A dilated cardiomyopathy mutation blunts adrenergic response and induces contractile dysfunction under chronic angiotensin II stress

Abstract

We investigated cardiac contractility in the ACTC E361G transgenic mouse model of dilated cardiomyopathy (DCM). No differences in cardiac dimensions or systolic function were observed in young mice, whereas young adult mice exhibited only mild diastolic abnormalities. Dobutamine had an inotropic and lusitropic effect on the mouse heart. In papillary muscle at 37°C, dobutamine increased relaxation rates [∼50% increase of peak rate of force decline normalized to force (dF/dtmin/F), 25% reduction of time to 90% relaxation (t90) in nontransgenic (NTG) mice], but in the ACTC E361G mouse, dF/dtmin/F was increased 20-30%, and t90 was only reduced 10% at 10 Hz. Pressure-volume measurements showed increases in maximum rate of pressure decline and decreases in time constant of left ventricular pressure decay in the ACTC E361G mouse that were 25-30% of the changes in the NTG mouse, consistent with blunting of the lusitropic response. The inotropic effect of dobutamine was also blunted in ACTC E361G mice, and the dobutamine-stimulated increase in cardiac output (CO) was reduced from 2,100 to 900 μl/min. Mice were treated with high doses of ANG II for 4 wk. The chronic stress treatment evoked systolic dysfunction in ACTC E361G mice but not in NTG. There was a significant reduction in rates of pressure increase and decrease, as well as reduced end-systolic pressure and increased volume. Ejection fraction and CO were reduced in the ACTC E361G mouse, indicating DCM. In vitro DCM-causing mutations uncouple the relationship between Ca(2+) sensitivity and troponin I phosphorylation. We conclude that this leads to the observed, reduced response to β1 agonists and reduced cardiac reserve that predisposes the heart to DCM under conditions of chronic stress.

Keywords: angiotensin II stress; dilated cardiomyopathy; dobutamine; heart muscle contractility.

Fig. 1.

Effect of stimulation frequency on force development and relaxation phase indices in isolated ACTC E361G and nontransgenic (NTG) papillary muscles contracting isometrically at 37°C. Data are presented as means ± SE of n = 5 ACTC E361G (open circles) muscles and n = 3 NTG (solid circles) muscles. Force (A), force normalized to force produced at 1 Hz (B), time to peak force (C), peak rate of force development (dF/dt_max/F; D), peak rate of force decline normalized to force (dF/dt_min/F; E), time to 50% relaxation (F), time to 90% relaxation (G), and relaxation time constant (tau; H). Data were analyzed by 2-way ANOVA; *P < 0.05, **P < 0.01, ***P < 0.001 E361G vs. NTG.

Fig. 2.

Effect of dobutamine on intact papillary muscle contractility. Papillary muscles, isolated from both ACTC E361G (n = 5) and NTG mice (n = 3–5), were stimulated with the addition of 10 μM dobutamine to the perfusion solution at 37°C. Means ± SE of the muscle function measurements (left); percent change with treatment at physiologically relevant frequencies (right) of the following: force production normalized to force produced at 1 Hz (A), time to peak tension (B), time to 90% relaxation (C), dF/dt_min/F (D), and relaxation rate constant (tau; E). Data were analyzed by 2-way ANOVA; *P < 0.05.

Fig. 3.

The effect of acute dobutamine treatment on cardiac performance. Data are presented as means ± SE of n = 6 ACTC E361G (open bars) mice and n = 7 NTG (solid bars) mice. Δ, change from the baseline. Heart rate (A), stroke volume (SV; B), cardiac output (CO; C), end-systolic pressure (ESP; D), maximum rate of pressure increase (dP/dt_max; E), end-systolic volume (ESV; F), maximum rate of pressure decline (dP/dt_min; G), tau (H), and end-diastolic volume (EDV; I). **P < 0.01, ***P < 0.001, ****P < 0.0001, unpaired Student's t-test.

Fig. 4.

Effect of 4 wk ANG II (1.4 mg·kg⁻¹·day⁻¹) infusion on cardiac dimensions of ACTC E361G and NTG mouse hearts measured by M-mode echocardiography. Data are presented as means ± SE of n = 8 ACTC E361G (open circles and lines) and n = 6 NTG (solid circles and lines) mice. Heart rate (A), CO (B), SV (C), ejection fraction (EF; D), end-diastolic left ventricle internal dimension at diastole (LVIDd; E), end-diastolic interventricular septum thickness (IVSd; F), ESV (G), and EDV (H). Data were analyzed by 2-way ANOVA (+P < 0.05, ++P < 0.01, +++P < 0.001 ACTC E361G vs. baseline; #P < 0.05, ##P < 0.01 NTG vs. baseline; *P < 0.05 ACTC E361G vs. NTG).

Fig. 5.

Effect of 4 wk ANG II (2.0 mg·kg⁻¹·day⁻¹) infusion on myocardial contraction indices of ACTC E361G and NTG mouse hearts measured by pressure-volume catheter. Data are presented as means ± SE of n = 4 ACTC E361G (open bars), n = 3 NTG (solid bars), and n = 4 ACTC E361G sham-operated (shaded bars) mice. Heart rate (A), CO (B), SV (C), EF (D), ESV (E), ESP (F), dP/dt_max (G), and peak rate of volume reduction (dV/dt_min; H). Data were analyzed by 1-way ANOVA (*P < 0.05); ns, not significant.