Effect of inotropic interventions on contraction and Ca2+ transients in the human heart

A detailed mathematical model of the human atrial cardiomyocyte: integration of electrophysiology and cardiomechanics

Mechano-electric regulations (MER) play an important role in the maintenance of cardiac performance. Mechano-calcium and mechano-electric feedback (MCF and MEF) pathways adjust the cardiomyocyte contractile force according to mechanical perturbations and affects electro-mechanical coupling. MER integrates all these regulations in one unit resulting in a complex phenomenon. Computational modelling is a useful tool to accelerate the mechanistic understanding of complex experimental phenomena. We have developed a novel model that integrates the MER loop for human atrial cardiomyocytes with proper consideration of feedforward and feedback pathways. The model couples a modified version of the action potential (AP) Koivumäki model with the contraction model by Quarteroni group. The model simulates iso-sarcometric and isometric twitches and the feedback effects on AP and Ca2+ -handling. The model showed a biphasic response of Ca2+ transient (CaT) peak to increasing pacing rates and highlights the possible mechanisms involved. The model has shown a shift of the threshold for AP and CaT alternans from 4.6 to 4 Hz under post-operative atrial fibrillation, induced by depressed SERCA activity. The alternans incidence was dependent on a chain of mechanisms including RyRs availability time, MCF coupling, CaMKII phosphorylation, and the stretch levels. As a result, the model predicted a 10% slowdown of conduction velocity for a 20% stretch, suggesting a role of stretch in creation of substrate formation for atrial fibrillation. Overall, we conclude that the developed model provides a physiological CaT followed by a physiological twitch. This model can open pathways for the future studies of human atrial electromechanics. KEY POINTS: With the availability of human atrial cellular data, interest in atrial-specific model integration has been enhanced. We have developed a detailed mathematical model of human atrial cardiomyocytes including the mechano-electric regulatory loop. The model has gone through calibration and evaluation phases against a wide collection of available human in-vitro data. The usefulness of the model for analysing clinical problems has been preliminaryly tested by simulating the increased incidence of Ca2+ transient and action potential alternans at high rates in post-operative atrial fibrillation condition. The model determines the possible role of mechano-electric feedback in alternans incidence, which can increase vulnerability to atrial arrhythmias by varying stretch levels. We found that our physiologically accurate description of Ca2+ handling can reproduce many experimental phenomena and can help to gain insights into the underlying pathophysiological mechanisms.

Small molecule drugs to improve sarcomere function in those with acquired and inherited myopathies

Muscle weakness is a hallmark of inherited or acquired myopathies. It is a major cause of functional impairment and can advance to life-threatening respiratory insufficiency. During the past decade, several small-molecule drugs that improve the contractility of skeletal muscle fibers have been developed. In this review, we provide an overview of the available literature and the mechanisms of action of small-molecule drugs that modulate the contractility of sarcomeres, the smallest contractile units in striated muscle, by acting on myosin and troponin. We also discuss their use in the treatment of skeletal myopathies. The first of three classes of drugs discussed here increase contractility by decreasing the dissociation rate of calcium from troponin and thereby sensitizing the muscle to calcium. The second two classes of drugs directly act on myosin and stimulate or inhibit the kinetics of myosin-actin interactions, which may be useful in patients with muscle weakness or stiffness.NEW & NOTEWORTHY During the past decade, several small molecule drugs that improve the contractility of skeletal muscle fibers have been developed. In this review, we provide an overview of the available literature and the mechanisms of action of small molecule drugs that modulate the contractility of sarcomeres, the smallest contractile units in striated muscle, by acting on myosin and troponin.

Cell to whole organ global sensitivity analysis on a four-chamber heart electromechanics model using Gaussian processes emulators

Cardiac pump function arises from a series of highly orchestrated events across multiple scales. Computational electromechanics can encode these events in physics-constrained models. However, the large number of parameters in these models has made the systematic study of the link between cellular, tissue, and organ scale parameters to whole heart physiology challenging. A patient-specific anatomical heart model, or digital twin, was created. Cellular ionic dynamics and contraction were simulated with the Courtemanche-Land and the ToR-ORd-Land models for the atria and the ventricles, respectively. Whole heart contraction was coupled with the circulatory system, simulated with CircAdapt, while accounting for the effect of the pericardium on cardiac motion. The four-chamber electromechanics framework resulted in 117 parameters of interest. The model was broken into five hierarchical sub-models: tissue electrophysiology, ToR-ORd-Land model, Courtemanche-Land model, passive mechanics and CircAdapt. For each sub-model, we trained Gaussian processes emulators (GPEs) that were then used to perform a global sensitivity analysis (GSA) to retain parameters explaining 90% of the total sensitivity for subsequent analysis. We identified 45 out of 117 parameters that were important for whole heart function. We performed a GSA over these 45 parameters and identified the systemic and pulmonary peripheral resistance as being critical parameters for a wide range of volumetric and hemodynamic cardiac indexes across all four chambers. We have shown that GPEs provide a robust method for mapping between cellular properties and clinical measurements. This could be applied to identify parameters that can be calibrated in patient-specific models or digital twins, and to link cellular function to clinical indexes.

