The crossroads of inflammation, fibrosis, and arrhythmia following myocardial infarction.

Optimal healing of damaged tissue following myocardial infarction (MI) requires a coordinated cellular response that can be divided into three phases: inflammatory, proliferative/reparative, and maturation. The inflammatory phase, characterized by rapid influx of cytokines, chemokines, and immune cells, is critical to the removal of damaged tissue. The onset of the proliferative/reparative phase is marked by increased proliferation of myofibroblasts and secretion of collagen to replace dead tissue. Lastly, crosslinking of collagen fibers and apoptosis of immune cells marks the maturation phase. Excessive inflammation or fibrosis has been linked to increased incidence of arrhythmia and other MI-related pathologies. This review describes the roles of inflammation and fibrosis in arrhythmogenesis and prospective therapies for anti-arrhythmic treatment.

Optical mapping of sarcoplasmic reticulum Ca2+ in the intact heart: ryanodine receptor refractoriness during alternans and fibrillation.

RATIONALE: Sarcoplasmic reticulum (SR) Ca(2+) cycling is key to normal excitation-contraction coupling but may also contribute to pathological cardiac alternans and arrhythmia. OBJECTIVE: To measure intra-SR free [Ca(2+)] ([Ca(2+)]SR) changes in intact hearts during alternans and ventricular fibrillation (VF). METHODS AND RESULTS: Simultaneous optical mapping of Vm (with RH237) and [Ca(2+)]SR (with Fluo-5N AM) was performed in Langendorff-perfused rabbit hearts. Alternans and VF were induced by rapid pacing. SR Ca(2+) and action potential duration (APD) alternans occurred in-phase, but SR Ca(2+) alternans emerged first as cycle length was progressively reduced (217±10 versus 190±13 ms; P<0.05). Ryanodine receptor (RyR) refractoriness played a key role in the onset of SR Ca(2+) alternans, with SR Ca(2+) release alternans routinely occurring without changes in diastolic [Ca(2+)]SR. Sensitizing RyR with caffeine (200 µmol/L) significantly reduced the pacing threshold for both SR Ca(2+) and APD alternans (188±15 and 173±12 ms; P<0.05 versus baseline). Caffeine also reduced the magnitude of spatially discordant SR Ca(2+) alternans, but not APD alternans, the pacing threshold for discordance, or threshold for VF. During VF, [Ca(2+)]SR was high, but RyR remained nearly continuously refractory, resulting in minimal SR Ca(2+) release throughout VF. CONCLUSIONS: In intact hearts, RyR refractoriness initiates SR Ca(2+) release alternans that can be amplified by diastolic [Ca(2+)]SR alternans and lead to APD alternans. Sensitizing RyR suppresses spatially concordant but not discordant SR Ca(2+) and APD alternans. Despite increased [Ca(2+)]SR during VF, SR Ca(2+) release was nearly continuously refractory. This novel method provides insight into SR Ca(2+) handling during cardiac alternans and arrhythmia.

Antiarrhythmic effects of interleukin 1 inhibition after myocardial infarction.

BACKGROUND: Interleukin 1ß (IL-1ß) is a key regulator of the inflammatory response after myocardial infarction (MI) by modulating immune cell recruitment, cytokine production, and extracellular matrix turnover. Elevated levels of IL-1ß are associated with adverse remodeling, and inhibition of IL-1 signaling after MI results in improved contractile function. OBJECTIVE: The goal of this study was to determine whether IL-1 signaling also contributes to post-MI arrhythmogenesis. METHODS: MI was created in 2 murine models of elevated inflammation: atherosclerotic on the Western diet or wild-type with a subseptic dose of lipopolysaccharide. The role of IL-1ß was assessed with the IL-1 receptor antagonist anakinra (10 mg/(kg·d), starting 24 hours post-MI). RESULTS: In vivo and ex vivo molecular imaging showed reduced myocardial inflammation after a 4-day course of anakinra treatment, despite no change in infarct size. At day 5 post-MI, high-speed optical mapping of transmembrane potential and intracellular Ca2+ in isolated hearts revealed that IL-1ß inhibition improved conduction velocity, reduced action potential duration dispersion, improved intracellular Ca2+ handling, decreased transmembrane potential and Ca2+ alternans magnitude, and reduced spontaneous and inducible ventricular arrhythmias. These functional improvements were linked to increased expression of connexin 43 and sarcoplasmic reticulum Ca2+-ATPase. CONCLUSION: This study revealed a novel mechanism for IL-1ß in contributing to defective excitation-contraction coupling and arrhythmogenesis in the post-MI heart. Our results suggest that inhibition of IL-1 signaling post-MI may represent a novel antiarrhythmic therapy.

