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Human induced pluripotent stem cell (iPSC)-derived cardiomyocytes are an established model for testing potential chemical hazards. Interindividual variability in toxicodynamic sensitivity has also been demonstrated in vitro; however, quantitative characterization of the population-wide variability has not been fully explored. We sought to develop a method to address this gap by combining a population-based iPSC-derived cardiomyocyte model with Bayesian concentration-response modeling. A total of 136 compounds, including 54 pharmaceuticals and 82 environmental chemicals, were tested in iPSC-derived cardiomyocytes from 43 nondiseased humans. Hierarchical Bayesian population concentration-response modeling was conducted for 5 phenotypes reflecting cardiomyocyte function or viability. Toxicodynamic variability was quantified through the derivation of chemical- and phenotype-specific variability factors. Toxicokinetic modeling was used for probabilistic in vitro-to-in vivo extrapolation to derive population-wide margins of safety for pharmaceuticals and margins of exposure for environmental chemicals. Pharmaceuticals were found to be active across all phenotypes. Over half of tested environmental chemicals showed activity in at least one phenotype, most commonly positive chronotropy. Toxicodynamic variability factor estimates for the functional phenotypes were greater than those for cell viability, usually exceeding the generally assumed default of approximately 3. Population variability-based margins of safety for pharmaceuticals were correctly predicted to be relatively narrow, including some below 10; however, margins of exposure for environmental chemicals, based on population exposure estimates, generally exceeded 1000, suggesting they pose little risk at current general population exposures even to sensitive subpopulations. Overall, this study demonstrates how a high-throughput, human population-based, in vitro-in silico model can be used to characterize toxicodynamic population variability in cardiotoxic risk.

Germidine and germerine, the Veratrum alkaloids lowered blood pressure accompanied with positive chronotropy and inotropy in mice. Germerine was more potent than germidine in both blood-pressure lowering and positive inotropy, whereas veratridine produced negative chronotropy and positive inotropy. An acyl group (an acetyl or a 2-methylbutyroyl group) at 3-O-R1 position and a 2-methylbutyroyl group at 15-O-R2 position in germine were important to produce the positive inotropy and chronotropy. The presence of a veratroyl group at 3-O-R1 position and a free hydroxyl group at 15-O-R2 position may be essential to produce the negative chronotropy by veratridine. The positive inotropy by germidine and veratridine may be due to TTX-resistant Na+ channel activation.

β-adrenergic stimulation increases the heart rate by accelerating the electrical activity of the pacemaker of the heart, the sinoatrial node (SAN). Ionic mechanisms underlying the actions of β-adrenergic stimulation are not yet fully understood. Isoprenaline (ISO), a β-adrenoceptor agonist, shifts voltage-dependent I(f) activation to more positive potentials resulting in an increase of I(f), which has been suggested to be the main mechanism underlying the effect of β-adrenergic stimulation. However, ISO has been found to increase the firing rate of rabbit SAN cells when I(f) is blocked. ISO also increases I(CaL), I(st), I(Kr), and I(Ks); and shifts the activation of I(Kr) to more negative potentials and increases the rate of its deactivation. ISO has also been reported to increase the intracellular Ca(2+) transient, which can contribute to chronotropy by modulating the "Ca(2+) clock." The aim of this study was to analyze the ionic mechanisms underlying the positive chronotropy of β-adrenergic stimulation using two distinct and well established computational models of the electrical activity of rabbit SAN cells.We modified the Boyett et al. (2001) and Kurata et al. (2008) models of electrical activity for the central and peripheral rabbit SAN cells by incorporating equations for the known dose-dependent actions of ISO on various ionic channel currents (I(CaL), I(st), I(Kr), and I(Ks)), kinetics of I(Kr) and I(f), and the intracellular Ca(2+) transient. These equations were constructed from experimental data. To investigate the ionic basis of the effects of ISO, we simulated the chronotropic effect of a range of ISO concentrations when ISO exerted all its actions or just a subset of them.In both the Boyett et al. and Kurata et al. SAN models, the chronotropic effect of ISO was found to result from an integrated action of ISO on I(CaL), I(f), I(st), I(Kr), and I(Ks), among which an increase in the rate of deactivation of I(Kr) plays a prominent role, though the effect of ISO on I(f) and [Ca(2+)](i) also plays a role.

Ionic mechanisms of spontaneous action potential in sinoatrial (SA) node pacemaker cells have been discussed for decades. Although a number of theoretical studies have proposed different mathematical models, no scientific consensus has been achieved yet, because of the complexity and variations in experimental findings used for developing models. Here, we introduce a theoretical method in simulation study, the lead potential analysis, which enabled us to isolate the contribution of individual currents from the secondary effect of modified channel activities. We compared three models, suggesting different ionic mechanisms (Himeno et al. model, Kurata et al. model, and Maltsev and Lakatta model), and contributions of Ca2+ through activation of I NaCa is estimated. Finally, the effect of catecholamine stimulation is discussed based on a SA node cell model with β1-adrenergic signaling cascade and mechanisms of the positive chronotropy are analyzed.

Exposure of rat atrial slices to 10 −5 M norepinephrine (NE) for 10 minutes increases 45 Ca ++ accumulation from 1.64 ± 0.10 to 2.23 ± 0.06 nmol/mg tissue. In the presence of leucine enkephalin (10 −8 M), NE-stimulated 45 Ca ++ uptake is reduced to 1.44 ± 0.10 nmol/mg tissue. The effect of leu-enkephalin is reversed in the presence of 10 −7 M naloxone, NE-stimulated 45 Ca ++ uptake being increased to 2.17 ± 0.15 nmol/mg tissue. The results support a direct interaction of leuenkephalin with β-agonist-stimulated Ca ++ flux in rat atria, and correlate with the previously reported enkephalin antagonism of NE-induced positive chronotropy in the same tissue.