Long-Term Cultivation of Human Atrial Myocardium

Organotypic culture of human ventricular myocardium is emerging in basic and translational cardiac research. However, few institutions have access to human ventricular tissue, whereas atrial tissue is more commonly available and important for studying atrial physiology. This study presents a method for long-term cultivation of beating human atrial myocardium. After written informed consent, tissues from the right-atrial appendage were obtained from patients with sinus rhythm undergoing open heart surgery with cardiopulmonary bypass. Trabeculae (pectinate muscles) prepared from the samples were installed into cultivation chambers at 37°C with a diastolic preload of 500 μN. After 2 days with 0.5 Hz pacing, stimulation frequency was set to 1 Hz. Contractile force was monitored continuously. Beta-adrenergic response, refractory period (RP) and maximum captured frequency (f_max) were assessed periodically. After cultivation, viability and electromechanical function were investigated, as well as the expression of several genes important for intracellular Ca²⁺ cycling and electrophysiology. Tissue microstructure was analyzed by confocal microscopy. We cultivated 19 constantly beating trabeculae from 8 patient samples for 12 days and 4 trabeculae from 3 specimen for 21 days. Functional parameters were compared directly after installation (0 d) with those after 12 d in culture. Contraction force was 384 ± 69 μN at 0 d and 255 ± 90 μN at 12 d (p = 0.8, n = 22), RP 480 ± 97 ms and 408 ± 78 ms (p = 0.3, n = 9), f_max 3.0 ± 0.5 Hz and 3.8 ± 0.5 Hz (p = 0.18, n = 9), respectively. Application of 100 nM isoprenaline to 11 trabeculae at 7 d increased contraction force from 168 ± 35 μN to 361 ± 60 μN (p < 0.01), f_max from 6.4 ± 0.6 Hz to 8.5 ± 0.4 Hz (p < 0.01) and lowered RP from 319 ± 22 ms to 223 ± 15 ms. CACNA1c (L-type Ca²⁺ channel subunit) and GJA1 (connexin-43) mRNA expressions were not significantly altered at 12 d vs 0 d, while ATP2A (SERCA) and KCNJ4 (Kir2.3) were downregulated, and KCNJ2 (Kir2.1) was upregulated. Simultaneous Ca²⁺ imaging and force recording showed preserved excitation-contraction coupling in cultivated trabeculae. Confocal microscopy indicated preserved cardiomyocyte structure, unaltered amounts of extracellular matrix and gap junctions. MTT assays confirmed viability at 12 d. We established a workflow that allows for stable cultivation and functional analysis of beating human atrial myocardium for up to 3 weeks. This method may lead to novel insights into the physiology and pathophysiology of human atrial myocardium.

Electro-Mechanical Whole-Heart Digital Twins: A Fully Coupled Multi-Physics Approach

Mathematical models of the human heart are evolving to become a cornerstone of precision medicine and support clinical decision making by providing a powerful tool to understand the mechanisms underlying pathophysiological conditions. In this study, we present a detailed mathematical description of a fully coupled multi-scale model of the human heart, including electrophysiology, mechanics, and a closed-loop model of circulation. State-of-the-art models based on human physiology are used to describe membrane kinetics, excitation-contraction coupling and active tension generation in the atria and the ventricles. Furthermore, we highlight ways to adapt this framework to patient specific measurements to build digital twins. The validity of the model is demonstrated through simulations on a personalized whole heart geometry based on magnetic resonance imaging data of a healthy volunteer. Additionally, the fully coupled model was employed to evaluate the effects of a typical atrial ablation scar on the cardiovascular system. With this work, we provide an adaptable multi-scale model that allows a comprehensive personalization from ion channels to the organ level enabling digital twin modeling.

Influence of atrial contraction dynamics on cardiac function

In recent years, there has been a move from monoventricular or biventricular models of the heart, to more complex models that incorporate the electromechanical function in all 4 chambers. However, the biophysical foundation is still underdeveloped, with most work in atrial cellular models having focused on electrophysiological properties. Here, we present a biophysical model of human atrial contraction at body temperature and use it to study the effects of atrial contraction on whole organ function and a study of the effects of remodelling due to atrial fibrillation on atrial and ventricular function.

Impact of sarcoplasmic reticulum calcium release on calcium dynamics and action potential morphology in human atrial myocytes: a computational study

Electrophysiological studies of the human heart face the fundamental challenge that experimental data can be acquired only from patients with underlying heart disease. Regarding human atria, there exist sizable gaps in the understanding of the functional role of cellular Ca²+ dynamics, which differ crucially from that of ventricular cells, in the modulation of excitation-contraction coupling. Accordingly, the objective of this study was to develop a mathematical model of the human atrial myocyte that, in addition to the sarcolemmal (SL) ion currents, accounts for the heterogeneity of intracellular Ca²+ dynamics emerging from a structurally detailed sarcoplasmic reticulum (SR). Based on the simulation results, our model convincingly reproduces the principal characteristics of Ca²+ dynamics: 1) the biphasic increment during the upstroke of the Ca²+ transient resulting from the delay between the peripheral and central SR Ca²+ release, and 2) the relative contribution of SL Ca²+ current and SR Ca²+ release to the Ca²+ transient. In line with experimental findings, the model also replicates the strong impact of intracellular Ca²+ dynamics on the shape of the action potential. The simulation results suggest that the peripheral SR Ca²+ release sites define the interface between Ca²+ and AP, whereas the central release sites are important for the fire-diffuse-fire propagation of Ca²+ diffusion. Furthermore, our analysis predicts that the modulation of the action potential duration due to increasing heart rate is largely mediated by changes in the intracellular Na+ concentration. Finally, the results indicate that the SR Ca²+ release is a strong modulator of AP duration and, consequently, myocyte refractoriness/excitability. We conclude that the developed model is robust and reproduces many fundamental aspects of the tight coupling between SL ion currents and intracellular Ca²+ signaling. Thus, the model provides a useful framework for future studies of excitation-contraction coupling in human atrial myocytes.

