Transmural Heterogeneity of Myofilament Function and Sarcomeric Protein Phosphorylation in Remodeled Myocardium of Pigs with a Recent Myocardial Infarction

Jolanda Van Der Velden,Daphne Merkus,Nicky M Boontje,Vincent De Beer,Wolfgang A Linke,Ger J M Stienen,Nazha Hamdani,Dirk J Duncker

doi:10.3389/fphys.2011.00083

Jolanda Van Der Velden, Daphne Merkus + Show 6 more

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https://doi.org/10.3389/fphys.2011.00083

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Abstract

Aim: Transmural differences in sarcomeric protein composition and function across the left ventricular (LV) wall have been reported. We studied in pigs sarcomeric function and protein phosphorylation in subepicardial (EPI) and subendocardial (ENDO) layers of remote LV myocardium after myocardial infarction (MI), induced by left circumflex coronary artery ligation. Methods: EPI and ENDO samples were taken 3 weeks after sham surgery (n = 12) or induction of MI (n = 12) at baseline (BL) and during β-adrenergic receptor (βAR) stimulation with dobutamine. Isometric force was measured in single cardiomyocytes at various [Ca2+] and 2.2 μm sarcomere length. Results: In sham hearts, no significant transmural differences were observed in myofilament function or protein phosphorylation. Myofilament Ca2+-sensitivity was significantly higher in both EPI and ENDO of MI compared to sham hearts. Maximal force was significantly reduced in MI compared to sham, but solely in ENDO cells. A higher passive force was observed in MI hearts, but only in EPI cells. The proportion of stiff N2B isoform was higher in EPI than in ENDO in both sham and MI hearts, and a trend toward increased N2B-proportion appeared in MI EPI, but not MI Endo. Analysis of myofilament protein phosphorylation did not reveal significant transmural differences in phosphorylation of myosin binding protein C, desmin, troponin T, troponin I (cTnI), and myosin light chain 2 (MLC-2) both at BL and during βAR stimulation with dobutamine infusion. A significant increase in MLC-2 phosphorylation was observed during dobutamine only in sham. In addition, the increase in cTnI phosphorylation upon dobutamine was twofold lower in MI than in sham. Conclusion: Myofilament dysfunction is present in both EPI and ENDO in post-MI remodeled myocardium, but shows a high degree of qualitative heterogeneity across the LV wall. These heterogeneous transmural changes in sarcomeric properties likely contribute differently to systolic vs. diastolic global LV dysfunction after MI.

Highlights

Upon electrical activation of cardiac muscle, cells calcium is released from the sarcoplasmic reticulum which initiates contraction of the sarcomeres upon binding of calcium to the troponin complex
We observed a significant increase in passive force development only in excised. Subepicardial (EPI) cells of post-myocardial infarction (MI) remodeled myocardium, which may involve a change in titin isoform composition or alterations in titin phosphorylation (Borbély et al, 2009; Krüger and Linke, 2009; Williams et al, 2009; LeWinter and Granzier, 2010)
In contrast to rodent studies we did not observe a transmural difference in myosin light chain 2 (MLC-2) phosphorylation in pig hearts both at BL and upon dobutamine infusion indicating that a transmural MLC-2 phosphorylation gradient is not prerequisite for proper cardiac pump function in large mammals

Summary

Introduction

Upon electrical activation of cardiac muscle, cells calcium is released from the sarcoplasmic reticulum which initiates contraction of the sarcomeres upon binding of calcium to the troponin complex. Excitation of the healthy heart is characterized by heterogeneity across the ventricular wall evident from the difference in duration of the action potential between subepicardial and subendocardial ventricular cells (Litovsky and Antzelevitch, 1989; Glukhov et al, 2010). This heterogeneity underlies optimal timing of ventricular excitation, and loss of this electrophysiologic heterogeneity is thought to contribute to arrhythmias in cardiac disease (Glukhov et al, 2010). A significantly higher expression of the fast α-myosin heavy chain isoform in subepicardial fibers (13% of total MHC) compared to subendocardial fibers (3% of total MHC) correlated with faster rates of delayed force development and force decay, and may underlie appropriate timing of force generation across the ventricular wall (Stelzer et al, 2008)

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