Increasing cardiac contractility after myocardial infarction exacerbates cardiac injury and pump dysfunction

Abstract

Rationale: Myocardial infarction (MI) leads to heart failure (HF) and premature death. The respective roles of myocyte death and depressed myocyte contractility in the induction of HF after MI have not been clearly defined and are the focus of this study.

Objectives: We developed a mouse model in which we could prevent depressed myocyte contractility after MI and used it to test the idea that preventing depression of myocyte Ca(2+)-handling defects could avert post-MI cardiac pump dysfunction.

Methods and results: MI was produced in mice with inducible, cardiac-specific expression of the β2a subunit of the L-type Ca(2+) channel. Myocyte and cardiac function were compared in control and β2a animals before and after MI. β2a myocytes had increased Ca(2+) current; sarcoplasmic reticulum Ca(2+) load, contraction and Ca(2+) transients (versus controls), and β2a hearts had increased performance before MI. After MI, cardiac function decreased. However, ventricular dilation, myocyte hypertrophy and death, and depressed cardiac pump function were greater in β2a versus control hearts after MI. β2a animals also had poorer survival after MI. Myocytes isolated from β2a hearts after MI did not develop depressed Ca(2+) handling, and Ca(2+) current, contractions, and Ca(2+) transients were still above control levels (before MI).

Conclusions: Maintaining myocyte contractility after MI, by increasing Ca(2+) influx, depresses rather than improves cardiac pump function after MI by reducing myocyte number.

Figure 1. β2a hearts are hypercontractile with mild hypertrophy

A, Bitransgenic inducible expression system to induce β2a expression. tTA is the tetracycline-controlled transactivator. B, Representative echocardiography M-mode image in control and β2a hearts. C through G, EF, FS, PWT, and septal wall thickness were greater, and left ventricle internal diameter (LVID) was smaller in β2a vs control mice (control, n=46; β2a, n=36). H, HW/BW ratio in β2a hearts (n=51) was greater than in controls (n=18). *P<0.05; **P<0.01.

Figure 2. MI and ischemia/reperfusion (I/R) causes more cell death in β2a hearts

A, AAR was not different in control (n=15) and β2a hearts (n=6). B, Infarct size (% of AAR) in hearts with 30 minutes of ischemia followed by 24 hours of reperfusion (I/R) was greater in β2a (n=6) than in control hearts (n=8). C, Infarct length after permanent occlusion was greater in β2a (n=6) than in controls (n=7) and. D through F, Myocytes per 10⁶ undergoing apoptosis in the border and remote zones of control (n=3) and β2a hearts (n=4). G, Caspase 3 activity in remote zone tissues (control, n=9; β2a, n=10). H, Fibrotic area (blue) in remote zones of trichrome-stained cardiac histological sections from control (n=3) and β2a hearts (n=3). I, Ki67⁺ myocytes per 10⁶ in control (n=3) and β2a hearts (n=4) after MI. *P<0.05; **P<0.01.

Figure 3. Ischemia/reperfusion depresses cardiac function in β2a hearts

A, Representative recordings of LV pressure in control and β2a isolated hearts. B through D, LVDP and LV end diastolic pressure (EDP) during and after ischemia/reperfusion in control (n=8) and β2a (n=5) isolated hearts. *P<0.05.

Figure 4. Mortality and cardiac remodeling is increased in β2a mice after MI

A, Kaplan–Meier survival curves during 6 weeks after MI in control (n=54) and β2a (n=36) mice. B and C, HW and lung weight (Lung W) were normalized to BW in control and β2a mice. D and E, Representative hematoxylin/eosin-stained sections from control and β2a hearts. F and G, Myocyte cross sectional area and perimeter in control (n=3) and β2a hearts (n=3). Numbers in the bars are the numbers of animals examined. **P<0.01; *P<0.05.

Figure 5. More severe cardiac failure in β2a mice after MI

Average cardiac EF (A), FS (B), LV internal diameter (C), PWT (D) were measured in control (n=46) and β2a (n=36) mice before and 1, 2, 4, or 6 weeks after MI and in sham-operated mice (β2a, n=11; control, n=11). *P<0.05 control MI vs β2a MI; #P<0.05 control sham vs β2a sham.

Figure 6. I_Ca,L is greater in β2a VMs from sham or MI hearts

A, Representative I_Ca,L recordings in control and β2a VMs±MI. B, Average voltage–current relationship of I_Ca,L in control (sham, n=11; MI, n=12) and β2a VMs (sham, n=13; MI, n=15). C, Peak I_Ca,L was significantly greater in β2a than control myocytes before and after MI. *P<0.05.

Figure 7. Effects of Iso (1μmol/L) on I_Ca,L in sham or post-MI myocytes

A and B, Current–voltage relationships in sham and MI (β2a and controls) with or without Iso. C, Iso increased peak I_Ca,L in control (n=4) but not in β2a ventricular myocytes (VMs) (n=7). D, Iso increased I_Ca,L in both control-MI (n=6) and β2a-MI VMs (n=5). E and F, Voltage dependence of I_Ca,L activation in sham or post-MI VMs with or without Iso. *P<0.05 between control and β2a VMs; #P<0.05 with or without Iso.

Figure 8. Contractions, [Ca²⁺]_i transients, and SR load with or without Iso

A, Comparison of FS between control and β2a in sham or post-MI myocytes with or without Iso (sham control, n=19 and β2a, n=9; post-MI control, n=9 and β2a, n=21). B, Comparison of [Ca²⁺]_i transients between control and β2a in sham or post-MI myocytes with or without Iso (sham control, n=8 and β2a, n=9; post-MI control, n=5 and β2a, n=9). C and D, Representative example of caffeine-induced [Ca²⁺]_i transients in control and β2a myocytes. E and F, Average data of peak caffeine-induced [Ca²⁺]_i transients and rate of decay in control and β2a myocytes. *P<0.05 between control and β2a VMs; #P<0.05 with or without Iso.