Predicting left ventricular contractile function via Gaussian process emulation in aortic-banded rats.

Cardiac contraction is the result of integrated cellular, tissue and organ function. Biophysical in silico cardiac models offer a systematic approach for studying these multi-scale interactions. The computational cost of such models is high, due to their multi-parametric and nonlinear nature. This has so far made it difficult to perform model fitting and prevented global sensitivity analysis (GSA) studies. We propose a machine learning approach based on Gaussian process emulation of model simulations using probabilistic surrogate models, which enables model parameter inference via a Bayesian history matching (HM) technique and GSA on whole-organ mechanics. This framework is applied to model healthy and aortic-banded hypertensive rats, a commonly used animal model of heart failure disease. The obtained probabilistic surrogate models accurately predicted the left ventricular pump function (R2 = 0.92 for ejection fraction). The HM technique allowed us to fit both the control and diseased virtual bi-ventricular rat heart models to magnetic resonance imaging and literature data, with model outputs from the constrained parameter space falling within 2 SD of the respective experimental values. The GSA identified Troponin C and cross-bridge kinetics as key parameters in determining both systolic and diastolic ventricular function. This article is part of the theme issue 'Uncertainty quantification in cardiac and cardiovascular modelling and simulation'.

Cardiomyocyte substructure reverts to an immature phenotype during heart failure.

KEY POINTS: As reactivation of the fetal gene program has been implicated in pathological remodelling during heart failure (HF), we examined whether cardiomyocyte subcellular structure and function revert to an immature phenotype during this disease. Surface and internal membrane structures appeared gradually during development, and returned to a juvenile state during HF. Similarly, dyadic junctions between the cell membrane and sarcoplasmic reticulum were progressively 'packed' with L-type Ca2+ channels and ryanodine receptors during development, and 'unpacked' during HF. Despite similarities in subcellular structure, dyads were observed to be functional from early developmental stages, but exhibited an impaired ability to release Ca2+ in failing cardiomyocytes. Thus, while immature and failing cardiomyocytes share similarities in subcellular structure, these do not fully account for the marked impairment of Ca2+ homeostasis observed in HF. ABSTRACT: Reactivation of the fetal gene programme has been implicated as a driver of pathological cardiac remodelling. Here we examined whether pathological remodelling of cardiomyocyte substructure and function during heart failure (HF) reflects a reversion to an immature phenotype. Using scanning electron microscopy, we observed that Z-grooves and t-tubule openings at the cell surface appeared gradually during cardiac development, and disappeared during HF. Confocal and super-resolution imaging within the cell interior revealed similar structural parallels; disorganization of t-tubules in failing cells was strikingly reminiscent of the late stages of postnatal development, with fewer transverse elements and a high proportion of longitudinal tubules. Ryanodine receptors (RyRs) were observed to be laid down in advance of developing t-tubules and similarly 'orphaned' in HF, although RyR distribution along Z-lines was relatively sparse. Indeed, nanoscale imaging revealed coordinated packing of L-type Ca2+ channels and RyRs into dyadic junctions during development, and orderly unpacking during HF. These findings support a 'last in, first out' paradigm, as the latest stages of dyadic structural development are reversed during disease. Paired imaging of t-tubules and Ca2+ showed that the disorganized arrangement of dyads in immature and failing cells promoted desynchronized and slowed Ca2+ release in these two states. However, while developing cells exhibited efficient triggering of Ca2+ release at newly formed dyads, dyadic function was impaired in failing cells despite similar organization of Ca2+ handling proteins. Thus, pathologically deficient Ca2+ homeostasis during HF is only partly linked to the re-emergence of immature subcellular structure, and additionally reflects lost dyadic functionality.

Evidence for heterogeneous subsarcolemmal Na+ levels in rat ventricular myocytes.

