LV wall rupture in the mouse occurs within a narrow time-window after AMI and is strain- and gender-dependent. Infarct expansion, regional hemorrhage with formation of hematoma and leuocyte accumulation are important pathological changes leading to reduced myocardial tensile strength.

Abstract-The antifibrotic effects of the peptide hormone relaxin on cardiac and renal fibrosis were studied in 9-to 10-month-old male spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Rats (nϭ8 to 9 per group) were allocated into 3 groups: WKY controls, vehicle-treated SHR (SHR-V), and relaxin-treated SHR (SHR-R). Relaxin (0.5 mg/kg per day) was administered via subcutaneously implanted osmotic mini-pumps over 2 weeks before hearts and kidneys were harvested for analysis. Collagen content was analyzed by hydroxyproline assay, gel electrophoresis, and quantitative histology. Zymography was used to determine matrix metalloproteinase (MMP) expression and Western blotting to determine proliferating cell nuclear antigen (PCNA) expression and ␣-smooth muscle actin (␣-SMA)/myofibroblast expression, whereas cardiac hypertrophy was assessed by myocyte size and real-time polymerase chain reaction of associated genes.

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BACKGROUND AND PURPOSEWhile maintaining cardiac performance, chronic b-adrenoceptor activation eventually exacerbates the progression of cardiac remodelling and failure. We examined the adverse signalling pathways mediated by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and reactive oxygen species (ROS) after chronic b2-adrenoceptor activation. EXPERIMENTAL APPROACHMice with transgenic b2-adrenoceptor overexpression (b2-TG) and non-transgenic littermates were either untreated or treated with an antioxidant (N-acetylcysteine, NAC) or NADPH oxidase inhibitors (apocynin, diphenyliodonium). Levels of ROS, phosphorylated p38 mitogen-activated protein kinase (MAPK), pro-inflammatory cytokines and collagen content in the left ventricle (LV) and LV function were measured and compared. KEY RESULTSb2-TG mice showed increased ROS production, phosphorylation of p38 MAPK and heat shock protein 27 (HSP27), expression of pro-inflammatory cytokines and collagen, and progressive ventricular dysfunction. b2-adrenoceptor stimulation similarly increased ROS production and phosphorylation of p38 MAPK and HSP27 in cultured cardiomyocytes. Treatment with apocynin, diphenyliodonium or NAC reduced phosphorylation of p38 MAPK and HSP27 in both cultured cardiomyocytes and the LV of b2-TG mice. NAC treatment (500 mg·kg) for 2 weeks eliminated the up-regulated expression of pro-inflammatory cytokines and collagen in the LV of b2-TG mice. Chronic NAC treatment to b2-TG mice from 7 to 10 months of age largely prevented progression of ventricular dilatation, preserved contractile function (fractional shortening 37 Ϯ 5% vs. 25 Ϯ 3%, ejection fraction 52 Ϯ 5% vs. 32 Ϯ 4%, both P < 0.05), reduced cardiac fibrosis and suppressed matrix metalloproteinase activity. CONCLUSION AND IMPLICATIONSb2-adrenoceptor stimulation provoked NADPH oxidase-derived ROS production in the heart. Elevated ROS activated p38 MAPK and contributed significantly to cardiac inflammation, remodelling and failure. Abbreviations a-MHC, a-myosin heavy chain; a-SMA, a-smooth muscle actin; b2-TG, b2-adrenoceptor transgenic; CMH, 1-hydroxy-3-methoxycarbonyl-2,2,5,5,tetramethylpyrrolidine; CTGF, connective tissue growth factor; DHE, BJP British Journal of Pharmacology LINKED ARTICLE

In the setting of myocardial infarction (MI), implanted stem cell viability is low and scar formation limits stem cell homing, viability, and integration. Thus, interventions that favorably remodel fibrotic healing may benefit stem cell therapies. However, it remains unclear whether it is feasible and safe to remodel fibrotic healing post-MI without compromising ventricular remodeling and dysfunction. This study, therefore, determined the anti-fibrotic and other effects of the hormone, relaxin in a mouse model of MI. Adult male mice underwent left coronary artery ligation-induced MI and were immediately treated with recombinant human relaxin (MI þ RLX) or vehicle (MI þ VEH) over 7 or 30 days, representing time points of early and mature fibrotic healing. Cardiac function was assessed by echocardiography and catheterization, while comprehensive immunohistochemistry, morphometry, and western blotting were performed to explore the relaxin-induced mechanisms of action post-MI. RLX significantly inhibited the MI-induced progression of cardiac fibrosis over 7 and 30 days, which was associated with a reduction in TGF-b1 expression, myofibroblast differentiation, and cardiomyocyte apoptosis in addition to a promotion of matrix metalloproteinase-13 levels and de novo blood vessel growth (all Po0.05 vs respective measurements from MI þ VEH mice). Despite the evident fibrotic healing post-MI, relaxin did not adversely affect the incidence of ventricular free-wall rupture or the extent of LV remodeling and dysfunction. These combined findings demonstrate that RLX favorably remodels the process of fibrotic healing post-infarction by lowering the density of mature scar tissue in the infarcted myocardium, border zone, and non-infarcted myocardium, and may, therefore, facilitate cell-based therapies in the setting of ischemic heart disease.