Involvement of sarcoplasmic reticulum in changing intracellular calcium due to Na+/K+-ATPase inhibition in cardiomyocytes

Earlier studies have demonstrated that ouabain-induced increase in [Ca2+]i, as a consequence of sarcolemma (SL) Na+/K+-ATPase inhibition, is associated with activation of both the SL Na+/Ca2+ exchanger and SL Ca2+ channels. In view of the importance of sarcoplasmic reticulum (SR) in the regulation of [Ca2+]i, this study examined the role of SR in ouabain-induced increase in [Ca2+]i in both quiescent and KCl-depolarized cardiomyocytes. For this purpose, adult rat cardiomyocytes were loaded with fura-2 and ouabain-induced changes in [Ca2+]i were monitored upon treatment with or without different agents that are known to influence Ca2+ handling by the intracellular organelles. Ouabain not only increased the basal [Ca2+]i and augmented KCl-induced increase in [Ca2+]i but also produced similar effects on the ATP-induced increase in [Ca2+]i. Treatments of cardiomyocytes with caffeine, ryanodine, or cyclopiazonic acid, which affect SR Ca2+ stores, attenuated the ouabain-induced increase in basal Ca2+ as well as augmentation of the KCl response. Both ryanodine and cyclopiazonic acid produced additional effects, when used in combination with a SL Ca2+ channel inhibitor (verapamil), but not with a Na+/Ca2+ exchange inhibitor (KB-R7943). Inhibitors of Ca2+/calmodulin kinase, protein kinase A, and inositol-3-phosphate receptors were also observed to depress the ouabain-induced increase in [Ca2+]i in cardiomyocytes. On the other hand, mitochondrial Ca2+ transport inhibitors did not exert any effect on the ouabain-induced alterations in [Ca2+]i in cardiomyocytes. Furthermore, ouabain did not show any direct effect on the Ca2+ uptake and Ca2+ release activities of SR or mitochondria. These results suggest an indirect involvement of SR Ca2+ stores in the ouabain-induced increase in [Ca2+]i in cardiomyocytes and indicate the participation of both Ca2+-induced Ca2+ release and regulatory mechanisms in this action.

Impact of ionic current variability on human ventricular cellular electrophysiology

Abnormalities in repolarization and its rate dependence are known to be related to increased proarrhythmic risk. A number of repolarization-related electrophysiological properties are commonly used as preclinical biomarkers of arrhythmic risk. However, the variability and complexity of repolarization mechanisms make the use of cellular biomarkers to predict arrhythmic risk preclinically challenging. Our goal is to investigate the role of ionic current properties and their variability in modulating cellular biomarkers of arrhythmic risk to improve risk stratification and identification in humans. A systematic investigation into the sensitivity of the main preclinical biomarkers of arrhythmic risk to changes in ionic current conductances and kinetics was performed using computer simulations. Four stimulation protocols were applied to the ten Tusscher and Panfilov human ventricular model to quantify the impact of +/-15 and +/-30% variations in key model parameters on action potential (AP) properties, Ca(2+) and Na(+) dynamics, and their rate dependence. Simulations show that, in humans, AP duration is moderately sensitive to changes in all repolarization current conductances and in L-type Ca(2+) current (I(CaL)) and slow component of the delayed rectifier current (I(Ks)) inactivation kinetics. AP triangulation, however, is strongly dependent only on inward rectifier K(+) current (I(K1)) and delayed rectifier current (I(Kr)) conductances. Furthermore, AP rate dependence (i.e., AP duration rate adaptation and restitution properties) and intracellular Ca(2+) and Na(+) levels are highly sensitive to both I(CaL) and Na(+)/K(+) pump current (I(NaK)) properties. This study provides quantitative insights into the sensitivity of preclinical biomarkers of arrhythmic risk to variations in ionic current properties in humans. The results show the importance of sensitivity analysis as a powerful method for the in-depth validation of mathematical models in cardiac electrophysiology.

Sarcolemmal cation channels and exchangers modify the increase in intracellular calcium in cardiomyocytes on inhibiting Na+-K+-ATPase

Although inhibition of the sarcolemmal (SL) Na+-K+-ATPase is known to cause an increase in the intracellular concentration of Ca2+([Ca2+]i) by stimulating the SL Na+/Ca2+exchanger (NCX), the involvement of other SL sites in inducing this increase in [Ca2+]iis not fully understood. Isolated rat cardiomyocytes were treated with or without different agents that modify Ca2+movements by affecting various SL sites and were then exposed to ouabain. Ouabain was observed to increase the basal levels of both [Ca2+]iand intracellular Na+concentration ([Na+]i) as well as to augment the KCl-induced increases in both [Ca2+]iand [Na+]iin a concentration-dependent manner. The ouabain-induced changes in [Na+]iand [Ca2+]iwere attenuated by treatment with inhibitors of SL Na+/H+exchanger and SL Na+channels. Both the ouabain-induced increase in basal [Ca2+]iand augmentation of the KCl response were markedly decreased when cardiomyocytes were exposed to 0–10 mM Na+. Inhibitors of SL NCX depressed but decreasing extracellular Na+from 105–35 mM augmented the ouabain-induced increase in basal [Ca2+]iand the KCl response. Not only was the increase in [Ca2+]iby ouabain dependent on the extracellular Ca2+concentration, but it was also attenuated by inhibitors of SL L-type Ca2+channels and store-operated Ca2+channels (SOC). Unlike the SL L-type Ca2+-channel blocker, the blockers of SL Na+channel and SL SOC, when used in combination with SL NCX inhibitor, showed additive effects in reducing the ouabain-induced increase in basal [Ca2+]i. These results support the view that in addition to SL NCX, SL L-type Ca2+channels and SL SOC may be involved in raising [Ca2+]ion inhibition of the SL Na+-K+-ATPase by ouabain. Furthermore, both SL Na+/H+exchanger and Na+channels play a critical role in the ouabain-induced Ca2+increase in cardiomyocytes.