The intracellular Na+ concentration ([Na+]) regulates cardiac contractility. Previous studies have suggested that subsarcolemmal [Na+] is higher than cytosolic [Na+] in cardiac myocytes, but this concept remains controversial. Here, we used electrophysiological experiments and mathematical modeling to test whether there are subsarcolemmal pools with different [Na+] and dynamics compared with the bulk cytosol in rat ventricular myocytes. A Na+ dependency curve for Na+-K+-ATPase (NKA) current was recorded with symmetrical Na+ solutions, i.e., the same [Na+] in the superfusate and internal solution. This curve was used to estimate [Na+] sensed by NKA in other experiments. Three experimental observations suggested that [Na+] is higher near NKA than in the bulk cytosol: 1) when extracellular [Na+] was high, [Na+] sensed by NKA was ~6 mM higher than the internal solution in quiescent cells; 2) long trains of Na+ channel activation almost doubled this gradient; compared with an even intracellular distribution of Na+, the increase of [Na+] sensed by NKA was 10 times higher than expected, suggesting a local Na+ domain; and 3) accumulation of Na+ near NKA after trains of Na+ channel activation dissipated very slowly. Finally, mathematical models assuming heterogeneity of [Na+] between NKA and the Na+ channel better reproduced experimental data than the homogeneous model. In conclusion, our data suggest that NKA-sensed [Na+] is higher than [Na+] in the bulk cytosol and that there are differential Na+ pools in the subsarcolemmal space, which could be important for cardiac contractility and arrhythmogenesis. NEW & NOTEWORTHY Our data suggest that the Na+-K+-ATPase-sensed Na+ concentration is higher than the Na+ concentration in the bulk cytosol and that there are differential Na+ pools in the subsarcolemmal space, which could be important for cardiac contractility and arrhythmogenesis. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/heterogeneous-sodium-in-ventricular-myocytes/ .

Overexpression of integrin α11 induces cardiac fibrosis in mice.

AIM: To understand the role of the collagen-binding integrin α11 in vivo, we have used a classical approach of creating a mouse strain overexpressing integrin α11. A transgenic mouse strain overexpressing α11 in muscle tissues was analysed in the current study with special reference to the heart tissue. METHODS: We generated and phenotyped integrin α11 transgenic (TG) mice by echocardiography, magnetic resonance imaging and histology. Wild-type (WT) mice were subjected to aortic banding (AB) and the expression of integrin α11 was measured in flow cytometry-sorted cardiomyocytes and non-myocytes. RESULTS: TG mice developed left ventricular concentric hypertrophy by 6 months, with increased collagen deposition and reactivation of mRNA encoding foetal genes associated with cardiovascular pathological remodelling compared to WT mice. Masson's trichrome staining revealed interstitial fibrosis, confirmed additionally by magnetic resonance imaging and was found to be most prominent in the cardiac septum of TG but not WT mice. TG hearts expressed increased levels of transforming growth factor-ß2 and transforming growth factor-ß3 and upregulated smooth muscle actin. Macrophage infiltration coincided with increased NF-κB signalling in TG but not WT hearts. Integrin α11 expression was increased in both cardiomyocytes and non-myocyte cells from WT AB hearts compared to sham-operated animals. CONCLUSION: We report for the first time that overexpression of integrin α11 induces cardiac fibrosis and left ventricular hypertrophy. This is a result of changes in intracellular hypertrophic signalling and secretion of soluble factors that increase collagen production in the heart.

Hypokalaemia induces Ca²âº overload and Ca²âº waves in ventricular myocytes by reducing Na⁺,K⁺-ATPase α2 activity.

KEY POINTS: Hypokalaemia is a risk factor for development of ventricular arrhythmias. In rat ventricular myocytes, low extracellular K(+) (corresponding to clinical moderate hypokalaemia) increased Ca(2+) wave probability, Ca(2+) transient amplitude, sarcoplasmic reticulum (SR) Ca(2+) load and induced SR Ca(2+) leak. Low extracellular K(+) reduced Na(+),K(+)-ATPase (NKA) activity and hyperpolarized the resting membrane potential in ventricular myocytes. Both experimental data and modelling indicate that reduced NKA activity and subsequent Na(+) accumulation sensed by the Na(+), Ca(2+) exchanger (NCX) lead to increased Ca(2+) transient amplitude despite concomitant hyperpolarization of the resting membrane potential. Low extracellular K(+) induced Ca(2+) overload by lowering NKA α2 activity. Triggered ventricular arrhythmias in patients with hypokalaemia may therefore be attributed to reduced NCX forward mode activity linked to an effect on the NKA α2 isoform. ABSTRACT: Hypokalaemia is a risk factor for development of ventricular arrhythmias. The aim of this study was to determine the cellular mechanisms leading to triggering of arrhythmias in ventricular myocytes exposed to low Ko. Low Ko, corresponding to moderate hypokalaemia, increased Ca(2+) transient amplitude, sarcoplasmic reticulum (SR) Ca(2+) load, SR Ca(2+) leak and Ca(2+) wave probability in field stimulated rat ventricular myocytes. The mechanisms leading to Ca(2+) overload were examined. Low Ko reduced Na(+),K(+)-ATPase (NKA) currents, increased cytosolic Na(+) concentration and increased the Na(+) level sensed by the Na(+), Ca(2+) exchanger (NCX). Low Ko also hyperpolarized the resting membrane potential (RMP) without significant alterations in action potential duration. Experiments in voltage clamped and field stimulated ventricular myocytes, along with mathematical modelling, suggested that low Ko increases the Ca(2+) transient amplitude by reducing NKA activity despite hyperpolarization of the RMP. Selective inhibition of the NKA α2 isoform by low dose ouabain abolished the ability of low Ko to reduce NKA currents, to increase Na(+) levels sensed by NCX and to increase the Ca(2+) transient amplitude. We conclude that low Ko, within the range of moderate hypokalaemia, increases Ca(2+) levels in ventricular myocytes by reducing the pumping rate of the NKA α2 isoform with subsequent Na(+) accumulation sensed by the NCX. These data highlight reduced NKA α2 -mediated control of NCX activity as a possible mechanism underlying triggered ventricular arrhythmias in patients with hypokalaemia.