Optical Mapping of Intra-Sarcoplasmic Reticulum Ca2+ and Transmembrane Potential in the Langendorff-perfused Rabbit Heart.

Sarcoplasmic reticulum (SR) Ca2+ handling plays a key role in normal excitation-contraction coupling and aberrant SR Ca2+ handling is known to play a significant role in certain types of arrhythmia. Because arrhythmias are spatially distinct, emergent phenomena, they must be investigated at the tissue level. However, methods for directly probing SR Ca2+ in the intact heart remain limited. This article describes the protocol for dual optical mapping of transmembrane potential (Vm) and free intra-SR [Ca2+] ([Ca2+]SR) in the Langendorff-perfused rabbit heart. This approach takes advantage of the low-affinity Ca2+ indicator Fluo-5N, which has minimal fluorescence in the cytosol where intracellular [Ca2+] ([Ca2+]i) is relatively low but exhibits significant fluorescence in the SR lumen where [Ca2+]SR is in the millimolar range. In addition to revealing SR Ca2+ characteristics spatially across the epicardial surface of the heart, this approach has the distinct advantage of simultaneous monitoring of Vm, allowing for investigations into the bidirectional relationship between Vm and SR Ca2+ and the role of SR Ca2+ in arrhythmogenic phenomena.

Atherosclerosis exacerbates arrhythmia following myocardial infarction: Role of myocardial inflammation.

BACKGROUND: Atherosclerotic animal models show increased recruitment of inflammatory cells to the heart after myocardial infarction (MI), which impacts ventricular function and remodeling. OBJECTIVE: The purpose of this study was to determine whether increased myocardial inflammation after MI also contributes to arrhythmias. METHODS: MI was created in 3 mouse models: (1) atherosclerotic (apolipoprotein E deficient [ApoE(-/-)] on atherogenic diet, n = 12); (2) acute inflammation (wild-type [WT] given daily lipopolysaccharide [LPS] 10 µg/day, n = 7); and (3) WT (n = 14). Sham-operated (n = 4) mice also were studied. Four days post-MI, an inflammatory protease-activatable fluorescent probe (Prosense680) was injected intravenously to quantify myocardial inflammation on day 5. Optical mapping with voltage-sensitive dye was performed on day 5 to assess electrophysiology and arrhythmia susceptibility. RESULTS: Inflammatory activity (Prosense680 fluorescence) was increased approximately 2-fold in ApoE+MI and LPS+MI hearts vs WT+MI (P<.05) and 3-fold vs sham (P<.05). ApoE+MI and LPS+MI hearts also had prolonged action potential duration, slowed conduction velocity, and increased susceptibility to pacing-induced arrhythmias (56% and 71% vs 13% for WT+MI and 0% for sham, respectively, P<.05, for ApoE+MI and LPS+MI groups vs both WT+MI and sham). Increased macrophage accumulation in ApoE+MI and LPS+MI hearts was confirmed by immunofluorescence. Macrophages were associated with areas of connexin43 (Cx43) degradation, and a 2-fold decrease in Cx43 expression was found in ApoE+MI vs WT+MI hearts (P<.05). ApoE+MI hearts also had a 3-fold increase in interleukin-1ß expression, an inflammatory cytokine known to degrade Cx43. CONCLUSION: Underlying atherosclerosis exacerbates post-MI electrophysiological remodeling and arrhythmias. LPS+MI hearts fully recapitulate the atherosclerotic phenotype, suggesting myocardial inflammation as a key contributor to post-MI arrhythmia.