Ca2+-sensitisers--a promising option to treat heart failure?

Because the number of transplants is still fewer than the number of patients waiting for a donor organ, new concepts of therapy are needed that allow patients to bridge the time gap until heart transplantation or even to improve symptoms while on treatment. Ca(2+)-sensitisers are agents that directly influence myofilaments and/or the cross-bridge-cycle. Depending on the molecular mechanisms underlying their action, Ca(2+)-sensitisers have been divided into three classes. While, a number of Ca(2+)-sensitising drugs have been described, currently only the Ca(2+)-sensitisers pimobendan and levosimendan are in clinical use. This review provides a survey on the molecular mechanisms and the therapeutic effectiveness of Ca(2+)-sensitisers for the treatment of human heart failure.

Alterations of tension-dependent ATP utilization in a transgenic rat model of hypertrophic cardiomyopathy

Although it is established that familial hypertrophic cardiomyopathy (FHC) is caused by mutations in several sarcomeric proteins, including cardiac troponin T (TnT), its pathogenesis is still not completely understood. Previously, we established a transgenic rat model of FHC expressing a human TnT molecule with a truncation mutation (DEL-TnT). This study investigated whether contractile dysfunction and electrical vulnerability observed in DEL-TnT rats might be due to alterations of intracellular Ca(2+) homeostasis, myofibrillar Ca(2+) sensitivity, and/or myofibrillar ATP utilization. Simultaneous measurements of the force of contraction and intracellular Ca(2+) transients were performed in right ventricular trabeculae of DEL-TnT hearts at 0.25 and 1.0 Hz. Rats expressing wild-type human TnT as well as nontransgenic rats served as controls. In addition, calcium-dependent ATPase activity and tension development were investigated in skinned cardiac muscle fibers. Force of contraction was significantly decreased in DEL-TnT compared with nontransgenic rats and TnT. Time parameters of Ca(2+) transients were unchanged at 0.25 Hz but prolonged at 1.0 Hz in DEL-TnT. The amplitude of the fura-2 transient was similar in all groups investigated, whereas diastolic and systolic fura-2 ratios were found elevated in rats expressing nontruncated human troponin T. In DEL-TnT rats, myofibrillar Ca(2+)-dependent tension development as well as Ca(2+) sensitivity of tension were significantly decreased, whereas tension-dependent ATP consumption ("tension cost") was markedly increased. Thus, a C-terminal truncation of the cardiac TnT molecule impairs the force-generating capacity of the cycling cross-bridges resulting in increased tension-dependent ATP utilization. Taken together, our data support the hypothesis of energy compromise as a contributing factor in the pathogenesis of FHC.

In Vitro Evidence That Carteolol Is a Nonconventional Partial Agonist of Guinea Pig Cardiac β1-Adrenoceptors: A Comparison with Xamoterol

The present study was designed to verify our previous hypothesis that carteolol, a beta1/beta2-adrenoceptor-blocking agent, is a nonconventional partial agonist of cardiac beta1-adrenoceptors. To this purpose, we characterized the effects of carteolol in guinea pig myocardial preparations and measured the affinities of carteolol for high- and low-affinity sites of beta1-adrenoceptors labeled by CGP12177 [(-)4-(3-t-butylamino-2-hydroxypropoxy)-2-benzimidazol-2-one]. All experiments were performed in comparison with xamoterol, a cardioselective beta1-adrenoceptor partial agonist. Both drugs caused cAMP-dependent positive inotropic and chronotropic effects, but carteolol was less effective and less potent than xamoterol, and its cardiac actions were not affected by conventional concentrations of the beta-blocker propranolol. Both carteolol and xamoterol antagonized the cardiac effects of isoprenaline, but although the antagonistic concentrations of xamoterol were almost equal to those producing cardiostimulation, the antagonistic concentrations of carteolol were 3 log units lower than those causing cardiostimulant effects. Both carteolol and xamoterol competed with (-)[3H]CGP12177 for a high-affinity site of beta1-adrenoceptors, but carteolol showed a higher affinity than xamoterol. Moreover, carteolol, unlike xamoterol, bound also to a low-affinity site of the receptors. The binding affinity constants of the drugs for the high-affinity site correlated well with the respective blocking potencies against isoprenaline, whereas the affinity constant of carteolol for the low-affinity site was well related to its agonist potency. In conclusion, our findings demonstrate that carteolol, unlike xamoterol, is a nonconventional partial agonist, which causes agonistic effects through interaction with the low-affinity propranolol-resistant site of beta1-adrenoceptors and antagonistic actions through the high-affinity site of the same receptors.

Altered hetero- and homeometric autoregulation in the terminally failing human heart

OBJECTIVE AND METHODS

To further investigate length-dependent force generation in human heart, nonfailing (donor hearts, NF) and terminally failing (heart transplants, dilated cardiomyopathy, DCM) left ventricular myocardium was studied under various preload (4-40 mN/mm2) or length conditions. In addition, morphological studies (van Giesson Trichrome staining, electron microscopy) were performed.

