mRNA isoform decreased in the low-T3 animals but was unchanged in the T3-treated animals. T3 supplementation normalized both cardiac function and phenotype of calorie-restricted animals, suggesting a role for the low-T3syndrome in the pathophysiological response to calorie restriction.