Ziqiu Ming

Reduced Phosphoinositide 3-Kinase (p110α) Activation Increases the Susceptibility to Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia presenting at cardiology departments. A limited understanding of the molecular mechanisms responsible for the development of AF has hindered treatment strategies. The purpose of this study was to assess whether reduced activation of phosphoinositide 3-kinase (PI3K, p110alpha) makes the compromised heart susceptible to AF. Risk factors for AF, including aging, obesity, and diabetes, have been associated with insulin resistance that leads to depressed/defective PI3K signaling. However, to date, there has been no link between PI3K(p110alpha) and AF. To address this question, we crossed a cardiac-specific transgenic mouse model of dilated cardiomyopathy (DCM) with a cardiac-specific transgenic mouse expressing a dominant negative mutant of PI3K (dnPI3K; reduces PI3K activity). Adult ( approximately 4.5 months) double-transgenic (dnPI3K-DCM), single-transgenic (DCM-Tg, dnPI3K-Tg), and nontransgenic mice were subjected to morphological, functional/ECG, microarray, and biochemical analyses. dnPI3K-DCM mice developed AF and had depressed cardiac function as well as greater atrial enlargement and fibrosis than DCM-Tg mice. AF was not detected in other groups. Aged DCM-Tg mice ( approximately 15 months) with a similar phenotype to dnPI3K-DCM mice (4.5 months) did not develop AF, suggesting loss of PI3K activity directly contributed to the AF phenotype. Furthermore, increasing PI3K activity reduced atrial fibrosis and improved cardiac conduction in DCM-Tg mice. Finally, in atrial appendages from patients with AF, PI3K activation was lower compared with tissue from patients in sinus rhythm. These results suggest a link between PI3K(p110alpha) and AF.

Relaxin remodels fibrotic healing following myocardial infarction

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+RLX) or vehicle (MI+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-β1 expression, myofibroblast differentiation, and cardiomyocyte apoptosis in addition to a promotion of matrix metalloproteinase-13 levels and de novo blood vessel growth (all P<0.05 vs respective measurements from MI+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.

Optimizing dosage of ketamine and xylazine in murine echocardiography

1 Ketamine and xylazine (KX) mixture is the most commonly used anaesthetic drug during echocardiography in mice to induce sedation and immobility. Nevertheless, the doses of KX reported in the literature vary substantially with associated significant difference in cardiac function. To explore the optimal KX dosage and observation time for murine echocardiography, we compared the effects of various KX combinations on echocardiographic measurement. 2 Mice were anaesthetized with ketamine (50 or 100 mg/kg) and xylazine (0–10 mg/kg). Echocardiography was performed 5, 10, 20 and 40 min after induction of anaesthesia. Also, cardiac function was assessed in mice with and without pressure‐overload induced left ventricle (LV) hypertrophy and dysfunction, either under anaesthesia with KX or whilst conscious. 3 Ketamine at 100 mg/kg alone or together with xylazine at 0.1 mg/kg was associated with a high and stable heart rate (HR), a high fractional shortening (FS) and produced the least effect on LV inner dimension at end of diastole (LVIDd). Ketamine and xylazine at 100 and 10 mg/kg, respectively, produced a lower and stable FS, but with a low and unstable HR. All other combinations resulted in depressed and unstable cardiac function during this period. 4 The dose‐dependent suppression of FS by xylazine was counteracted partly by ketamine. 5 Although in the chronic pressure‐overload model LV hypertrophy can be detected accurately in both the anaesthetized or conscious state, systolic dysfunction was masked partially by higher doses of xylazine (2.5 or 10 mg/kg) combined with ketamine at 100 mg/kg. 6 With KX anaesthesia, both the dose of xylazine and the anaesthetic duration are critical in achieving an ideal condition for murine echocardiography. Ketamine at 100 mg/kg alone produces acceptable anaesthesia, stable cardiac function with a minimal depressant effect and is therefore recommended if single‐dose anaesthetic is to be used.

Infarct size and post-infarct inflammation determine the risk of cardiac rupture in mice.

BACKGROUND/OBJECTIVES Infarct size (IS) is a determinant of pathophysiological events after myocardial infarction (MI), but its relation to the risk of cardiac rupture remains undefined. METHODS MI was induced in 129sv and C57Bl/6 mice. Left ventricular (LV) remodelling was examined by echocardiography prior to the onset of rupture. Changes in muscle tensile strength and expression of inflammatory factors were determined. Autopsy was performed and IS measured. RESULTS Rupture incidence was higher in 129sv than C57Bl/6 mice (62% vs. 33%, P<0.001). Rupture occurred in mice with IS over a threshold, which was smaller in 129sv than C57Bl/6 mice (20% vs. 30%). 129sv mice with IS>30% had a higher incidence of rupture than those with IS 20-30%. Echocardiography revealed IS-dependent LV remodelling and dysfunction and 129sv mice had a better-preserved function compared with C57Bl/6 counterparts. 129sv but not C57Bl/6 mice that subsequently developed rupture showed more severe regional dysfunction and remodelling compared with IS-matched non-ruptured hearts. Tensile strength of the infarcted myocardium was reduced significantly, which was IS-related. 129sv mice had higher expression levels of inflammatory mediators in the infarcted myocardium or circulating inflammatory cells, underlying the higher risk of rupture in this strain than C57Bl/6. CONCLUSIONS A critical IS level is necessary for post-MI rupture and IS correlates with the reduction in muscle tensile strength. Strain differences exist in global function and regional or systemic inflammation that explain the different risk of rupture or heart failure between strains. Limiting IS or minimizing inflammation would lower the risk of ventricular rupture.