IL-18 neutralization during alveolar hypoxia improves left ventricular diastolic function in mice.

AIM: In patients, an association exists between pulmonary diseases and diastolic dysfunction of the left ventricle (LV). We have previously shown that alveolar hypoxia in mice induces LV diastolic dysfunction and that mice exposed to hypoxia have increased levels of circulating interleukin-18 (IL-18), suggesting involvement of IL-18 in development of diastolic dysfunction. IL-18 binding protein (IL-18BP) is a natural inhibitor of IL-18. In this study, we hypothesized that neutralization of IL-18 during alveolar hypoxia would improve LV diastolic function. METHODS: Mice were exposed to 10% oxygen for 2 weeks while treated with IL-18BP or vehicle. Cardiac function and morphology were measured using echocardiography, intraventricular pressure measurements and magnetic resonance imaging (MRI). For characterization of molecular changes in the heart, both real-time PCR and Western blotting were performed. ELISA technique was used to measure levels of circulating cytokines. RESULTS: As expected, exposure to hypoxia-induced LV diastolic dysfunction, as shown by prolonged time constant of isovolumic relaxation (τ). Improved relaxation with IL-18BP treatment was demonstrated by a significant reduction towards control τ values. Decreased levels of phosphorylated phospholamban (P-PLB) in hypoxia, but normalization by IL-18BP treatment suggest a role for IL-18 in regulation of calcium-handling proteins in hypoxia-induced diastolic dysfunction. In addition, MRI showed less increase in right ventricular (RV) wall thickness in IL-18BP-treated animals exposed to hypoxia, indicating an effect on RV hypertrophy. CONCLUSION: Neutralization of IL-18 during alveolar hypoxia improves LV diastolic function and partly prevents RV hypertrophy.

Impaired left ventricular mechanical and energetic function in mice after cardiomyocyte-specific excision of Serca2.

Sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA)2 transports Ca2+ from the cytosol into the sarcoplasmic reticulum of cardiomyocytes and is essential for maintaining myocardial Ca2+ handling and thus the mechanical function of the heart. SERCA2 is a major ATP consumer in excitation-contraction coupling but is regarded to contribute to energetically efficient Ca2+ handling in the cardiomyocyte. Previous studies using cardiomyocyte-specific SERCA2 knockout (KO) mice have demonstrated that decreased SERCA2 activity reduces the Ca2+ transient amplitude and induces compensatory Ca2+ transport mechanisms that may lead to more inefficient Ca2+ transport. In this study, we examined the relationship between left ventricular (LV) function and myocardial O2 consumption (MVo2) in ex vivo hearts from SERCA2 KO mice to directly measure how SERCA2 elimination influences mechanical and energetic features of the heart. Ex vivo hearts from SERCA2 KO hearts developed mechanical dysfunction at 4 wk and demonstrated virtually no working capacity at 7 wk. In accordance with the reported reduction in Ca2+ transient amplitude in cardiomyocytes from SERCA2 KO mice, work-independent MVo2 was decreased due to a reduced energy cost of excitation-contraction coupling. As these hearts also showed a marked impairment in the efficiency of chemomechanical energy transduction (contractile efficiency, i.e, work-dependent MVo2), hearts from SERCA2 KO mice were found to be mechanically inefficient. This ex vivo evaluation of mechanical and energetic function in hearts from SERCA2 KO mice brings together findings from previous experimental and mathematical modeling-based studies and demonstrates that reduced SERCA2 activity not only leads to mechanical dysfunction but also to energetic dysfunction.

SERCA2 activity is involved in the CNP-mediated functional responses in failing rat myocardium.