RESULTS

In NF, a biphasic increase in force of contraction (FOC) was observed after elevating the preload (4-40 mN/mm2): there was an immediate fast increase (FOCf,), followed by a slow increase over several minutes (FOCs), which was paralleled by an increase in the systolic fura-2 transient. In DCM, FOCf, FOCs and the systolic fura-2 transient were blunted and diastolic tension was increased at increasing muscle length. Only in NF, a stretched induced increase in diastolic fura-2 ratio was observed. In DCM, no obvious interstitial fibrosis and no difference in basement membrane structure and attachment were observed.

CONCLUSIONS

Since FOCf has been attributed to the Frank-Starling mechanism, whereas FOCs represents a length-dependent increase in the intracellular Ca2+-transient, the impaired length-dependent force generation in failing myocardium results from a dysregulation of both myofibrillar Ca2+-sensitivity as well as the intracellular Ca2+-homeostasis. Interstitial fibrosis may have only minor impact on force generation in human end-stage heart failure.

Characterization of intrinsic sympathomimetic activity of carteolol in rat cardiovascular preparations

We evaluated in vitro, in myocardial and vascular preparations isolated from reserpine-treated rats, the intrinsic sympathomimetic activity (ISA) of carteolol, a beta(1)/beta(2)-adrenoceptor blocking agent used in cardiovascular and non-cardiovascular diseases. In spontaneously beating atria, carteolol, at low concentrations (0.01 and 0.1 microM), antagonized the positive inotropic effect of isoprenaline, whereas at higher concentrations (1 microM to 1 mM), it caused an increase in the force of contraction (EC(50): 4.6 +/- 0.1 microM, E(max): 17.1 +/- 1.1%, with respect to the maximum isoprenaline response) and a slight increase (7.8 +/- 1.9% over basal values) in the heart rate. The positive inotropic effect of carteolol was abolished by concentrations of propranolol or timolol (10 microM) much higher than those blocking isoprenaline effects in the same preparations. A similar positive inotropic effect was also observed in electrically driven left atrium and in Langendorff perfused hearts. Functional and biochemical evidences supported the involvement of cAMP in the cardiac action of carteolol. In peripheral arteries (femoral and tail) pre-contracted with phenylephrine, carteolol exerted ISA-related relaxing effects, independent of the presence of endothelium and sensitive to high concentrations (10 microM) of conventional beta-blockers. On the basis of these results, we propose to categorize carteolol as a non-conventional partial agonist of both cardiac and vascular beta-adrenoceptors.

Ouabain increases myofibrillar Ca 2+ sensitivity but does not influence the Ca 2+ release in human skinned fibres

The present study investigated the impact of the Na(+) pump inhibitor ouabain (g-strophanthin) on Ca(2+) sensitivity and Ca(2+) release in human right auricular trabeculae (coronary bypass) and in skinned muscle fibres from left ventricular myocardium (cardiac transplantation, dilated cardiomyopathy). A time-dependent increase in force of contraction was observed in right auricular trabeculae in response to ouabain (100 nM) before the intracellular Ca(2+) transient (fura-2) increased (n=6). In triton X-skinned fibres (no sarcoplasmic reticulum) of human failing myocardium, ouabain (1-100 nM) concentration-dependently increased tension at a free extracellular Ca(2+) concentration of 1 microM and the Hill coefficient of the Ca(2+)-dependent tension development. Ouabain (1-100 nM) did not directly induce a Ca(2+) release out of the sarcoplasmic reticulum, nor did it alter the caffeine (10 mM) induced sarcoplasmic reticulum Ca(2+) release in saponin-skinned fibre preparations in which the sarcoplasmic reticulum had been Ca(2+)-loaded. In conclusion, ouabain increases myofibrillar Ca(2+) sensitivity possibly due to an increase in the cooperativity of the thick and thin myofilaments. This mechanism may additionally contribute to the positive inotropic effect of ouabain.

Ser16-, but not Thr17-phosphorylation of phospholamban influences frequency-dependent force generation in human myocardium

Beta-adrenoceptor/cAMP-dependent Ser16-phosphorylation as well as Ca(2+)-dependent Thr17-phosphorylation of phospholamban (PLB) influences SERCA 2a activity and thus myocardial contractility. To determine the cross-signaling between Ca2+ and cAMP pathways, the phosphorylation of Ser16-PLB and Thr17-PLB was studied at increasing stimulation frequencies as well as in the presence of beta-adrenergic stimulation in isolated ventricular trabeculae from failing (dilative cardiomyopathy, DCM, heart transplants, n=9) and non-failing human myocardium (donor hearts, NF, n=9). In addition, we measured the intracellular Ca(2+)-transient (fura-2) at increasing stimulation frequencies (0.5-3.0 Hz). Protein expression of SERCA 2a and phospholamban was similar in DCM and NF. In DCM, diastolic [Ca2+]i was increased and systolic [Ca2+]i as well as Ser16 PLB-phosphorylation were decreased as compared to NF at 0.5 Hz. The positive force-frequency relationship in human non-failing myocardium was accompanied by a frequency-dependent increase in Ser16-PLB, but not Thr17-PLB phosphorylation. In DCM, Ser16-PLB as well as Thr17-PLB phosphorylation were not altered at higher stimulation frequencies. After application of isoprenaline (1 microM), a profound increase in Ser16-PLB phosphorylation was accompanied by a small increase in Thr17-PLB phosphorylation, only in NF. The frequency-dependent phosphorylation of Ser16-PLB may favor an increase in Ca2+ transient and force generation in humans. Cross talk signaling of Ser16/Thr17-PLB phosphorylation after beta-adrenergic stimulation exists in non-failing, but not in failing human myocardium. The Ca(2+)-dependent CaM-kinase activity may be altered in human heart failure.

