Obesity increases the incidence of cardiac arrhythmias and impairs wound healing. However, it is presently unknown whether a high-fat diet affects arrhythmic risk or wound healing before the onset of ...overt obesity or hyperlipidemia. After 8 wk of feeding a high-fat diet to adult female rats, a nonsignificant increase in body weight was observed and associated with a normal plasma lipid profile. Following ischemia/reperfusion injury, scar length (standard diet 0.29 +/- 0.09 vs. high-fat 0.32 +/- 0.13 cm), thickness (standard diet 0.047 +/- 0.02 vs. high-fat 0.059 +/- 0.01 cm), and collagen alpha(1) type 1 content (standard diet 0.21 +/- 0.04 vs. high-fat 0.20 +/- 0.04 arbitrary units/mm(2)) of infarcted hearts were not altered by the high-fat diet. However, the mortality rate was greatly increased 24 h postinfarction (from 5% to 46%, P < 0.01 for ischemia/reperfusion rats; from 20% to 89%, P < 0.0001, in complete-occlusion rats) in high-fat fed rats, in association with a higher prevalence of ventricular arrhythmias. Ventricular arrhythmia inducibility was also significantly increased in noninfarcted rats fed a high-fat diet. In the hearts of rats fed a high-fat diet, connexin-40 expression was absent, connexin-43 was hypophosphorylated and lateralized, and neurofilament-M immunoreactive fiber density (standard diet 2,020 +/- 260 vs. high-fat diet 2,830 +/- 250 microm(2)/mm(2)) and tyrosine hydroxylase protein expression were increased (P < 0.05). Thus, in the absence of overt obesity and hyperlipidemia, sympathetic hyperinnervation and an aberrant pattern of gap junctional protein expression and regulation in the heart of female rats fed a high-fat diet may have contributed in part to the higher incidence of inducible cardiac arrhythmias.
Atrial Fibrillation Promotion by Endurance Exercise Guasch, Eduard, MD; Benito, Begoña, MD; Qi, Xiaoyan, PhD ...
Journal of the American College of Cardiology,
07/2013, Letnik:
62, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Objectives The goal of this study was to assess mechanisms underlying atrial fibrillation (AF) promotion by exercise training in an animal model. Background High-level exercise training promotes AF, ...but the underlying mechanisms are unclear. Methods AF susceptibility was assessed by programmed stimulation in rats after 8 (Ex8) and 16 (Ex16) weeks of daily 1-h treadmill training, along with 4 and 8 weeks after exercise cessation and time-matched sedentary (Sed) controls. Structural remodeling was evaluated by using serial echocardiography and histopathology, autonomic nervous system with pharmacological tools, acetylcholine-regulated potassium current (IKACh ) with patch clamp recording, messenger ribonucleic acid expression with quantitative polymerase chain reaction, and regulators of G protein–signaling (RGS) 4 function in knockout mice. Results AF inducibility increased after 16 weeks of training (e.g., AF >30 s in 64% of Ex16 rats vs 15% of Sed rats; p < 0.01) and rapidly returned to baseline levels with detraining. Atropine restored sinus rhythm in 5 of 5 Ex rats with AF sustained >15 min. Atrial dilation and fibrosis developed after 16 weeks of training and failed to fully recover with exercise cessation. Parasympathetic tone was increased in Ex16 rats and normalized within 4 weeks of detraining. Baroreflex heart rate responses to phenylephrine-induced blood pressure elevation and IKACh sensitivity to carbachol were enhanced in Ex16 rats, implicating both central and end-organ mechanisms in vagal enhancement. Ex rats showed unchanged cardiac adrenergic and cholinergic receptor and IKACh -subunit gene expression, but significant messenger ribonucleic acid downregulation of IKACh -inhibiting RGS proteins was present at 16 weeks. RGS4 knockout mice showed significantly enhanced sensitivity to AF induction in the presence of carbachol. Conclusions Chronic endurance exercise increased AF susceptibility in rats, with autonomic changes, atrial dilation, and fibrosis identified as potential mechanistic contributors. Vagal promotion is particularly important and occurs via augmented baroreflex responsiveness and increased cardiomyocyte sensitivity to cholinergic stimulation, possibly due to RGS protein downregulation.
Aims
Previous studies suggested that T-type Ca2+-current (ICaT)-blockers improve cardiac remodelling, but all available ICaT-blockers have non-specific actions on other currents and/or functions. To ...clarify the role of ICaT in cardiac remodelling, we studied mice with either of the principal cardiac ICaT-subunits (Cav3.1 or Cav3.2) knocked out.
