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Proarrhythmic cardiotoxicity remains a substantial barrier to drug development as well as a major global health challenge. In vitro human pluripotent stem cell-based new approach methodologies have been increasingly proposed and employed as alternatives to existing in vitro and in vivo models that do not accurately recapitulate human cardiac electrophysiology or cardiotoxicity risk. In this study, we expanded the capacity of our previously established 3D human cardiac microtissue model to perform quantitative risk assessment by combining it with a physiologically based pharmacokinetic model, allowing a direct comparison of potentially harmful concentrations predicted in vitro to in vivo therapeutic levels. This approach enabled the measurement of concentration responses and margins of exposure for 2 physiologically relevant metrics of proarrhythmic risk (i.e. action potential duration and triangulation assessed by optical mapping) across concentrations spanning 3 orders of magnitude. The combination of both metrics enabled accurate proarrhythmic risk assessment of 4 compounds with a range of known proarrhythmic risk profiles (i.e. quinidine, cisapride, ranolazine, and verapamil) and demonstrated close agreement with their known clinical effects. Action potential triangulation was found to be a more sensitive metric for predicting proarrhythmic risk associated with the primary mechanism of concern for pharmaceutical-induced fatal ventricular arrhythmias, delayed cardiac repolarization due to inhibition of the rapid delayed rectifier potassium channel, or hERG channel. This study advances human-induced pluripotent stem cell-based 3D cardiac tissue models as new approach methodologies that enable in vitro proarrhythmic risk assessment with high precision of quantitative metrics for understanding clinically relevant cardiotoxicity. [ABSTRACT FROM AUTHOR]

Environmental factors play a substantial role in determining cardiovascular health, but data informing the risks presented by environmental toxicants is insufficient. In vitro new approach methodologies (NAMs) offer a promising approach with which to address the limitations of traditional in vivo and in vitro assays for assessing cardiotoxicity. Driven largely by the needs of pharmaceutical toxicity testing, considerable progress in developing NAMs for cardiotoxicity analysis has already been made. As the scientific and regulatory interest in NAMs for environmental chemicals continues to grow, a thorough understanding of the unique features of environmental cardiotoxicants and their associated cardiotoxicities is needed. Here, we review the key characteristics of as well as important regulatory and biological considerations for fit-for-purpose NAMs for environmental cardiotoxicity. By emphasizing the challenges and opportunities presented by NAMs for environmental cardiotoxicity we hope to accelerate their development, acceptance, and application. [ABSTRACT FROM AUTHOR]

In contrast to the other heterotrimeric GTP-binding proteins (G proteins) [G.sub.s] and [G.sub.i], the functional role of Go is still poorly defined. To investigate the role of G[[alpha].sub.o] in the heart, we generated transgenic mice with cardiac-specific expression of a constitutively active form of G[[alpha].sub.o1] * (G[[alpha].sub.o] *), the predominant G[[alpha].sub.o] isoform in the heart. G[[alpha].sub.o] expression was increased 3- to 15-fold in mice from 5 independent lines, all of which had a normal life span and no gross cardiac morphological abnormalities. We demonstrate enhanced contractile function in G[[alpha].sub.o] * transgenic mice in vivo, along with increased L-type [Ca.sup.2+] channel current density, calcium transients, and cell shortening in ventricular G[[alpha].sub.o] *-expressing myocytes compared with wild-type controls. These changes were evident at baseline and maintained after isoproterenol stimulation. Expression levels of all major [Ca.sup.2+] handling proteins were largely unchanged, except for a modest reduction in [Na.sup.+]/[Ca.sup.2+] exchanger in transgenic ventricles. In contrast, phosphorylation of the ryanodine receptor and phospholamban at known PKA sites was increased 1.6- and 1.9-fold, respectively, in G[[alpha].sub.o] * ventricles. Density and affinity of [beta]-adrenoceptors, cAMP levels, and PKA activity were comparable in G[[alpha].sub.o] * and wild-type myocytes, but protein phosphatase 1 activity was reduced upon G[[alpha].sub.o] * expression, particularly in the vicinity of the ryanodine receptor. We conclude that G[[alpha].sub.o] * exerts a positive effect on [Ca.sup.2+] cycling and contractile function. Alterations in protein phosphatase 1 activity rather than PKA-mediated phosphorylation might be involved in hyperphosphorylation of key [Ca.sup.2+] handling proteins in hearts with constitutive [G[alpha].sub.o] activation. G proteins; signal transduction; calcium; contraction; transgenic mice