BACKGROUND AND PURPOSES: Myocardial C-type natriuretic peptide (CNP) levels are increased in heart failure. CNP can induce negative inotropic (NIR) and positive lusitropic responses (LR) in normal hearts, but its effects in failing hearts are not known. We studied the mechanism of CNP-induced NIR and LR in failing hearts and determined whether sarcoplasmatic reticulum Ca(2+) ATPase2 (SERCA2) activity is essential for these responses. EXPERIMENTAL APPROACH: Contractility, cGMP levels, Ca(2+) transient amplitudes and protein phosphorylation were measured in left ventricular muscle strips or ventricular cardiomyocytes from failing hearts of Wistar rats 6 weeks after myocardial infarction. KEY RESULTS: CNP increased cGMP levels, evoked a NIR and LR in muscle strips, and caused phospholamban (PLB) Ser(16) and troponin I (TnI) Ser(23/24) phosphorylation in cardiomyocytes. Both the NIR and LR induced by CNP were reduced in the presence of a PKG blocker/cGMP analogue (Rp-8-Br-Pet-cGMPS) and the SERCA inhibitor thapsigargin. CNP increased the amplitude of the Ca(2+) transient and increased SERCA2 activity in cardiomyocytes. The CNP-elicited NIR and LR were not affected by the L-type Ca(2+) channel activator BAY-K8644, but were abolished in the presence of isoprenaline (induces maximal activation of cAMP pathway). This suggests that phosphorylation of PLB and TnI by CNP causes both a NIR and LR. The NIR to CNP in mouse heart was abolished 8 weeks after cardiomyocyte-specific inactivation of the SERCA2 gene. CONCLUSIONS AND IMPLICATIONS: We conclude that CNP-induced PLB and TnI phosphorylation by PKG in concert mediate both a predictable LR as well as the less expected NIR in failing hearts.

The functional activity of inhibitory G protein (G(i)) is not increased in failing heart ventricle.

Beta-adrenergic receptor (ßAR) inotropic effects are attenuated and muscarinic receptor-mediated inhibition thereof is enhanced in heart failure. We investigated if increased G(i) activity contributes to attenuated ßAR-inotropic effects and potentiates muscarinic accentuated antagonism in failing rat ventricle. Contractility was measured in ventricular strips and adenylyl cyclase (AC) activity in ventricular membranes from rats with post-infarction heart failure (HF) or Sham-operated controls (Sham). The maximal ßAR-mediated inotropic effect of isoproterenol was reduced by ~70% and basal, ßAR- & forskolin-stimulated AC activity was significantly lower in HF vs. Sham. Carbachol-evoked antagonism of the ßAR-mediated inotropic response was complete only in HF despite a ~40% reduction in the ability of carbachol to inhibit ßAR-stimulated AC. However, neither the relative efficacy (contractility decreased by ~46%) nor the potency of carbachol to inhibit the ßAR inotropic response differed between Sham and HF ventricle. Pertussis toxin (PTX) inactivation of G(i) did not increase the maximal ßAR inotropic effect or the attenuated basal, ßAR- & forskolin-stimulated AC activity in HF, but increased the potency of isoproterenol only in Sham (~0.5 log unit). In HF ventricle pretreated with PTX, simultaneous inhibition of phosphodiesterases 3,4 (PDE3,4) alone produced a larger inotropic response than isoproterenol in ventricle untreated with PTX (84% and 48% above basal respectively). In the absence of PTX, PDE3,4 inhibition evoked negligible inotropic effects in HF. These data are not consistent with the hypothesis that increased G(i) activity contributes to the reduced ßAR-mediated inotropic response and AC activity in failing ventricle. The data, however, support the hypothesis that G(i), through chronic receptor independent inhibition of AC, together with PDE3,4 activity, is necessary to maintain a low basal level of contractility.

ACE inhibition prevents diastolic Ca2+ overload and loss of myofilament Ca2+ sensitivity after myocardial infarction.