Methods and results
Adult male Cav3.1- or Cav3.2-knockout (Cav3.1−/−, Cav3.2−/−) mice and respective wild-type (WT) littermate controls were subjected to left anterior descending coronary artery ligation to create myocardial infarction (MI). Echocardiography and programmed electrical stimulation were performed at baseline and 4 weeks post-MI. At baseline, Cav3.1−/− mice had slowed heart rates and longer PR intervals vs. WT, but no other electrophysiological and no haemodynamic differences. Cav3.2−/− showed no differences vs. WT. Contractile indices (left ventricular fractional shortening and ejection fraction) decreased more post-MI in Cav3.1−/− mice than in Cav3.1+/+ (e.g. by 34 and 29% for WT; 50 and 45% for Cav3.1−/−, respectively; P < 0.05 for each). Cav3.1−/− mice had increased ventricular tachycardia (VT) inducibility post-MI (9 of 11, 82%) vs. WT (3 of 10, 30%; P < 0.05). Cav3.2−/− mice were not different in cardiac function or VT inducibility vs. WT. Quantitative polymerase chain reaction showed that Cav3.1 is the major ICaT-subunit and that no compensatory Cav3.2 up-regulation occurs in Cav3.1−/− mice. Cav3.1−/− and Cav3.2−/− mice had no mRNA expression for the knocked-out gene, at baseline or post-MI.
Conclusion
Our findings suggest that, contrary to suggestions from previous studies with (imperfectly selective) pharmacological agents having T-type Ca2+-channel-blocking actions, elimination of Cav3.1 expression leads to impaired cardiac function and enhanced arrhythmia vulnerability post-MI, whereas Cav3.2 elimination has no effect.
MK5, a member of the MAPK-activated protein kinase family, is highly expressed in the heart. Whereas MK2 and MK3 are activated by p38 MAPK, MK5 has also been shown to be activated by ERK3 and ERK4. ...We studied the regulation of MK5 in mouse heart. mRNA for 5 splice variants (MK5.1–5.5), including the original form (MK5.1), was detected. MK5 comprises 14 exons: exon 12 splicing was modified in MK5.2, MK5.3, and MK5.5. MK5.2 and MK5.5 lacked 6 bases at the 3′-end of exon 12, whereas MK5.3 lacked exon 12, resulting in a frame shift and premature termination of translation at codon 3 of exon 13. MK5.4 and MK5.5 lacked exons 2–6, encoding kinase subdomains I–VI, and were kinase-dead. All 5 MK5 variants were detected at the mRNA level in all mouse tissues examined; however, their relative abundance was tissue-specific. Furthermore, the relative abundance of variant mRNA was altered both during hypertrophy and postnatal cardiac development, suggesting that the generation or the stability of MK5 variant mRNAs is subject to regulation. When expressed in HEK293 cells, MK5.1, MK5.2 and MK5.3 were nuclear whereas MK5.4 and MK5.5 were cytoplasmic. A p38 MAPK activator, anisomycin, induced the redistribution of each variant. In contrast, MK5 co-immunoprecipitated ERK3, but not ERK4 or p38α, in control and hypertrophying hearts. GST pull-down assays revealed unbound ERK4 and p38α but no free MK5 or ERK3 in heart lysates. Hence, 1) in heart MK5 complexes with ERK3 and 2) MK5 splice variants may mediate distinct effects thus increasing the functional diversity of ERK3–MK5 signaling.
We studied temperature homeostasis in male mice lacking all thyroid hormone receptor-α gene products (TRα-0/0). As other TRα-deficient mice, TRα-0/0 mice have lower core body temperature (TC) than ...cognate wild-type controls. We found that obligatory thermogenesis is normal in TRα-0/0 and that the lower TC at room temperature (RT, 20–22 C) is caused by a down setting of the hypothalamic thermostat. However, TRα-0/0 mice are cold intolerant due to impaired facultative thermogenesis. Norepinephrine-induced brown adipose tissue (BAT) thermogenesis is blunted, even though BAT-relevant genes and T4 deiodinase respond normally to cold stimulation, as do serum T3, serum glycerol (marker of lipolysis), and heart rate. BAT normally contributes to maintain TC at RT, 9 C below thermoneutrality, yet TRα-0/0 mice do not show signs of being cold stressed at 20–22 C. Instead, oxygen consumption is greater in TRα-0/0 than in wild-type mice at RT, suggesting the recruitment of an alternate, cold-activated form of thermogenesis to compensate for the lack of BAT thermogenesis. These results indicate that TRα is necessary for T3 to modulate the central control of TC and for an essential step in norepinephrine activation of BAT thermogenesis but not to sustain obligatory thermogenesis. In addition, the results provide evidence for an alternate form of facultative thermogenesis, which probably originates in skeletal muscle and that is less effective and more energy demanding than BAT thermogenesis.