The preferential beta3-adrenoceptor agonist BRL 37344 increases force via beta1-/beta2-adrenoceptors and induces endothelial nitric oxide synthase via beta3-adrenoceptors in human atrial myocardium

1 The present study investigated the effects of the preferential beta(3)-AR agonist BRL 37344 (BRL) on force of contraction (FOC), Ca(2+)-transient and eNOS-activity in human right atrial myocardium. 2 BRL concentration-dependently caused an increase in FOC that was paralleled by an increase in Ca(2+)-transient and a shortening of time to half peak relaxation (T0.5T). These effects were abolished in the presence of propranolol (0.3 micro M). 3 BRL acted as a competitive antagonist towards isoprenaline and in binding experiments it was shown to have a distinct affinity towards beta(1/2)-AR. 4 In immunohistochemical experiments BRL (10 micro M) increased detection of activated eNOS. This effect remained constant in the presence of propranolol (0.3 micro M). 5 BRL increased directly detected NO in DAF-staining experiments. This increase was significantly smaller in the presence of the NO-inhibitor L-NAME. 6 The inotropic effects of BRL were not changed in the presence of L-NMA. 7 These results suggest that the inotropic effects of BRL in human atrium are mediated via beta(1/2)-AR, whereas the increase of atrial eNOS-activity is due to beta(3)- adrenergic stimulation. This increase in eNOS-activity did not influence atrial myocardial contractility. In conclusion, this study shows that beta(3)-adrenergic stimulation is present in human atrium, but may not be functionally as significant as in the left ventricle.

Bucindolol exerts agonistic activity on the propranolol-insensitive state of beta1-adrenoceptors in human myocardium

In congestive heart failure patients, treatment with beta-adrenoceptor antagonists improves symptoms and decreases mortality. However, intrinsic sympathomimetic activity of beta-adrenoceptor antagonists might be disadvantageous in chronic heart failure. The nonselective beta1- and beta2-adrenoceptor antagonist bucindolol has failed to decrease mortality in clinical trials. A putative beta4-adrenoceptor, which mediates positive inotropic effects by activation of the adenylate cyclase has been described. Recently, this putative beta4-adrenoceptor has been identified to be a propranolol-insensitive state of the beta1-adrenoceptor. The present study aimed to characterize whether bucindolol exhibits agonistic activity on this atypical beta1-adrenoceptor state as one possible reason for clinical inefficiency. For comparison (S)-4-(3'-t-butylamino-1'-hydroxypropoxy)-benzimidozole-2 (CGP 12177), metoprolol, and nebivolol were investigated. Bucindolol did not reveal intrinsic sympathomimetic activity in electrically driven (1 Hz, 37 degrees C), forskolin-stimulated, left ventricular papillary muscle strips (donor hearts, nonfailing; n = 5) and in right auricular trabeculae (bypass operation; n = 4). Functional studies on the propranolol-insensitive state of beta1-adrenoceptors were performed in isolated muscle preparations after beta1- and beta2-adrenoceptor antagonism (propranolol, 1 microM), inhibition of beta3-mediated inotropic effects (N-nitro-L-arginine, 100 microM) and forskolin treatment (0.3 microM). Positive inotropic response to stimulation of atypical state beta1-adrenoceptors could be demonstrated in right auricular as well as left ventricular human myocardium (CGP 12177 treatment, 10 microM). Under these conditions, also bucindolol, but not metoprolol and nebivolol, significantly increased contractility (all 10 microM). In conclusion, bucindolol but not metoprolol or nebivolol mediate positive inotropic effects in human myocardium due to activation of atypical state beta1-adrenoceptors. Thus, the agonistic activity of bucindolol may influence outcome in heart failure patients.

Beneficial effects of the Ca(2+) sensitizer levosimendan in human myocardium

Levosimendan has been reported to increase cardiac Ca(2+) sensitivity, thereby not enhancing intracellular Ca(2+) or diastolic tension. This may be advantageous for the treatment of heart failure patients. Therefore, the present study investigates the mode of action of levosimendan in both failing and nonfailing (NF) human myocardium. The effects of levosimendan on contractile force, Ca(2+) transient (fura 2), and the force-frequency relationship (0.5-3 Hz) were studied in left ventricular terminally failing [dilated cardiomyopathy (DCM; n = 18)] and nonfailing (NF) myocardium (donor hearts, n = 6). Levosimendan (0.03-10 micromol/l) increased contractile force in NF (EC(50): 0.38 micromol/l). In left ventricular failing myocardium, levosimendan only increased force after prestimulation with isoprenaline (0.1 micromol/l, EC(50) levosimendan: 0.062 micromol/l) or after elevation of the extracellular Ca(2+) concentration from 1.8 to 3.2 mmol/l. After application of isoprenaline, levosimendan shortened relaxation and contraction kinetics. Levosimendan did not change the systolic Ca(2+) transient but it improved the force-frequency relationship in DCM. In conclusion, levosimendan improves contraction in failing human myocardium under conditions with already increased intracellular Ca(2+).