Prevention of adverse cardiac remodeling after myocardial infarction (MI) remains a therapeutic challenge. Angiotensin-converting enzyme inhibitors (ACE-I) are a well-established first-line treatment. ACE-I delay fibrosis, but little is known about their molecular effects on cardiomyocytes. We investigated the effects of the ACE-I delapril on cardiomyocytes in a mouse model of heart failure (HF) after MI. Mice were randomly assigned to three groups: Sham, MI, and MI-D (6 weeks of treatment with a non-hypotensive dose of delapril started 24h after MI). Echocardiography and pressure-volume loops revealed that MI induced hypertrophy and dilation, and altered both contraction and relaxation of the left ventricle. At the cellular level, MI cardiomyocytes exhibited reduced contraction, slowed relaxation, increased diastolic Ca2+ levels, decreased Ca2+-transient amplitude, and diminished Ca2+ sensitivity of myofilaments. In MI-D mice, however, both mortality and cardiac remodeling were decreased when compared to non-treated MI mice. Delapril maintained cardiomyocyte contraction and relaxation, prevented diastolic Ca2+ overload and retained the normal Ca2+ sensitivity of contractile proteins. Delapril maintained SERCA2a activity through normalization of P-PLB/PLB (for both Ser16- PLB and Thr17-PLB) and PLB/SERCA2a ratios in cardiomyocytes, favoring normal reuptake of Ca2+ in the sarcoplasmic reticulum. In addition, delapril prevented defective cTnI function by normalizing the expression of PKC, enhanced in MI mice. In conclusion, early therapy with delapril after MI preserved the normal contraction/relaxation cycle of surviving cardiomyocytes with multiple direct effects on key intracellular mechanisms contributing to preserve cardiac function.

Preserved metabolic reserve capacity in skeletal muscle of post-infarction heart failure patients.

It has been proposed that exercise capacity during whole body exercise in post-infarction congestive heart failure (CHF) patients is limited by skeletal muscle function. We therefore investigated the balance between cardiopulmonary and muscular metabolic capacity. CHF patients (n=8) and healthy subjects (HS, n=12) were included. Patients with coronary artery disease (CAD, n=8) were included as a control for medication. All subjects performed a stepwise incremental load test during bicycling (∼24 kg muscle mass), two-legged knee extensor (2-KE) exercise (∼4 kg muscle mass) and one-legged knee extensor (1-KE) exercise (∼2 kg muscle mass). Peak power and peak pulmonary oxygen uptake (VO(2peak) ) increased and muscle-specific VO(2peak) decreased with an increasing muscle mass involved in the exercise. Peak power and VO(2peak) were lower for CHF patients than HS, with values for CAD patients falling between CHF patients and HS. During bicycling, all groups utilized 24-29% of the muscle-specific VO(2peak) as measured during 1-KE exercise, with no difference between the groups. Hence, the muscle metabolic reserve capacity during whole body exercise is not different between CHF patients and HS, indicating that appropriately medicated and stable post-infarction CHF patients are not more limited by intrinsic skeletal muscle properties during whole body exercise than HS.

Intrinsic skeletal muscle alterations in chronic heart failure patients: a disease-specific myopathy or a result of deconditioning?

Chronic heart failure (CHF) patients frequently experience impaired exercise tolerance due to skeletal muscle fatigue. Studies suggest that this in part is due to intrinsic alterations in skeletal muscle of CHF patients, often interpreted as a disease-specific myopathy. Knowledge about the mechanisms underlying these skeletal muscle alterations is of importance for the pathophysiological understanding of CHF, therapeutic approach and rehabilitation strategies. We here critically review the evidence for skeletal muscle alterations in CHF, the underlying mechanisms of such alterations and how skeletal muscle responds to training in this patient group. Skeletal muscle characteristics in CHF patients are very similar to what is reported in response to chronic obstructive pulmonary disease (COPD), detraining and deconditioning. Furthermore, skeletal muscle alterations observed in CHF patients are reversible by training, and skeletal muscle of CHF patients seems to be at least as trainable as that of matched controls. We argue that deconditioning is a major contributor to the skeletal muscle dysfunction in CHF patients and that further research is needed to determine whether, and to what extent, the intrinsic skeletal muscle alterations in CHF represent an integral part of the pathophysiology in this disease.

A mouse model of reverse cardiac remodelling following banding-debanding of the ascending aorta.

AIM: Myocardial remodelling during pressure overload might contribute to development of heart failure. Reverse remodelling normally occurs following aortic valve replacement for aortic stenosis; however, the details and regulatory mechanisms of reverse remodelling remain unknown. Thus, an experimental model of reverse remodelling would allow for studies of this process. Although models of aortic banding are widely used, only few reports of debanding models exist. The aim of this study was to establish a banding-debanding model in the mouse with repetitive careful haemodynamic evaluation by high-resolution echocardiography. METHODS: C57Bl/6 mice were subjected to ascending aortic banding and subsequent debanding. Cardiac geometry and function were evaluated by echocardiography, and left ventricular myocardium was analysed by histology and quantitative real-time polymerase chain reaction. RESULTS: The degree of aortic banding was controlled by non-invasive estimation of the gradient, and we found a close correlation between left ventricular mass estimated by echocardiography and weight at the time of killing. Aortic banding led to left ventricular hypertrophy, fibrosis and expression of foetal genes, indicating myocardial remodelling. Echocardiography revealed concentric left ventricular remodelling and myocardial dysfunction. Following debanding, performed via a different incision, there was rapid regression of left ventricular weight and normalization of both cardiac geometry and function by 14 days. CONCLUSIONS: We have established a reproducible and carefully characterized mouse model of reverse remodelling by banding and debanding of the ascending aorta. Such a model might contribute to increased understanding of the reversibility of cardiac pathology, which in turn might give rise to new strategies in heart failure treatment.