The renin-angiotensin and sympathetic nervous systems play critical interlinked roles in the development of left ventricular hypertrophy, fibrosis, and dysfunction. These studies investigated the ...hemodynamic and cardiac effects of monoblockade and coblockade of renin-angiotensin and sympathetic nervous systems. Stroke-prone spontaneously hypertensive rats (16 weeks old; male; n=12 per group) received the sympatholytic imidazoline compound, moxonidine (2.4 mg/kg per day); the angiotensin-receptor blocker eprosartan (30 mg/kg per day), separately or in combination; or saline vehicle for 8 weeks, SC, via osmotic minipumps. Blood pressure and heart rate were continuously measured by radiotelemetry. After 8 weeks, in vivo cardiac function and structure were measured by transthoracic echocardiography and a Millar conductance catheter, and the rats were then euthanized and blood and heart ventricles collected for various determinations. Compared with vehicle, the subhypotensive dose of moxonidine resulted in lower (P<0.01) heart rate, left ventricular hypertrophy, cardiomyocyte cross-sectional area, interleukin 1β, tumor necrosis factor-α, and mRNA for natriuretic peptides. Eprosartan reduced pressure (P<0.01), as well as extracellular signal–regulated kinase (ERK) 44 phosphorylation, Bax/Bcl-2, and collagen I/III, and improved left ventricular diastolic function (P<0.03). Combined treatment resulted in greater reductions in blood pressure, heart rate, left ventricular hypertrophy, collagen I/III, and inhibited inducible NO synthase and increased endothelial NO synthase phosphorylation, as well as reduced left ventricular anterior wall thickness, without altering the other parameters. Thus, in advanced hypertension complicated with cardiac fibrosis, sympathetic inhibition and angiotensin II blockade resulted in greater reduction in blood pressure and heart rate, inhibition of inflammation, and improved left ventricular pathology but did not add to the benefits of angiotensin II blockade on cardiac function.
Abstract only Cardiac hypertrophy, a common consequence of cardiopathologies such as hypertension and myocardial infarcts, involves formation of excessive interstitial fibrosis, which may impair ...cardiac function. Fibroblasts are the primary source of extracellular matrix protein. Extracellular-regulated kinase 4 (ERK4) is an atypical mitogen-activated protein kinase (MAPK). The regulation and role of ERK4 in the heart are currently unidentified and its only known target is MAP kinase-activated protein kinase 5 (MK5), a kinase involved in regulating fibroblast function. Following constriction of the transverse aorta (TAC), MK5 haplodeficient mice showed an attenuation of the TAC-induced increase in collagen 1-α 1 mRNA at 2-wk post-TAC and reduced hypertrophy 8-wk post-TAC. Further studies revealed MK5 immunoreactivity in cardiac fibroblasts but not myocytes. MK5 immunoprecipitates from whole heart contain ERK3 immunoreactivity, but not that of ERK4 or p38 MAPK. This study was to examine the role of ERK4 in myocardial structure, function, and remodeling 3-wk post-TAC. At 12 wk of age, echocardiographic imaging revealed systolic and diastolic function in male ERK4 -/- mice were similar to wild-type littermates (ERK4 +/+ ). Three weeks post-TAC, hypertrophy was similar in ERK4 +/+ and ERK4 -/- mice. Transcripts for BNP and βMHC increased to similar extent in TAC- ERK4 +/+ and TAC- ERK4 -/- mice. Two-way ANOVA indicated that ERK4 deficiency altered the effect of TAC on TGFβ 1 and collagen 1-α 1 transcript levels with each being higher in TAC-ERK4 -/- mice. Furthermore, MK5 immunoprecipitates from cardiac fibroblast lysates did not contain ERK4 immunoreactivity. Additional experiments revealed the presence of ERK4 immunoreactivity in myocytes but not fibroblasts. These results suggest 1) ERK4 may be involved in myocyte - fibroblast communication during myocardial remodeling and 2) in cardiac myocytes, ERK4 is part of a novel signaling cascade that does not involve MK5.
Abstract only
Introduction
Angiopoietin‐like 2 (ANGPTL2) is a pro‐inflammatory and pro‐oxidative glycoprotein that induces vascular endothelial dysfunction in mice and promotes atherosclerosis. ...Higher circulating levels of ANGPTL2 have been reported in patients with heart failure, but the potentially deleterious effect of ANGPTL2 on cardiac function is ill defined. Our aim was to investigate the cardiac function of adult mice knockdown for
angptl2
(KD mice).
Hypothesis
Based on the available data in the literature, we hypothesized that KD for
angptl2
would be cardioprotective.
Methods
Cardiac function was measured by high frequency echocardiography in adult 7‐month old KD and wild‐type (WT) littermates. Gene expression was measured by quantitative RT‐PCR.
