Thyroid State and Tolerance of Mammalian Myocardium to Hypoxia

Thyroid hormone is known to affect myocardial glycogen stores and thereby possibly limit anaerobic performance of mammalian cardiac muscle. Thyroid hormone administration (3,5,T-triiodo-L-thyroxine, 300 mu g/kg/day, sc) for 10 days decreased left ventricle (LV) glycogen concentration relative to euthyroid animals (2.78 +/- 0.46 vs. 4.28 +/- 0.29 mg/g of LV (mean +/- SEM)) while increasing the percent of V(1) myosin isozyi-ne, contractile activity and cardiac mass. In contrast, thyroidectomy increased myocardial glycogen stores (8.50 +/- 0.56 mg/g of LV) and shifted the myosin isozyme toward V(3), prolonged contractile activity and decreased LV mass. Thyroxine administration for 3, 7 and 10 days to thyroidectomized animals progressively decreased contractile duration and increased LV mass. Thyroxine administration for 3 or 7 days to thyroidectomized rats did not reduce glycogen stores (7.75 +/- 1.02 and 9.62 +/- 1.16 mg/g of LV, respectively), whereas myocardial glycogen declined to 3.30 +/- 0.58 mg/g of LV after 10 days of treatment. During hypoxia, cardiac muscle from thyroidectomized rats maintained greater active force and developed less contracture relative to euthyroid and, to a greater extent, than hyperthyroid rats. Removal of glucose from the bath decreased anaerobic performance and impaired recovery; however, myocardium from thyroidectomized rats remained more tolerant to hypoxia than the euthyroid group. Overall, the intrinsic LV glycogen content was positively correlated to anaerobic performance. These data demonstrate that the thyroid state profoundly affects myocardial growth, contractility and anaerobic performance of rat myocardium. Although energy demand may affect function during hypoxia, anaerobic substrate reserve (cardiac glycogen concentration) appears to be the primary factor determining tolerance to hypoxic stress. J. Exp. Zool. 311A:399-407, 2009. (C) 2009 Wiley-Liss, Inc.

Force patterns of hypoxic myocardium applied to oxygenated muscle preparations: Comparison with effects of regional ischemia on the contraction of non-ischemic myocardium

Objective: To examine the basis for local wall motion abnormalities commonly seen in patients with ischemic heart disease, computer-controlled isolated muscle studies were carried out. Methods: Force patterns of physiologically sequenced contractions (PSCs) from rat left ventricular muscle preparations under well-oxygenated conditions and during periods of hypoxia and reoxygenation were recorded and stored in a computer. Force patterns of hypoxic-reoxygenating and oxygenated myocardium were applied to oxygenated and hypoxic-reoxygenating myocardium, respectively. Results: Observed patterns of shortening and lengthening closely resemble those obtained from ischemic and non-ischemic myocardial segments using ultrasonic crystals in intact dog hearts during coronary occlusion and reperfusion, and are similar to findings reported in angiographic studies of humans with coronary artery disease. Conclusion: The current study, demonstrating motions of oxygenated isolated muscle preparations which are similar to those in perfused segments of intact hearts with regional ischemia, supports the concept that the multiple motions of both ischemic and non-ischemic segments seen in regional myocardial disease can be explained by interactions of strongly and weakly contracting muscle during the physiologic cardiac cycle.

A comparative study of myocardial function and morphology during fasting/refeeding and food restriction in rats

Background: This study compared the influence of fasting/refeeding cycles and food restriction on rat myocardial performance and morphology. Methods: Sixty-day-old male Wistar rats were submitted to food ad libitum (C), 50% food restriction (R50), and fasting/refeeding cycles (RF) for 12 weeks. Myocardial function was evaluated under baseline conditions and after progressive increase in calcium and isoproterenol. Myocardium ultrastructure was examined in the papillary muscle. Results: Fasting/refeeding cycles maintained rat body weight and left ventricle weight between control and food-restricted rats. Under baseline conditions, the time to peak tension (TPT) was more prolonged in R50 than in RF and C rats. Furthermore, the maximum tension decline rate (-dT/dt) increased less in R50 than in RF with calcium elevation. While the R50 group showed focal changes in many muscle fibers, such as the disorganization or loss of myofilaments, polymorphic mitochondria with disrupted cristae, and irregular appearance or infolding of the plasma membrane, the RF rats displayed few alterations such as loss or disorganization of myofibrils. Conclusion: Food restriction promotes myocardial dysfunction, not observed in RF rats, and higher morphological damage than with fasting/refeeding. The increase in TPT may be attributed possibly to the disorganization and loss of myofibrils; however, the mechanisms responsible for the alteration in -dT/dt in R50 needs to be further clarified. © 2010 Elsevier Inc. All rights reserved.