Signal-transducing function of Na+-K+-ATPase is essential for ouabain's effect on [Ca2+]i in rat cardiac myocytes

We showed before that Na+-K+-ATPase is also a signal transducer in neonatal rat cardiac myocytes. Binding of ouabain to the enzyme activates multiple signal pathways that regulate cell growth. The aims of this work were to extend such studies to adult cardiac myocytes and to determine whether the signal-transducing function of Na+/K+-ATPase regulates the well-known effects of ouabain on intracellular Ca2+ concentration ([Ca2+]i). In adult myocytes, ouabain activated protein tyrosine phosphorylation and p42/44 mitogen-activated protein kinases (MAPKs), increased production of reactive oxygen species (ROS), and raised both systolic and diastolic [Ca2+]i. Pretreatment of myocytes with several Src kinase inhibitors, or overexpression of a dominant negative Ras, antagonized ouabain-induced activation of MAPKs and increases in [Ca2+]i. Treatment with PD-98059 (a MAPK kinase inhibitor) or overexpression of a dominant negative MAPK kinase 1 also ablated the effect of ouabain on MAPKs and [Ca2+]i. N-acetyl-cysteine, which blocks the effect of ouabain on ROS, did not prevent the ouabain-induced rise in [Ca2+]i. Clearly, the activation of the Ras/MAPK cascade, but not ROS generation, is necessary for ouabain-induced increases in [Ca2+]i in rat cardiac myocytes.

The in vitro effects of remifentanil, sufentanil, fentanyl, and alfentanil on isolated human right atria

Because some clinical studies have suggested that opioids used in anesthesia may have different deleterious hemodynamic effects, we compared the direct myocardial effects of cumulative concentrations of remifentanil, sufentanil, fentanyl, and alfentanil on inotropic and lusitropic variables of isolated human myocardium in vitro. Human right atrial trabeculae, obtained from patients scheduled for coronary bypass surgery or aortic valve replacement, were suspended vertically in an oxygenated (95% oxygen/5% CO(2)) Tyrode's modified solution ([Ca(2+)](o) = 2.0 mM, 37 degrees C, pH 7.40, stimulation frequency 1 Hz). The effects of cumulative concentrations (10(-11), 10(-10), 10(-9), 10(-8), 10(-7), and 10(-6) M) of remifentanil (n = 8), sufentanil (n = 8), fentanyl (n = 8), and alfentanil (n = 8) on inotropic and lusitropic variables of isometric twitches were measured. Remifentanil, sufentanil, and fentanyl did not modify active isometric force and peak of the positive force derivative as compared with the Control group. Alfentanil induced a dose-dependent decrease in active isometric force and peak of the positive force derivative. This effect was abolished in the presence of [Ca(2+)](o) = 4.0 mM. None of these opioids altered lusitropic variables.

Human embryonic stem cells can differentiate into myocytes with structural and functional properties of cardiomyocytes

The study of human cardiac tissue development is hampered by the lack of a suitable in vitro model. We describe the phenotypic properties of cardiomyocytes derived from human embryonic stem (ES) cells. Human ES cells were cultivated in suspension and plated to form aggregates termed embryoid bodies (EBs). Spontaneously contracting areas appeared in 8.1% of the EBs. Cells from the spontaneously contracting areas within EBs were stained positively with anti-cardiac myosin heavy chain, anti--alpha-actinin, anti-desmin, anti--cardiac troponin I (anti-cTnI), and anti-ANP antibodies. Electron microscopy revealed varying degrees of myofibrillar organization, consistent with early-stage cardiomyocytes. RT-PCR studies demonstrated the expression of several cardiac-specific genes and transcription factors. Extracellular electrograms were characterized by a sharp component lasting 30 +/- 25 milliseconds, followed by a slow component of 347 +/- 120 milliseconds. Intracellular Ca(2+) transients displayed a sharp rise lasting 130 +/- 27 milliseconds and a relaxation component lasting 200--300 milliseconds. Positive and negative chronotropic effects were induced by application of isoproterenol and carbamylcholine, respectively. In conclusion, the human ES cell--derived cardiomyocytes displayed structural and functional properties of early-stage cardiomyocytes. Establishment of this unique differentiation system may have significant impact on the study of early human cardiac differentiation, functional genomics, pharmacological testing, cell therapy, and tissue engineering.

Nebivolol, carvedilol and metoprolol do not influence cardiac Ca(2+) sensitivity

It has been argued that some beta-adrenoceptor antagonists may directly influence myofibrillar cross-bridge interaction in cardiac skinned fiber preparations of animal models. The present study investigates the effects of nebivolol, metoprolol and carvedilol on tension development of Triton X-100 skinned fibers obtained from human failing myocardium as well as on force of contraction and intracellular Ca(2+) transient in isolated trabeculae. In skinned fiber preparations, none of the beta-adrenoceptor antagonists (10 microM) influenced Ca(2+) sensitivity of tension development or maximal Ca(2+) activated tension (DT(max)): control: EC(50) for Ca(2+): 1.28+/-0.05 microM, DT(max): 14.09+/-0.59 mN/mm(2); nebivolol: 1.36+/-0.1 microM, 14.14+/-0.95 mN/mm(2); carvedilol: 1.32+/-0.11 microM, 13.83+/-0.90 mN/mm(2); metoprolol: 1.34+/-0.14 microM, 13.72+/-0.36 mN/mm(2). Simultaneous measurement of force and Ca(2+) transient in the presence of the beta-adrenoceptor antagonists (3 microM) showed that the decrease in force of contraction was paralleled by a similar decrease in the intracellular Ca(2+) transient. In conclusion, none of the investigated beta-adrenoceptor antagonists influenced Ca(2+) sensitivity of myofibrillar tension development in human failing myocardium.

cAMP-dependent protein kinase A-stimulated sarcoplasmic reticulum function in heart failure