Results
In contrast to our hypothesis, when compared to WT mice (n=32), KD mice (n=31) exhibited a left ventricular (LV) systolic dysfunction, characterized by a reduced fractional shortening (35.8±1.0 vs 32.2±1.2%, p<0.05), a reduced ejection fraction (71.6±1.3 vs 66.4±1.8%, p<0.05), a lower lateral (2.3±0.1 vs 2.1±0.1 cm/s, p<0.05) and septal (2.5±0.1 vs 2.2±0.1 cm/s, p<0.05) contractility. This LV systolic dysfunction observed in KD mice was reproduced in WT mice exposed to a cardiac pressure overload generated by transverse aortic constriction (TAC); in KD mice, TAC did not further alter cardiac function. Interestingly, we observed that cardiac NADPH oxidase
Nox4
mRNA expression tended to be higher in KD mice (+13%, p>0.05). NOX4 is known to produce H
2
O
2
, a deleterious hypertrophic stimulus in cardiomyocytes. Cardiac
Nox4
was strongly and negatively correlated with fractional shortening (r=−0.836, p<0.001) and ejection fraction (r=−0.839, p<0.001) in KD mice only (sham and TAC mice, n=15), i.e. the higher
Nox4
, the worse LV dysfunction. In contrast,
Nox4
expression was not correlated with markers of LV dysfunction in WT mice (sham and TAC, n=12; p>0.05). We then tested the effects of repressing, or not, cardiac expression of
Nox4
by a single injection of cardiac specific associated adenovirus AAV9 delivering a NOX4 targeted shRNA (shNOX4) or a scramble shRNA (SCR), in both WT and KD mice. The shNOX4, which significantly reduced cardiac
Nox4
mRNA expression (‐34%, p<0.05), fully reversed LV systolic dysfunction in KD mice (Table
). In contrast, in WT mice AAV9‐shNOX4 had no effect (Table
).
Conclusion
Knockdown of ANGPTL2 promotes LV systolic dysfunction that can be reversed by the decrease of NOX4 expression in the heart of KD mice.
Support or Funding Information
This work was funded by grants from the Canadian Institutes of Health Research (PJT‐162446) and by the Foundation of the Montreal Heart Institute.
WT‐SCR (n=5)
KD‐SCR (n=7)
WT‐shNOX4 (n=8)
KD‐shNOX4 (n=5)
Fractional shortening (%)
38.6±2.8
27.7±1.7
*
35.3±1.8
37.4±2.4
†
Ejection fraction (%)
74.9±3.5
60.1±2.7
*
71.1±2.2
73.7±3.0
†
Lateral contractility (cm/s)
2.5±0.1
1.7±0.1
*
2.4±0.1
2.3±0.1
†
Septal contractility (cm/s)
2.7±0.1
2.0±0.1
*
2.6±0.1
2.7±0.1
†
*
p<0.05 WT‐SCR vs KD‐SCR;
†
p<0.05 KD‐SCR vs KD‐shNOX4 (Two‐way ANOVA Bonferroni’s multiple comparisons test).
Abstract only
Obesity and elevated body fat are generally associated with a higher risk of metabolic diseases such as type 2 diabetes, but the mechanisms involved are not clear. Angiopoietin‐like 2 ...(ANGPTL2), a circulating pro‐inflammatory glycoprotein mainly secreted by adipose tissues, is now considered as a key mediator linking obesity to insulin resistance. Accordingly, high levels of ANGPTL2 in humans are associated with obesity and the development of diabetes, and in Angptl2
−/−
mice exposed to a high fat diet, body fat is lower and insulin sensitivity better than in wild‐type (WT) mice. Various approaches have been developed to promote weight loss and increase insulin sensitivity such as intermittent fasting, an efficient but strenuous lifestyle intervention that involves alternating cycles of fasting and eating. Our objective was to investigate whether the knockdown of ANGPTL2 in mice reproduces or improves the benefits of intermittent fasting in WT mice on weight and insulin sensitivity. Intermittent fasting (one day of fasting/one day of free access to food) for 4 months reduced (p<0.05)
Angptl2
mRNA expression by 53% in adipose tissue from WT mice. Both intermittent fasting for 4 months and knockdown of ANGPTL2 similarly decreased (p<0.05) feeding efficiency by 55%, weight gain by 55%, liver (38%) and adipose (around 55%) tissue weights and similarly increased (p<0.05) insulin sensitivity (measured by the insulin tolerance test). All measured parameters were similar between WT mice fed with an intermittent fasting regimen and ANGPTL2 knockdown mice fed
ad libitum
, suggesting that knockdown of ANGPTL2 reproduces the benefits of intermittent fasting on weight loss and insulin sensitivity. Finally, intermittent fasting in ANGPTL2 knockdown mice did not further reduce weight gain, adipose tissue and liver weight, and did not further improve insulin sensitivity. We therefore propose that lowering ANGPTL2 could be a useful and promising strategy to limit obesity and prevent insulin resistance. The development of pharmacological inhibitors of ANGTPL2 is needed.
Support or Funding Information
Canadian Institutes of Health Research
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