Obesity Preserves Myocardial Function During Blockade of the Glycolytic Pathway

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Direct effects of colchicine on myocardial function: studies in hypertrophied and failing spontaneously hypertensive rats

The aging spontaneously hypertensive rat (SHR) is a model in which the transition from chronic stable left ventricular hypertrophy to overt heart failure can be observed. Although the mechanisms for impaired function in hypertrophied and failing cardiac muscle from the SHR have been studied, none accounts fully for the myocardial contractile abnormalities. The cardiac cytoskeleton has been implicated as a possible cause for myocardial dysfunction. If an increase in microtubules contributes to dysfunction, then myocardial microtubule disruption by colchicine should promote an improvement in cardiac performance. We studied the active and passive properties of isolated left ventricular papillary muscles from 18- to 24-month-old SHR with evidence of heart failure (SHR-F, n=6), age-matched SHR without heart failure (SHR-NF, n=6), and age-matched normotensive Wistar-Kyoto rats (WKY, n=5). Mechanical parameters were analyzed before and up to 90 minutes after the addition of colchicine (10(-5), 10(-4), and 10(-3) mol/L). In the baseline state, active tension (AT) developed by papillary muscles from the WKY group was greater than for SHR-NF and SHR-F groups (WKY 5.69+/-1.47 g/mm2 [mean+/-SD], SHR-NF 3.41+/-1.05, SHR-F 2.87+/-0.26; SHR-NF and SHR-F P<0.05 versus WKY rats). The passive stiffness was greater in SHR-F than in the WKY and SHR-NF groups (central segment exponential stiffness constant, Kcs: SHR-F 70+/-25, SHR-NF 44+/-17, WKY 41+/-13 [mean+/-SD]; SHR-F P<0.05 versus SHR-NF and WKY rats). AT did not improve after 10, 20, and 30 minutes of exposure to colchicine (10(-5), 10(-4), and 10(-3) mol/L) in any group. In the SHR-F group, AT and passive stiffness did not change after 30 to 90 minutes of colchicine exposure (10(-4) mol/L). In summary, the data in this study fail to demonstrate improvement of intrinsic muscle function in SHR with heart failure after colchicine. Thus, in the SHR there is no evidence that colchicine-induced cardiac microtubular depolymerization affects the active or passive properties of hypertrophied or failing left ventricular myocardium.

Cell therapy prevents structural, functional and molecular remodeling of remote non-infarcted myocardium

Background/objectives: Therapy using bone marrow (BM) cells has been tested experimentally and clinically due to the potential ability to restore cardiac function by regenerating lost myocytes or increasing the survival of tissues at risk after myocardial infarction (MI). In this study we aimed to evaluate whether BM-derived mononuclear cell (MNC) implantation can positively influence the post-MI structural remodeling, contractility and Ca(2 +)-handling proteins of the remote non-infarcted tissue in rats. Methods and results: After 48 h of MI induction, saline or BM-MNC were injected. Six weeks later, MI scars were slightly smaller and thicker, and cardiac dilatation was just partially prevented by cell therapy. However, the cardiac performance under hemodynamic stress was totally preserved in the BM-MNC treated group if compared to the untreated group, associated with normal contractility of remote myocardium as analyzed in vitro. The impaired post-rest potentiation of contractile force, associated with decreased protein expression of the sarcoplasmic reticulum Ca2 +-ATPase and phosphorylated-phospholamban and overexpression of Na(+)/Ca(2 +) exchanger, were prevented by BM-MNC, indicating preservation of the Ca(2 +) handling. Finally, pathological changes on remodeled remote tissue such as myocyte hypertrophy, interstitial fibrosis and capillary rarefaction were also mitigated by cell therapy. Conclusions: BM-MNC therapy was able to prevent cardiac structural and molecular remodeling after MI, avoiding pathological changes on Ca(2 +)-handling proteins and preserving contractile behavior of the viable myocardium, which could be the major contributor to the improvements of global cardiac performance after cell transplantation despite that scar tissue still exists.

betaARKct: a therapeutic approach for improved adrenergic signaling and function in heart disease

One of the most powerful regulators of cardiovascular function is catecholamine-stimulated adrenergic receptor (AR) signaling. The failing heart is characterized by desensitization and impaired beta-AR responsiveness as a result of upregulated G protein-coupled receptor kinase-2 (GRK2) present in injured myocardium. Deterioration of cardiac function is progressively enhanced by chronic adrenergic over-stimulation due to increased levels of circulating catecholamines. Increased GRK2 activity contributes to this pathological cycle of over-stimulation but lowered responsiveness. Over the past two decades the GRK2 inhibitory peptide betaARKct has been identified as a potential therapy that is able to break this vicious cycle of self-perpetuating deregulation of the beta-AR system and subsequent myocardial malfunction, thus halting development of cardiac failure. The betaARKct has been shown to interfere with GRK2 binding to the betagamma subunits of the heterotrimeric G protein, therefore inhibiting its recruitment to the plasma membrane that normally leads to phosphorylation and internalization of the receptor. In this article we summarize the current data on the therapeutic effects of betaARKct in cardiovascular disease and report on recent and ongoing studies that may pave the way for this peptide towards therapeutic application in heart failure and other states of cardiovascular disease.