Cyclic AMP-dependent inotropic effects are differentially regulated by muscarinic G(i)-dependent constitutive inhibition of adenylyl cyclase in failing rat ventricle.

BACKGROUND AND PURPOSE: ß-Adrenoceptor (ß-AR)-mediated inotropic effects are attenuated and G(i) proteins are up-regulated in heart failure (HF). Muscarinic receptors constitutively inhibit cAMP formation in normal rat cardiomyocytes. We determined whether constitutive activity of muscarinic receptors to inhibit adenylyl cyclase (AC) increases in HF and if so, whether it modifies the reduced ß-AR- or emergent 5-HT4-mediated cAMP-dependent inotropic effects. EXPERIMENTAL APPROACH: Contractility and AC activity were measured and related to each other in rat ventricle with post-infarction HF and sham-operated (Sham) controls with or without blockade of muscarinic receptors by atropine and inactivation of G(i) protein by pertussis toxin (PTX). KEY RESULTS: Isoprenaline-mediated inotropic effects were attenuated and basal, isoprenaline- and forskolin-stimulated AC activity was reduced in HF compared with Sham. Atropine or PTX pretreatment increased forskolin-stimulated AC activity in HF hearts. ß-AR-stimulated AC and maximal inotropic response were unaffected by atropine in Sham and HF. In HF, the potency of serotonin (5-HT) to evoke an inotropic response was increased in the presence of atropine with no change in the maximal inotropic response. Interestingly, PTX pretreatment reduced the potency of 5-HT to evoke inotropic responses while increasing the maximal inotropic response. CONCLUSIONS AND IMPLICATIONS: Although muscarinic constitutive inhibition of AC is increased in HF, it does not contribute to the reduced ß-AR-mediated inotropic effects in rat ventricle in HF. The data support the hypothesis that there are differences in the functional compartmentation of 5-HT4 and ß-AR AC signalling in myocardium during HF.

Activation of muscarinic receptors elicits inotropic responses in ventricular muscle from rats with heart failure through myosin light chain phosphorylation.

BACKGROUND AND PURPOSE: Muscarinic stimulation increases myofilament Ca(2+) sensitivity with no apparent inotropic response in normal rat myocardium. Increased myofilament Ca(2+) sensitivity is a molecular mechanism promoting increased contractility in failing cardiac tissue. Thus, muscarinic receptor activation could elicit inotropic responses in ventricular myocardium from rats with heart failure, through increasing phosphorylation of myosin light chain (MLC). EXPERIMENTAL APPROACH: Contractile force was measured in left ventricular papillary muscles from male Wistar rats, 6 weeks after left coronary artery ligation or sham surgery. Muscles were also frozen, and MLC-2 phosphorylation level was quantified. KEY RESULTS: Carbachol (10 micromol.L(-1)) evoked a positive inotropic response only in muscles from rats with heart failure approximating 36% of that elicited by 1 micromol.L(-1) isoproterenol (20 +/- 1.5% and 56 +/- 6.1% above basal respectively). Carbachol-evoked inotropic responses did not correlate with infarction size but did correlate with increased left ventricular end diastolic pressure, heart weight/body weight ratio and lung weight, primary indicators of the severity of heart failure. Only muscarinic receptor antagonists selective for M(2) receptors antagonized carbachol-mediated inotropic effects with the expected potency. Carbachol-evoked inotropic responses and increase in phosphorylated MLC-2 were attenuated by MLC kinase (ML-9) and Rho-kinase inhibition (Y-27632), and inotropic responses were abolished by Pertussis toxin pretreatment. CONCLUSION AND IMPLICATIONS: In failing ventricular muscle, muscarinic receptor activation, most likely via M(2) receptors, provides inotropic support by increasing MLC phosphorylation and consequently, myofilament Ca(2+) sensitivity. Enhancement of myofilament Ca(2+) sensitivity, representing a less energy-demanding mechanism of inotropic support may be particularly advantageous in failing hearts.