Cardiac hypertrophy is a common response to injury and hemodynamic stress and an important harbinger of heart failure and death. Herein, we identify the Kruppel-like factor 15 (KLF15) as an inhibitor of cardiac hypertrophy. Myocardial expression of KLF15 is reduced in rodent models of hypertrophy and in biopsy samples from patients with pressure-overload induced by chronic valvular aortic stenosis. Overexpression of KLF15 in neonatal rat ventricular cardiomyocytes inhibits cell size, protein synthesis and hypertrophic gene expression. KLF15-null mice are viable but, in response to pressure overload, develop an eccentric form of cardiac hypertrophy characterized by increased heart weight, exaggerated expression of hypertrophic genes, left ventricular cavity dilatation with increased myocyte size, and reduced left ventricular systolic function. Mechanistically, a combination of promoter analyses and gel-shift studies suggest that KLF15 can inhibit GATA4 and myocyte enhancer factor 2 function. These studies identify KLF15 as part of a heretofore unrecognized pathway regulating the cardiac response to hemodynamic stress. heart | transcription

Impaired parasympathetic heart rate control in mice with a reduction of functional G protein [beta][gamma]-subunits. Am J Physiol Heart Circ Physiol 282: H445-H456, 2002; 10.1152/ajpheart. 00565.2001.--Acetylcholine released on parasympathetic stimulation slows heart rate through activation of muscarinic receptors on the sinus nodal cells and subsequent opening of the atrial muscarinic potassium channel ([K.sub.ACh]). [K.sub.ACh] is directly activated by G protein [beta][gamma]-subunits. To elucidate the physiological role of G[beta][gamma] for the regulation of heart rate and electrophysiological function in vivo, we created transgenic mice with a reduced amount of membrane-bound G[beta] protein by overexpressing nonprenylated [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-subunits in their hearts using the [alpha]-myosin heavy chain promoter. At baseline and after muscarinic stimulation with carbachol, heart rate and heart rate variability were determined with electrocardiogram telemetry in conscious mice and in vivo intracardiac electrophysiological studies in anesthetized mice. Reduction of the amount of functional G[beta][gamma] protein by >50% caused a pronounced blunting of the carbachol-induced bradycardia as well as the increases in time- and frequency-domain indexes of heart rate variability and baroreflex sensitivity that were observed in wild types. In addition, sinus node recovery time and inducibility of atrial arrhythmias were reduced in transgenic mice. Our data demonstrate in vivo that G[beta][gamma] plays a crucial role for parasympathetic heart rate control, sinus node automaticity, and atrial arrhythmia vulnerability. atrial arrhythmia; heart rate regulation; in vivo electrophysiology; sinus node pacemaker activity; transgenic mice

The chronic iso infusion causes changes in the cardiac gene expression which corresponds to the growth of myocyte hypertrophy and interstital fibrosis. The differences in the hemodynamic parameters do not influence the activation of common mechanisms in pressure overload and beta-adrenergic receptor-mediated cardiac hypertrophy. The mechanisms controlling the gene induce different status of sensitivity to beta-adrenergic receptor stimulation.

The heart is a complex pluricellular organ composed of cardiomyocytes and non-myocytes including fibroblasts, endothelial cells and immune cells. Myocytes are responsible for electrical conduction and contractile force generation, while the other cell types are responsible for matrix deposition, vascularization, and injury response. Myocytes and non-myocytes are known to communicate and exert mutual regulatory effects. In concert, they determine the structural, electrical and mechanical characteristics in the healthy and remodelled myocardium. Dynamic crosstalk between myocytes and non-myocytes plays a crucial role in stress/injury-induced hypertrophy and fibrosis development that can ultimately lead to heart failure and arrhythmias. Investigations of heterocellular communication in the myocardium are hampered by the intricate interspersion of the different cell types and the complexity of the tissue architecture. In vitro models have facilitated investigations of cardiac cells in a direct and controllable manner and have provided important functional and mechanistic insights. However, these cultures often lack regulatory input from the other cell types as well as additional topographical, electrical, mechanical and biochemical cues from the cardiac microenvironment that all contribute to modulating cell differentiation, maturation, alignment, function and survival. Advancements in the development of more complex pluricellular physiological platforms that incorporate diverse cues from the myocardial microenvironment are expected to lead to more physiologically relevant cardiac tissue-like in vitro models for mechanistic biological research, disease modelling, therapeutic target identification, drug testing and regeneration. [ABSTRACT FROM AUTHOR]