Cell therapies for heart function recovery: focus on myocardial tissue engineering and nanotechnologies

Cell therapies have gained increasing interest and developed in several approaches related to the treatment of damaged myocardium. The results of multiple clinical trials have already been reported, almost exclusively involving the direct injection of stem cells. It has, however, been postulated that the efficiency of injected cells could possibly be hindered by the mechanical trauma due to the injection and their low survival in the hostile environment. It has indeed been demonstrated that cell mortality due to the injection approaches 90%. Major issues still need to be resolved and bed-to-bench followup is paramount to foster clinical implementations. The tissue engineering approach thus constitutes an attractive alternative since it provides the opportunity to deliver a large number of cells that are already organized in an extracellular matrix. Recent laboratory reports confirmed the interest of this approach and already encouraged a few groups to investigate it in clinical studies. We discuss current knowledge regarding engineered tissue for myocardial repair or replacement and in particular the recent implementation of nanotechnological approaches.

Acute myocardial infarction in early pregnancy: definition of myocardium at risk with noncontrast T2-weighted cardiac magnetic resonance

We report a case of a 34-year-old woman who had a left anterior wall myocardial infarction develop in the first trimester of pregnancy. Despite urgent and successful revascularization, she demonstrated persistent segmental wall motion abnormalities by transthoracic echocardiography. To manage this patient safely through pregnancy with a better definition of myocardium at risk, a cardiac magnetic resonance examination was performed. This identified a large territory of acutely edematous myocardium in addition to providing accurate volumetric measurements of left ventricular size and function. Because of her gravid state, gadolinium was not administered nor was it required to delineate the region of myocardium at risk.

Contractile function is preserved in unloaded hearts despite atrophic remodeling

OBJECTIVE: Recent studies have shown that mechanically unloading a failing heart may induce reverse remodeling and functional improvement. However, these benefits may be balanced by an unloading-related remodeling including myocardial atrophy that might lead to decrease in function. Using a model of heterotopic heart transplantation, we aimed to characterize the myocardial changes induced by long-term unloading. MATERIAL AND METHODS: Macroscopic as well as cellular and functional changes were followed in normal hearts unloaded for a 3-month period. Microscopic parameters were evaluated with stereologic methodology. Myocardial contractile function was quantified with a Langendorff isolated, perfused heart technique. RESULTS: Atrophy was macroscopically obvious and accompanied by a 67% reduction of the myocyte volume and a 43% reduction of the interstitial tissue volume, thus accounting for a shift of the myocyte/connective tissue ratio in favor of noncontractile tissue. The absolute number of cardiomyocyte nuclei decreased from 64.7 +/- 5.1 x 10(7) in controls to 22.6 +/- 3.7 x 10(7) (30 days) and 21.6 +/- 3.1 x 10(7) (90 days) after unloading (P < .05). The numeric nucleic density in the unloaded myocardium, as well as the mean cardiomyocyte volume per cardiomyocyte nucleus, remained constant throughout the 90 days of observation. Functional data indicated an increase in ventricular stiffness, although contractile function was preserved, as confirmed by unaltered maximal developed pressure and increased contractility (maximum rate of left ventricular pressure development) and relaxation (minimum rate of left ventricular pressure development). CONCLUSION: Atrophic remodeling involves both the myocyte and interstitial tissue compartment. These data suggest that although there is decreased myocardial volume and increased stiffness, contractile capacity is preserved in the long-term unloaded heart.

Plasma-functionalized electrospun matrix for biograft development and cardiac function stabilization.

Cardiac tissue engineering approaches can deliver large numbers of cells to the damaged myocardium and have thus increasingly been considered as a possible curative treatment to counteract the high prevalence of progressive heart failure after myocardial infarction (MI). Optimal scaffold architecture and mechanical and chemical properties, as well as immune- and bio-compatibility, need to be addressed. We demonstrated that radio-frequency plasma surface functionalized electrospun poly(ɛ-caprolactone) (PCL) fibres provide a suitable matrix for bone-marrow-derived mesenchymal stem cell (MSC) cardiac implantation. Using a rat model of chronic MI, we showed that MSC-seeded plasma-coated PCL grafts stabilized cardiac function and attenuated dilatation. Significant relative decreases of 13% of the ejection fraction (EF) and 15% of the fractional shortening (FS) were observed in sham treated animals; respective decreases of 20% and 25% were measured 4 weeks after acellular patch implantation, whereas a steadied function was observed 4 weeks after MSC-patch implantation (relative decreases of 6% for both EF and FS).

