Proarrhythmic Electrical Remodeling by Noncardiomyocytes at Interfaces With Cardiomyocytes Under Oxidative Stress

Myofibroblasts impair myocardial impulse propagation by heterocellular connexin43 gap-junctional coupling through micropores

Aim: Composite population of myofibroblasts (MFs) within myocardial tissue is known to alter impulse propagation, leading to arrhythmias. However, it remains unclear whether and how MFs alter their propagation patterns when contacting cardiomyocytes (CMs) without complex structural insertions in the myocardium. We attempted to unveil the effects of the one-sided, heterocellular CM-MF connection on the impulse propagation of CM monolayers without the spatial insertion of MFs as an electrical or mechanical obstacle. Methods and results: We evaluated fluo8-based spatiotemporal patterns in impulse propagation of neonatal rat CM monolayers cultured on the microporous membrane having 8-μm diameter pores with co-culture of MFs or CMs on the reverse membrane side (CM-MF model or CM-CM model, respectively). During consecutive pacing at 1 or 2 Hz, the CM monolayers exhibited forward impulse propagation from the pacing site with a slower conduction velocity (θ) and a larger coefficient of directional θ variation in the CM-MF model than that in the CM-CM model in a frequency-dependent manner (2 Hz >1 Hz). The localized placement of an MF cluster on the reverse side resulted in an abrupt segmental depression of the impulse propagation of the upper CM layer, causing a spatiotemporally non-uniform pattern. Dye transfer of the calcein loaded in the upper CM layer to the lower MF layer was attenuated by the gap-junction inhibitor heptanol. Immunocytochemistry identified definitive connexin 43 (Cx43) between the CMs and MFs in the membrane pores. MF-selective Cx43 knockdown in the MF layer improved both the velocity and uniformity of propagation in the CM monolayer. Conclusion: Heterocellular Cx43 gap junction coupling of CMs with MFs alters the spatiotemporal patterns of myocardial impulse propagation, even in the absence of spatially interjacent and mechanosensitive modulations by MFs. Moreover, MFs can promote pro-arrhythmogenic impulse propagation when in face-to-face contact with the myocardium that arises in the healing infarct border zone.

Effects of salt and stress on blood pressure parameters and antioxidant enzyme function in the heart and aorta of borderline hypertensive rats

NEW FINDINGS

What is the central question of this study? Although the involvement of reactive oxidative species in triggering hypertension has been documented, there are no data about the role of antioxidant enzymes in the heart and aorta of borderline hypertensive rats kept in baseline conditions or exposed to high salt with or without repeated stress. What is the main finding and its importance? In borderline hypertensive rats, high salt intake and stress contribute significantly to increase blood pressure and antioxidative defence in the aorta but decrease it in the heart. Elucidating the early changes that accompany elevated blood pressure could provide new therapeutical venues for prevention and treatment of the condition.

ABSTRACT

Hypertension and its complications are a leading cause of death in the human population. Several factors can contribute to development of hypertension, such as genetic predisposition, high salt intake and environmental stressors, underlying oxidative stress as one of its key trademarks. We studied the effects of increased salt intake and chronic stress on blood pressure parameters and the activity and protein levels of antioxidant enzymes in the heart and aorta of borderline hypertensive rats (BHRs) with genetic susceptibility to hypertension. All animals were randomized into four groups: (1) Wistar rats kept in baseline conditions; (2) BHRs kept in baseline conditions; (3) BHRs drinking 0.9% saline solution; and (4) BHRs drinking 0.9% saline solution and exposed to repeated heterotypic stress. The BHRs exhibited significantly higher blood pressure, mitochondrial superoxide dismutase (SOD2) and catalase (CAT) protein levels and lower glutathione peroxidase (GPx) and glutathione reductase (GR) activities in the aorta, followed by lower CAT and GPx protein levels and higher CAT and GR activities in the heart, compared with normotensive Wistar rats. In the BHR aorta, high salt intake elevated CAT and GPx activities, and when combined with stress it increased GPx and GR activities. In BHR hearts, high salt intake provoked lower CAT activity. Adding repeated stress to salt treatment further decreased CAT activity, in addition to Cu2+ -Zn2+ superoxide dismutase (SOD1) and GR activities. The protein level of CAT was lower, whereas SOD2 and GPx increased. Overall, our results suggest that BHR hearts are better adapted to oxidative pressure, compared with the aorta, when exposed to salt and stress.

Glycolysis Inhibition Alleviates Cardiac Fibrosis After Myocardial Infarction by Suppressing Cardiac Fibroblast Activation

Objective: To explore the role of glycolysis in cardiac fibroblast (CF) activation and cardiac fibrosis after myocardial infarction (MI).

Method:In vivo: 2-Deoxy-D-glucose (2-DG), a glycolysis inhibitor, was injected into the abdominal cavity of the MI or sham mice every day. On the 28th day, cardiac function was measured by ultrasonic cardiography, and the hearts were harvested. Masson staining and immunofluorescence (IF) were used to evaluate the fibrosis area, and western blot was used to identify the glycolytic level. In vitro, we isolated the CF from the sham, MI and MI with 2-DG treatment mice, and we also activated normal CF with transforming growth factor-β1 (TGF-β1) and block glycolysis with 2-DG. We then detected the glycolytic proteins, fibrotic proteins, and the concentrations of lactate and glucose in the culture medium. At last, we further detected the fibrotic and glycolytic markers in human fibrotic and non-fibrotic heart tissues with masson staining, IF and western blot.

Result: More collagen and glycolytic protein expressions were observed in the MI mice hearts. The mortality increased when mice were treated with 2-DG (100 mg/kg/d) after the MI surgery (Log-rank test, P < 0.05). When the dosage of 2-DG declined to 50 mg/kg/d, and the treatment was started on the 4th day after MI, no statistical difference of mortality between the two groups was observed (Log-rank test, P = 0.98). The collagen volume fraction was smaller and the fluorescence signal of α-smooth muscle actin (α-SMA) was weaker in mice treated with 2-DG than PBS. In vitro, 2-DG could significantly inhibit the increased expression of both the glycolytic and fibrotic proteins in the activated CF.

Conclusion: Cardiac fibrosis is along with the enhancement of CF activation and glycolysis. Glycolysis inhibition can alleviate cardiac fibroblast activation and cardiac fibrosis after myocardial infarction.