Thyroid hormone (TH) signaling is altered in response to various stresses including myocardial ischemia. The present study investigated potential implication of TH signaling in the pathophysiology of postischemic remodeling. Acute myocardial infarction was induced in rats by coronary artery ligation (AMI). After 34 weeks, 6 animals were on congestive heart failure (CHF) as indicated by measurements in lung and right ventricular weight. 7 animals were in compensated state (Non-CHF) and 8 sham-operated animals (SHAM) served as controls. Progression to congestive heart failure was characterized by marked decrease in EF% and all other functional echocardiographic parameters. Furthermore, beta-MHC expression was significantly increased in CHF. A distinct pattern of thyroid hormone receptor (TR) expression was observed in the course of postischemic remodeling; TR alpha 1 was upregulated and TR beta 1 was downregulated in Non-CHF, and TR alpha 1 expression was markedly decreased during the transition from Non-CHF to CHF resulting in tissue hypothyroidism. Circulating T3 and T4 remained unchanged. This response was associated with marked decrease in mTOR activation. A potential link between mTOR and TR alpha 1 expression was shown in a neonatal cardiomyocytes model of PE (phenylephrine)-induced pathological growth. Phenylephrine increased the expression of TR alpha 1 in nucleus and this response was abrogated in the case of mTOR inhibition by rapamycin. In conclusion, progression to congestive heart failure after myocardial infarction is associated with suppressed expression of TR alpha 1 and results in tissue hypothyroidism. This process may, at least in part, be mTOR dependent.

The present study explored the effects of thyroid hormone (TH) treatment on post-ischemic cardiac function and potential implicated mechanisms. Acute myocardial infarction (AMI) was induced in mice by coronary artery ligation while sham-operated animals served as controls. This procedure resulted in a marked depression of cardiac function and significant reduction in TH levels in plasma. TH was given at a dose aiming to normalize T3 levels in plasma [AMI-TH (A)] and also at higher doses. The group of animals treated with the highest dose of TH, which displayed significantly increased mortality rate was included in the study [AMI-TH (B)]. In AMI-TH (A) mice, TH significantly improved left ventricular (LV) ejection fraction (EF%), [27.9% (1.4) in AMI versus 38.0 (3.1) in AMI-TH (A), P < 0.05], and favorably remodeled LV chamber while α-MHC was the dominant isoform expressed. In AMI-TH (B) mice, TH treatment resulted in increased mortality as compared to untreated mice (73% vs 47%, P < 0.05), while the favorable effect of TH was not evident in the survived animals. At the molecular level, TH, at the replacement dose, modestly increased p-Akt levels in the myocardium without any change in p-ERK levels. On the contrary, TH at the higher dose resulted in further increase in p-Akt along with an increase in p-ERK levels. In conclusion, TH appears to have a dose-dependent bimodal effect on post-ischemic cardiac performance and this effect may, at least in part, be mediated by a distinct pattern of activation of Akt and ERK signaling.

We have previously shown that acute thyroid hormone treatment could limit reperfusion injury and increase post-ischemic recovery of function. In the present study, we further explore potential initiating mechanisms of this response. Thus, isolated rat hearts were subjected to 30 min zero-flow global ischemia (I) followed by 60-min reperfusion (R). Reperfusion injury was assessed by post-ischemic recovery of left ventricular developed pressure (LVDP%) and LDH release. T3 at a dose of 60 nM which had no effect on contractile function of non-ischemic myocardium, significantly increased LVDP% [48% (2.9) vs. 30.2% (3.3) for untreated group, P < 0.05] and reduced LDH release [8.3 (0.3) vs. 10 (0.42) for untreated group, P < 0.05] when administered at R. T4 (60 and 400 nM) had no effect on contractile function either in non-ischemic or ischemic myocardium. Administration of debutyl-dronedarone (DBD), a TRα1 antagonist abolished the T3-limiting effect on reperfusion injury: Thus, co-administration of T3 and DBD resulted in significantly lower LVDP%, [23% (4.7) vs. 48% (2.9) for T3 group, P < 0.05] and higher LDH release [9.9 (0.3) vs. 8.3 (0.3), for T3 group, P < 0.05]. In conclusion, acute T3 and not T4 treatment will be able to protect against reperfusion injury. T3 can exert this beneficial effect on ischemic myocardium at a dose that has no effects on non-ischemic myocardium. Acute T3-limiting effect on reperfusion injury is mediated, at least in part, via TRα1 receptor.