Essential role of β-adrenergic receptor kinase 1 in cardiac development and function

The β-adrenergic receptor kinase 1 (βARK1) is a member of the G protein-coupled receptor kinase (GRK) family that mediates the agonist-dependent phosphorylation and desensitization of G protein-coupled receptors. We have cloned and disrupted the βARK1 gene in mice by homologous recombination. No homozygote βARK1−/− embryos survive beyond gestational day 15.5. Prior to gestational day 15.5, βARK1−/− embryos display pronounced hypoplasia of the ventricular myocardium essentially identical to the “thin myocardium syndrome” observed upon gene inactivation of several transcription factors (RXRα, N-myc, TEF-1, WT-1). Lethality in βARK1−/− embryos is likely due to heart failure as they exhibit a >70% decrease in cardiac ejection fraction determined by direct in utero intravital microscopy. These results along with the virtual absence of endogenous GRK activity in βARK1−/− embryos demonstrate that βARK1 appears to be the predominant GRK in early embryogenesis and that it plays a fundamental role in cardiac development.

Neurally-mediated increase in calcineurin activity regulates cardiac contractile function in absence of hypertrophy

Objective: The calcineurin pathway has been involved in the development of cardiac hypertrophy, yet it remains unknown whether calcineurin activity can be regulated in myocardium independently from hypertrophy and cardiac load. Methods: To test that hypothesis, we measured calcineurin activity in a rat model of infrarenal aortic constriction (IR), which affects neurohormonal pathways without increasing cardiac afterload. Results: In this model, there was no change in arterial pressure over the 4-week experimental period, and the left ventricle/body weight ratio did not increase. At 2 weeks after IR, calcineurin activity was increased 1.8-fold (P

Effects of revascularisation and contractile reserve on left ventricular remodelling in patients with impaired left ventricular function

Objective: We sought to define the influence of revascularisation and contractile reserve on left ventricular (LV) remodelling in patients with LV dysfunction after myocardial infarction. Revascularisation of viable myocardium is associated with improved regional function, but the effect on remodelling is undefined. Methods: We studied 70 patients with coronary artery disease and LV dysfunction, 31 of whom underwent revascularisation. A standard dobutamine stress echocardiogram (DbE) was carried out. All patients underwent standard medical treatment; the decision to revascularise was made clinically, independent of this study. LV volumes and ejection fraction were measured by 3D echocardiography at baseline and after an average of 40 weeks. Results: There was no significant difference in baseline ejection fraction or volumes between patients who underwent revascularisation and the remainder. Compared to medically treated patients, revascularised patients had significant improvements in ejection fraction and end-systolic volume in follow-up. The impact of baseline variables on remodelling was assessed by dividing patients into tertiles of LV ejection fraction and volumes. Revascularised patients in the lowest tertile of ejection fraction at baseline (<38%) had a significant improvement in end-systolic volume and ejection fraction, larger than obtained in medically treated patients with low ejection fraction. Revascularised patients with an ejection fraction >38% did not show significant improvement in volumes compared to baseline. Revascularised patients in the largest tertiles of end-systolic (>88 ml) or end-diastolic volume (>149 ml) at baseline had a significant improvement in end-systolic volume. Conclusion: Remodeling appears to occur independent of the presence of regional contractile reserve but does correlate with the volume response to low-dose dobutamine. (C) 2003 Elsevier Ireland Ltd. All rights reserved.

Should we be evaluating the ventricle or the myocardium? Advances in tissue characterization

The assessment of left ventricular (LV) dysfunction has become the most frequent indication for echocardiography, a growth that has been driven by the epidemic of heart failure. The value of echocardiography for assessing LV dysfunction is unquestionable, the quantification of both LV systolic and diastolic dysfunction being a reliable indicator of mortality. 1,2 Nonetheless, whereas the ejection fraction and diastolic assessment are important clinical parameters, they are highly dependent on loading and may produce abnormal results under unusual loading conditions. Moreover, in a number of situations where the LV is evaluated, although the overall function is an important finding, the referring clinician is really requesting an assessment of the nature of the underlying myocardial tissue (Table 1). Indeed, in some situations (eg, among family members of patients with a cardiomyopathy) questions arise about the presence of pathology despite the presence of normal ventricular function. Traditionally, it has been difficult to obtain this information because of the lack of sufficiently sensitive parameters, but a number of new developments have shown such success in this area that the clinical application of tools to assess the myocardium in routine practice appears finally to be a realistic proposition.