Abstract
Rationale: Our previous studies showed that increased endoplasmic reticulum (ER) stress in the myocardium impairs cytoprotective signaling in obese type 2 diabetes mellitus (T2DM). Objective: We examined roles of ER stress in alterations of ventricular function and mortality after myocardial infarction by T2DM. Methods and Results: SERCA2a protein level was significantly lower by 45% and Ser16- and Thr17-phosphorylated phospholamban levels tended to be lower in a rat model of T2DM (OLETF) than those in non-diabetic control rats (LETO). Treatment with an ER stress modulator, 4-phenylbutyric acid (4-PBA) or tauroursodeoxycholic acid (TUDCA), restored SERCA2a protein in OLETF to the level of that in LETO. Although left ventricular (LV) ejection fractions were comparable in OLETF and LETO, LV end-systolic elastance (1685±86 vs. 1953±68 mmHg/ml) and LVdP/dtmax (5886±337 vs. 7389±457 mmHg/s) were lower and tau was larger (12.1±0.9 vs. 8.9±0.8 msec) in OLETF. These impairments of LV function in OLETF were restored by treatment with 4-PBA (LV end-systolic elastance: 1813±58 mmHg/ml, LVdP/dtmax: 7053±546 mmHg/s and tau: 8.5±0.5 msec). Elevation of LV systolic pressure and dP/dtmax levels by dobutamine infusion was reduced, and inhibitory effects of verapamil on the dobutamine-induced positive inotropism were blunted in OLETF. In patch-clamp experiments using isolated cardiomyocytes, increase in L-type Ca 2+ channel current after isoproterenol treatment was markedly attenuated in OLETF as compared with the change in LETO. Mortality rate within 48 hrs after myocardial infarction was significantly higher in OLETF than in LETO (64.0% vs. 7.7%). Telemetric recording indicated that rapid progression of heart failure but not arrhythmia was responsible for the high mortality rate in OLETF. In contrast to the improvement of baseline ventricular dysfunction, neither 4-PBA nor TUDCA significantly reduced mortality rate after infarction in OLETF. Conclusions: ER stress plays a major role in reduction of SERCA2a by T2DM. Blunted response of cardiac L-type Ca 2+ channels to adrenoceptor activation, rather than reduced SERCA2a, may contribute to increase in lethal heart failure after myocardial infarction in T2DM.
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