Increased sarcoplasmic/endoplasmic reticulum calcium ATPase 2a activity underlies the mechanism of the positive inotropic effect of ivabradine

Chamber-specific contractile responses of atrial and ventricular hiPSC-cardiomyocytes to GPCR and ion channel targeting compounds: A microphysiological system for cardiac drug development

Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CMs) have found utility for conducting in vitro drug screening and disease modelling to gain crucial insights into pharmacology or disease phenotype. However, diseases such as atrial fibrillation, affecting >33 M people worldwide, demonstrate the need for cardiac subtype-specific cells. Here, we sought to investigate the base characteristics and pharmacological differences between commercially available chamber-specific atrial or ventricular hiPSC-CMs seeded onto ultra-thin, flexible PDMS membranes to simultaneously measure contractility in a 96 multi-well format. We investigated the effects of GPCR agonists (acetylcholine and carbachol), a Ca2+ channel agonist (S-Bay K8644), an HCN channel antagonist (ivabradine) and K+ channel antagonists (4-AP and vernakalant). We observed differential effects between atrial and ventricular hiPSC-CMs on contractile properties including beat rate, beat duration, contractile force and evidence of arrhythmias at a range of concentrations. As an excerpt of the compound analysis, S-Bay K8644 treatment showed an induced concentration-dependent transient increase in beat duration of atrial hiPSC-CMs, whereas ventricular cells showed a physiological increase in beat rate over time. Carbachol treatment produced marked effects on atrial cells, such as increased beat duration alongside a decrease in beat rate over time, but only minimal effects on ventricular cardiomyocytes. In the context of this chamber-specific pharmacology, we not only add to contractile characterization of hiPSC-CMs but propose a multi-well platform for medium-throughput early compound screening. Overall, these insights illustrate the key pharmacological differences between chamber-specific cardiomyocytes and their application on a multi-well contractility platform to gain insights for in vitro cardiac liability studies and disease modelling.

Ivabradine acutely improves cardiac Ca handling and function in a rat model of Duchenne muscular dystrophy

The muscular dystrophies caused by dystrophin deficiency, the so-called dystrophinopathies, are associated with impaired cardiac contractility and arrhythmias, which considerably contribute to disease morbidity and mortality. Impaired Ca handling in ventricular cardiomyocytes has been identified as a causative factor for complications in the dystrophic heart, and restoration of normal Ca handling in myocytes has emerged as a promising new therapeutic strategy. In the present study, we explored the hypothesis that ivabradine, a drug clinically approved for the treatment of heart failure and stable angina pectoris, improves Ca handling in dystrophic cardiomyocytes and thereby enhances contractile performance in the dystrophic heart. Therefore, ventricular cardiomyocytes were isolated from the hearts of adult dystrophin-deficient DMD^mdx rats, and the effects of acutely applied ivabradine on intracellular Ca transients were tested. In addition, the drug's acute impact on cardiac function in DMD^mdx rats was assessed by transthoracic echocardiography. We found that administration of ivabradine to DMD^mdx rats significantly improved cardiac function. Moreover, the amplitude of electrically induced intracellular Ca transients in ventricular cardiomyocytes isolated from DMD^mdx rats was increased by the drug. We conclude that ivabradine enhances Ca release from the sarcoplasmic reticulum in dystrophic cardiomyocytes and thereby improves contractile performance in the dystrophic heart.

No evidence for pericardial restraint in the snapping turtle (Chelydra serpentina) following pharmacologically-induced bradycardia at rest or during exercise

Most animals elevate cardiac output during exercise through a rise in heart rate (f_H), whilst stroke volume (V_S) remains relatively unchanged. Cardiac pacing reveals that elevating f_H alone does not alter cardiac output, which is instead largely regulated by the peripheral vasculature. In terms of myocardial oxygen demand, an increase in f_H is more costly than that which would incur if V_S instead were to increase. We hypothesized that f_H must increase because any substantial rise in V_S would be constrained by the pericardium. To investigate this hypothesis, we explored the effects of pharmacologically-induced bradycardia, with ivabradine treatment, on V_S at rest and during exercise in the common snapping turtle (Chelydra serpentina) with intact or opened pericardium. We first showed that, in isolated myocardial preparations, ivabradine exerted a pronounced positive inotropic effect on atrial tissue, but only minor effects on ventricle. Ivabradine reduced f_H in vivo, such that exercise tachycardia was attenuated. Pulmonary and systemic V_S rose in response to ivabradine. The rise in pulmonary V_S largely compensated for the bradycardia at rest, leaving total pulmonary flow unchanged by ivabradine, although ivabradine reduced pulmonary blood flow during swimming (exercise x ivabradine interaction, P<0.05). Although systemic V_S increased, systemic blood flow was reduced by ivabradine both at rest and during exercise, in spite of ivabradine's potential to increase cardiac contractility. Opening the pericardium had no effect on f_H, V_S or blood flows before or after ivabradine, indicating that the pericardium does not constrain V_S in turtles, even during pharmacologically-induced bradycardia.

