Detrimental effects of thyroid hormone analog DITPA in the mouse heart: increased mortality with in vivo acute myocardial ischemia-reperfusion

Abstract

There is emerging evidence that treatment with thyroid hormone (TH) can improve postischemic cardiac function. 3,5-Diiodothyropropionic acid (DITPA), a TH analog, has been proposed to be a safer therapeutic agent than TH because of its negligible effects on cardiac metabolism and heart rate. However, conflicting results have been reported for the cardiac effects of DITPA. Importantly, recent clinical trials demonstrated no symptomatic benefit in patients with DITPA despite some improved hemodynamic and metabolic parameters. To address these issues, dose-dependent effects of DITPA were investigated in mice for baseline cardiovascular effects and postischemic myocardial function and/or salvage. Mice were treated with subcutaneous DITPA at 0.937, 1.875, 3.75, or 7.5 mg·kg(-1)·day(-1) for 7 days, and the results were compared with untreated mice for ex vivo and/or in vivo myocardial ischemia-reperfusion (I/R). DITPA had no effects on baseline body temperature, body weight, or heart rate; however, it mildly increased blood pressure. In isolated hearts, baseline contractile function was significantly impaired in DITPA-pretreated mice; however, postischemic recovery was comparable between untreated and DITPA-treated groups. In vivo baseline cardiac parameters were significantly affected by DITPA, with increased ventricular dimensions and decreased contractile function. Importantly, DITPA-treated mice demonstrated high prevalence of fatal cardiac rhythm abnormalities during in vivo ischemia and/or reperfusion. There were no improvements in myocardial infarction and postischemic fractional shortening with DITPA. Myocardial sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA), phospholamban (PLB), and heat shock protein (HSP) levels remained unchanged with DITPA treatment. Thus DITPA administration impairs baseline cardiac parameters in mice and can be fatal during in vivo acute myocardial I/R.

Fig. 1.

Time courses for the postischemic recovery of coronary flow (CF; A), myocardial relaxation [left ventricular end-diastolic pressure (LVEDP); B], heart rate (HR; C), and myocardial contraction [left ventricular developed pressure (LVDP); D] in untreated and 3,5-diiodothyropropionic acid (DITPA)-treated [DITPA 3.75 mg·kg⁻¹·day⁻¹ (DITPA-3.75)] isolated hearts. CF is expressed as ml·min⁻¹·g heart wt⁻¹. HR is expressed as beats per minute (bpm). IS, ischemia; PI, preischemia. Values are means ± SD; n = 8/group.

Fig. 2.

Myocardial infarction and survival following 30-min left anterior descending coronary artery (LAD) ligation and 24-h reperfusion with or without DITPA treatment. A: % of area at risk (AAR) over left ventricle (LV) and infarct area (IA) over AAR are shown for different groups. Values are means ± SD; n = 6–14/group. DITPA-1.875, DITPA-3.75, DITPA-7.5, 1.875, 3.75, and 7.5 mg·kg⁻¹·day⁻¹ DITPA, respectively. B: Kaplan-Meier survival curve analysis of 4 different groups showed significant differences in survival during in vivo myocardial ischemia and reperfusion (*P < 0.05). n = 10–20/group.

Fig. 3.

Representative ECG tracings obtained before (baseline) and during in vivo myocardial ischemia (IS) and reperfusion (RP). A: typical ECG tracings in an untreated mouse during baseline (BL), IS with developing ST elevation (arrow), and RP with disappearing ST elevation. B: ECG tracings of a DITPA-1.875 mouse during BL, IS with developing ST elevation (arrow), and RP with disappearing ST elevation. C: ECG tracings of a DITPA-3.75 mouse showing IS with developing ST elevation (arrow) and low-voltage signals with fatal atrioventricular (A-V) blocks from 5 min of RP. D: ECG tracings of a DITPA-7.5 mouse showing ST elevation (arrow) in early IS followed by low-voltage signals and fatal A-V blocks from 10 min of IS.

Fig. 4.

Representative ECG tracings obtained from a DITPA-7.5 mouse during in vivo myocardial ischemia (IS) and reperfusion (RP). Immediately after LAD ligation, ST segment elevation developed within 5 min of IS. At 20 min of IS agonal ventricular rhythm appeared spontaneously, and it progressed to partial A-V block with low-voltage signals at 30 min of IS and finally complete A-V block developed at 10 min of RP.

Fig. 5.

Immunoblots for sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA)2a, phospholamban (PLB), and heat shock protein (HSP)70 levels in cardiac homogenates of untreated (control) and DITPA-treated mice. A: representative blots for cardiac SERCA2a, GAPDH, and PLB proteins (top) and their relative mean densitometric value (bottom). B: representative blots for cardiac HSP70 (top) and their relative mean densitometric value (bottom). y-Axis: changes in optical density expressed as % change from normalized value where untreated hearts were assigned 100%. GAPDH served as internal control for respective blot. Values are means ± SD; n = 4/group.