Thyroid hormone at supra-physiological dose optimizes cardiac geometry and improves cardiac function in rats with old myocardial infarction.

Thyroid hormone (TH) is critical in cardiac cell differentiation (regulating contractile proteins and cell geometry) and this effect could be potentially exploited therapeutically in reversing the process of de-differentiation which underlies postischemic cardiac remodeling. Acute myocardial infarction was induced in male Wistar rats by ligating left coronary artery (AMI, n=8), while sham operated animals served as control (SHAM, n=8). 13 weeks after AMI, TH was administered in a group of animals for 4 weeks (AMI-THYR, n=9). TH significantly increased beta-MHC and decreased alpha-MHC expression in the myocardium. This response was accompanied by changes in cardiac geometry: sphericity index, (SI, long to short axis ratio) was found to be 1.95 (SEM, 0.02) in SHAM, 1.51(0.03) in AMI and 1.64(0.03) in AMI-THYR, p<0.05. As a consequence, cardiac function was significantly improved: left ventricular ejection fraction (EF%) was 74.5% (SEM, 2.8) in SHAM vs 29.5% (2.1) in AMI, and 40.0% in AMI-THYR, p<0.05. Furthermore, +dp/dt and -dp/dt were 4250 (127) and 2278 (55) in SHAM vs 2737(233) and 1508 (95) in AMI vs 3866 (310) and 2137(111) in AMI -THYR, respectively, p<0.05. TH treatment partially reverses cardiac dysfunction in rats with old myocardial infarction by favorably changing cardiac chamber geometry and expression of myosin isoforms. Thyroid hormone, unlike current treatments, appears to be a paradigm of therapeutic intervention which aims at restoring cardiac geometry and may prove new effective treatment for heart failure.