It is unclear whether decreased protein expression of SERCA2 (SR-Ca(2+)-ATPase) and phospholamban (PLB), or alterations in the phosphorylation state of PLB leading to increased inhibition of SERCA2 are responsible for the reduced SERCA2 function in failing human myocardium. In crude membrane preparations from patients with terminal heart failure due to idiopathic dilated cardiomyopathy (DCM) and control hearts (NF), SERCA2 activity was measured with a NADH coupled assay. Protein expression of SERCA2 and PLB and the phosphorylation state at the two phosphorylation sites, serine-16-PLB and threonine-17-PLB, were investigated with specific (phosphorylation) antibodies and Western blot technique. In NF, the Vmax and the Ca2+ sensitivity of SERCA2 activity were significantly higher compared to DCM. Protein expression of SERCA2 and PLB were unchanged, whereas the phosphorylation status at both serine-16-PLB and threonine-17-PLB were significantly reduced in DCM. The native phosphorylation status of PLB measured by the back-phosphorylation technique was reduced in DCM as well. After stimulation with protein kinase A only the Ca2+ sensitivity, but not Vmax, increased. The reduced phosphorylation state of PLB may lead to decreased Ca2+ sensitivity of SERCA2 in failing human myocardium. The altered regulation of the SR-CA(2+)-ATPase in human heart failure may offer an opportunity for an improvement in the therapy of heart failure.

Potent vasodilatory with minor cardiodepressant actions of mibefradil in human cardiac tissue

1. The present study compared the cardiovascular effects of mibefradil (MIB), a novel Ca2+-channel antagonist with high selectivity for T-type Ca2+-channels to the effect of the L-type Ca2+-channel-antagonists nifedipine (NIF) and diltiazem (DIL) in left ventricular myocardium and coronary arteries of hearts obtained from patients suffering from dilated cardiomyopathy (NYHA IV). Right atrial myocardium from patients undergoing aortocoronary bypass surgery without signs of cardiac failure was studied as well. 2. NIF and DIL (100 micromol l(-1)) completely depressed force of contraction (FOC) in electrically driven left ventricular myocardium (NIF 6.5+/-1.4% and DIL 7.1+/-1.2% of control), whereas a similar concentration of MIB only reduced force of contraction to 55.1+/-4.0% of the basal FOC. The negative inotropic potency as measured by the concentration needed to reduce basal FOC for 25% was NIF (0.0095 micromol l(-1))>DIL (0.041 micromol l(-1))>MIB (9.47 micromol l(-1)). 3. All three Ca2+-channel antagonists were more potent in human atrial compared to human left ventricular myocardium to reduce FOC. 4. The rank order of Ca+-antagonistic moiety as measured by the decrease of the intracellular Ca2+-transient (fura-2 ratio method) was NIF>DIL>MIB. 5. All Ca2+-channel antagonists completely relaxed human coronary arteries (% of papaverine effect: MIB 81.7+/-5.5%, DIL 91.3+/-0.9%, NIF 96.4+/-3.7%) precontracted with PGF2alpha (0.3 micromol l(-1)). The rank order of vasodilatory potency was NIF (EC50; 0.02 micromol l(-1))>DIL (0.13 micromol l(-1))>MIB (2.05 micromol l(-1)). 6. The vasoselectivity measured by the ratio of the concentration needed to achieve a 25% decrease in force and the concentration needed for 25% vasodilatation was 316 for MIB, 1.5 for NIF and 1.0 for DIL. 7. The present study provides evidence that blockade of T-type Ca2+-channels (e.g. mibefradil) results in potent vasodilatory properties with only minor cardiodepressant effects.

Effect of dopamine and atrial pacing on stunned myocardium

OBJECTIVE

Post-ischaemic stunned myocardium shows an impaired function at restored coronary blood flow, but performance can be normalized by positive inotropic stimulation. The power of stunned myocardium, however, is not augmented with increasing heart rate by atrial pacing, which is in contrast to intact areas. This pathological response is mitigated by inhibiting the degradation of cyclic AMP. The present experiments studied the effect of stimulating cyclic AMP formation by dopamine on the response of stunned myocardium to atrial pacing.

METHODS

In anaesthetized (piritramide) open chest pigs, heart rate, left ventricular and aortic pressure, left descending (LAD) and circumflex (LCX) coronary artery and aortic blood flow, myocardial systolic shortening in the LAD and LCX area were monitored, and myocardial power was calculated. The LAD region was subjected to ischaemia and reperfused for 2 h. Subsequently, heart rate was raised by right atrial pacing before and during intravenous infusion of dopamine (10 microg/kg per min). The ischaemic/reperfused area was sliced post mortem and stained by triphenyl tetrazolium chloride to exclude myocardial infarction. Data from 11 experiments are presented.

RESULTS

After 2 h LAD reperfusion, LAD blood flow and power were 100% and 36% of pre-ischaemic control, respectively, indicating myocardial stunning. The power of the intact area was not changed significantly (111% of control). Increasing heart rate by +36 and +70 from 94 beats/min increased the power of the intact area to 161% and 183% of control; the power of the stunned myocardium decreased to 34% and 19% of pre-stunning control. Dopamine increased the power of the stunned region to 143% of the pre-stunning level and the power of the intact area to 206% of control. Increasing heart rate by +34 and +70 from 113 beats/min during dopamine, increased the power of the intact myocardium to 288% and 344% of control and the power of the stunned region to 177% and 174% of the pre-stunning level.

CONCLUSIONS

The data confirm the pathological response of stunned myocardium to atrial pacing and the recruitment of a functional reserve by catecholamines. The adverse effect of pacing on the function of stunned myocardium is abolished by positive inotropic stimulation. Possibly, the cyclic AMP system is involved in the normal response to pacing and this pathway is disturbed in stunned myocardium; other defects are not excluded or supported, however. Physiologically increased heart rate by an increased activity of the sympathetic nervous system, is probably not accompanied by a reduced power of stunned myocardium, due to the associated positive inotropic stimulation.