5-HT4-elicited positive inotropic response is mediated by cAMP and regulated by PDE3 in failing rat and human cardiac ventricles.

BACKGROUND AND PURPOSE: The left ventricle in failing hearts becomes sensitive to 5-HT parallelled by appearance of functional G(s)-coupled 5-HT(4) receptors. Here, we have explored the regulatory functions of phosphodiesterases in the 5-HT(4) receptor-mediated functional effects in ventricular muscle from failing rat and human heart. EXPERIMENTAL APPROACH: Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats. Contractility was measured in left ventricular papillary muscles of rat, 6 weeks after surgery and in left ventricular trabeculae from explanted human hearts. cAMP was quantified by RIA. KEY RESULTS: In papillary muscles from postinfarction rat hearts, 5-HT(4) stimulation exerted positive inotropic and lusitropic effects and increased cAMP. The inotropic effect was increased by non-selective PDE inhibition (IBMX, 10 microM) and selective inhibition of PDE3 (cilostamide, 1 microM), but not of PDE2 (EHNA, 10 microM) or PDE4 (rolipram, 10 microM). Combined PDE3 and PDE4 inhibition enhanced inotropic responses beyond the effect of PDE3 inhibition alone, increased the sensitivity to 5-HT, and also revealed an inotropic response in control (sham-operated) rat ventricle. Lusitropic effects were increased only during combined PDE inhibition. In failing human ventricle, the 5-HT(4) receptor-mediated positive inotropic response was regulated by PDEs in a manner similar to that in postinfarction rat hearts. CONCLUSIONS AND IMPLICATIONS: 5-HT(4) receptor-mediated positive inotropic responses in failing rat ventricle were cAMP-dependent. PDE3 was the main PDE regulating this response and involvement of PDE4 was disclosed by concomitant inhibition of PDE3 in both postinfarction rat and failing human hearts. 5-HT, PDE3 and PDE4 may have pathophysiological functions in heart failure.

Effects of treatment with a 5-HT4 receptor antagonist in heart failure.

BACKGROUND AND PURPOSE: Positive inotropic responses (PIR) to 5-hydroxytryptamine (5-HT) are induced in the left ventricle (LV) in rats with congestive heart failure (CHF); this is associated with upregulation of the G(s)-coupled 5-HT(4) receptor. We investigated whether chronic 5-HT(4) receptor blockade improved cardiac function in CHF rats. EXPERIMENTAL APPROACH: Rats were given either the 5-HT(4) antagonist SB207266 (0.5 mg kg(-1) 24h(-1); MI(int)) or placebo (MI(pl)) through mini-osmotic pumps for 6 weeks subsequent to induction of post-infarction CHF. In vivo cardiac function and ex vivo responses to isoprenaline or 5-HT were evaluated using echocardiography and isolated LV papillary muscles, respectively. mRNA levels were investigated using real-time quantitative RT-PCR. KEY RESULTS: LV diastolic function improved, with 4.6% lower LV diastolic diameter and 24.2% lower mitral flow deceleration in MI(int) compared to MI(pl). SB207266 reduced LV systolic diameter by 6.1%, heart weight by 10.2% and lung weight by 13.1%. The changes in posterior wall thickening and shortening velocity, cardiac output, LV systolic pressure and (dP/dt)(max), parameters of LV systolic function, did not reach statistical significance. The PIR to isoprenaline (10 microM) increased by 36% and the response to 5-HT (10 microM) decreased by 57% in MI(int) compared to MI(pl). mRNA levels for ANP, 5-HT(4(b)) and 5-HT(2A) receptors, MHCbeta, and the MHCbeta/MHCalpha -ratio were not significantly changed in MI(int) compared to MI(pl). CONCLUSIONS AND IMPLICATIONS: Treatment with SB207266 to some extent improved in vivo cardiac function and ex vivo myocardial function, suggesting a possible beneficial effect of treatment with a 5-HT(4) receptor antagonist in CHF.

Temperature-dependent skeletal muscle dysfunction in rats with congestive heart failure.