Impact of ivabradine in decompensated heart failure due to cancer therapy-related cardiac dysfunction

In cases of decompensated heart failure related to cancer therapy-related cardiac dysfunction, ivabradine administration could lead to an increased stroke volume by reducing the sinus heart rate, resulting in favorable hemodynamics. Assessment of the overlap between the E- and A-waves facilitates understanding the effects of ivabradine in such cases.

Two cases of dilated cardiomyopathy with blood pressure-limited tolerability of cardioprotective agents improved by ivabradine

The titration of cardioprotective agents is essential for successful treatment of heart failure (HF) patients with reduced left ventricular ejection fraction. However, hypotension is one of the limiting factors for titration. Ivabradine reduces heart rate without compromising systolic function by prolonging diastolic filling time. Herein two cases of dilated cardiomyopathy (DCM) are presented in which ivabradine improved blood pressure (BP)-limited tolerability and allowed for further titration of cardioprotective agents. In both cases, the introduction of ivabradine raised the BP, which permitted further increase of the dose of renin-angiotensin system inhibitors or beta-blockers. One major hypothesized mechanism of ivabradine-induced BP elevation has been postulated to be an increase in stroke volume due to prolonged ventricular diastolic filling time. However, ivabradine is not expected to increase BP for all HF patients. In those with small and poorly compliant ventricles with severe diastolic or restricted dysfunction, decreased heart rate and prolonged diastole may excessively suppress compensatory mechanisms, and thus may not lead to increased cardiac output and BP. In contrast, ivabradine potentially increases BP and improves BP-limited tolerability of cardioprotective agents in DCM patients with a large and compliant heart. In addition, subsequent titration of cardioprotective agents may provide additional cardiac reverse remodeling. Learning objective: Ivabradine is usually used for heart failure patients with reduced ejection fraction when the tolerability of cardioprotective agents is maximized. This agent has no direct cardiac contractility-suppressing action. It potentially increases blood pressure and improves tolerability of cardioprotective agents in patients with a large and compliant heart such as dilated cardiomyopathy. Furthermore, subsequent titration of cardioprotective agents may provide additional cardiac reverse remodeling.

Bay 60-7550, a PDE2 inhibitor, exerts positive inotropic effect of rat heart by increasing PKA-mediated phosphorylation of phospholamban

This study investigated the hemodynamic effect of Bay 60-7550, a phosphodiesterase type 2 (PDE2) inhibitor, in healthy rat hearts both in vivo and ex vivo and its underlying mechanisms. In vivo rat left ventricular pressure-volume loop, Langendorff isolated rat heart, Ca²⁺ transient of left ventricular myocyte and Western blot experiments were used in this study. The results demonstrated that Bay 60-7550 (1.5 mg/kg, i. p.) increased the in vivo rat heart contractility by enhancing stroke work, cardiac output, stroke volume, end-diastolic volume, heart rate, and ejection fraction. The simultaneous aortic pressure recording indicated that the systolic blood pressure was increased and diastolic blood pressure was decreased by Bay 60-7550. Also, the arterial elastance which is proportional to the peripheral vessel resistance was significantly decreased. Bay 60-7550 (0.001, 0.01, 0.1, 1 μmol/l) also enhanced the left ventricular development pressure in non-paced and paced modes with a decrease of heart rate in non-paced model. Bay 60-7550 (1 μmol/l) increased SERCA2a activity and SR Ca²⁺ content and reduced SR Ca²⁺ leak rate. Furthermore, Bay 60-7550 (0.1 μmol/l) increased the phosphorylation of phospholamban at 16-serine without significantly changing the phosphorylation levels of phospholamban at 17-threonine and RyR2. Bay 60-7550 increased the rat heart contractility and reduced peripheral arterial resistance may be mediated by increasing the phosphorylation of phospholamban and dilating peripheral vessels. PDE2 inhibitors which result in a positive inotropic effect and a decrease in peripheral resistance might serve as a target for developing agents for the treatment of heart failure in clinical settings.

The bradycardic agent ivabradine decreases conduction velocity in the AV node and in the ventricles in-vivo

Ivabradine blocks hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels, thereby lowering the heart rate, an action that is used clinically for the treatment of heart failure and angina pectoris. We and others have shown previously that ivabradine, in addition to its HCN channel blocking activity, also inhibits voltage-gated Na channels in vitro at concentrations that may be clinically relevant. Such action may reduce conduction velocity in cardiac atria and ventricles. Here, we explore the effect of administration of ivabradine on parameters of ventricular conduction and repolarization in the surface ECG of anesthetized mice. We found that 5 min after i.p. administration of 10 mg/kg ivabradine spontaneous heart rate had declined by ~13%, which is within the range observed in human clinical studies. At the same time a significant increase in QRS duration by ~18% was observed, suggesting a reduction in ventricular conduction velocity. During transesophageal pacing at heart rates between 100 and 220 beats/min there was no obvious rate-dependence of ivabradine-induced QRS prolongation. On the other hand, ivabradine produced substantial rate-dependent slowing of AV nodal conduction. We conclude that ivabradine prolongs conduction in the AV-node and in the ventricles in vivo.