Abnormalities in the excitation-contraction coupling of slow-twitch muscle seem to explain the slowing and increased fatigue observed in congestive heart failure (CHF). However, it is not known which elements of the excitation-contraction coupling might be affected. We hypothesize that the temperature sensitivity of contractile properties of the soleus muscle might be altered in CHF possibly because of alterations of the temperature sensitivity of intracellular Ca(2+) handling. We electrically stimulated the in situ soleus muscle of anesthetised rats that had 6-wk postinfarction CHF using 1 and 50 Hz and using a fatigue protocol (5-Hz stimulation for 30 min) at 35, 37, and 40 degrees C. Ca(2+) uptake and release were measured in sarcoplasmic reticulum vesicles at various temperatures. Contraction and relaxation rates of the soleus muscle were slower in CHF than in sham at 35 degrees C, but the difference was almost absent at 40 degrees C. The fatigue protocol revealed that force development was more temperature sensitive in CHF, whereas contraction and relaxation rates were less temperature sensitive in CHF than in sham. The Ca(2+) uptake and release rates did not correlate to the difference between CHF and sham regarding contractile properties or temperature sensitivity. In conclusion, the discrepant results regarding altered temperature sensitivity of contraction and relaxation rates in the soleus muscle of CHF rats compared with Ca(2+) release and uptake rates in vesicles indicate that the molecular cause of slow-twitch muscle dysfunction in CHF is not linked to the intracellular Ca(2+) cycling.

Defective excitation-contraction coupling in hearts of rats with congestive heart failure.

AIM: We examined the cellular basis for depressed cardiac contractility in rats with congestive heart failure (CHF) secondary to myocardial infarction. METHODS: Six weeks after ligation of the left coronary artery, CHF was confirmed by haemodynamic measures and echocardiographic demonstration of reduced myocardial contractility in vivo. Papillary muscles from CHF animals developed less force than those from sham operated (SHAM) animals. Cell shortening was measured in isolated ventricular myocytes voltage-clamped with high resistance electrodes. Ca2+ transients were measured in fluo-4 loaded myocytes. RESULTS: Contractions triggered by depolarizing test steps from a post conditioning potential of -70 mV were significantly smaller and had significantly reduced velocity of shortening in CHF compared with SHAM myocytes. However, contractions initiated from -40 mV, were similar in amplitude and velocity of shortening in CHF and SHAM cells. L-type Ca2+ current was not significantly different between CHF and SHAM cells, whether activated from -70 or -40 mV. Therefore, in SHAM cells, excitation-contraction coupling exhibited higher gain when contractions were initiated from negative (-70 mV), as compared with depolarized potentials (-40 mV). However, in CHF myocytes, excitation-contraction coupling gain was selectively depressed with steps from -70 mV. This depression of gain in CHF was not accompanied by a significant reduction in sarcoplasmic reticulum Ca2+ content. Isoproterenol increased Ca2+ transients less in CHF than SHAM myocytes. CONCLUSION: In this post-infarction model of CHF, the contractile deficit was voltage dependent and the gain of excitation-contraction coupling was selectively depressed for contractions initiated negative to -40 mV.

Pentobarbital versus medetomidine-ketamine-fentanyl anaesthesia: effects on haemodynamics and the incidence of ischaemia-induced ventricular fibrillation in swine.

The present study was performed to compare haemodynamic variables at baseline and the incidence of ventricular fibrillation during the early phase of ischaemia in swine during pentobarbital or medetomidine-ketamine-fentanyl anaesthesia. Twenty-two swine (mean +/- SD: 29+/- 3 kg) were anaesthetized with sodium pentobarbital (induction with 36 mg/kg intraperitoneally, and maintenance with 5-20 mg/kg/h intravenously [i.v.]) and 6 swine (27+/- 3 kg) were anaesthetized with ketamine and fentanyl (premedicated with medetomidine 0.1 mg/kg and ketamine 10 mg/kg intramuscularly, induction with ketamine 20 mg/kg and fentanyl 0.025 mg/kg i.v., and maintenance with ketamine 20 mg/kg/h and fentanyl 0.025 mg/kg/h i.v.). After a stabilization period of 30 min, the left anterior descending coronary artery (LAD) was occluded for 10 min. Haemodynamic data and occurrence of ventricular fibrillation were recorded. The ischaemic area was measured by fluorescing microspheres. Swine anaesthetized with medetomidine-ketamine-fentanyl had significantly lower heart rate, myocardial contractility, peak left ventricular pressure, arterial blood pressure, aortic blood flow, myocardial blood flow and cardiac index at baseline, than swine anaesthetized with pentobarbital. Whereas none of the swine anaesthetized with pentobarbital fibrillated during the LAD occlusion, ventricular fibrillation occurred in 83% of the animals anaesthetized with medetomidine-ketamine-fentanyl (P< 0.001). No significant difference was found in size of ischaemic area between the two groups. Thus, we show a depression in haemodynamic variables at baseline and a higher incidence of ventricular fibrillation during the early phase of ischaemia in swine anaesthetized with medetomidine-ketamine-fentanyl compared to swine anaesthetized with pentobarbital.