Action potential responses to changes in stimulation frequency and isoproterenol in rat ventricular myocytes

Ionic current changes underlying action potential repolarization responses to physiological pacing and adrenergic stimulation in adult rat ventricular myocytes

This study aimed to simulate ventricular responses to elevations in myocyte pacing and adrenergic stimulation using a novel electrophysiological rat model and investigate ion channel responses underlying action potential (AP) modulations. Peak ion currents and AP repolarization to 50% and 90% of full repolarization (APD_50-90 ) were recorded during simulations at 1-10 Hz pacing under control and adrenergic stimulation conditions. Further simulations were performed with incremental ion current block (L-type calcium current, I_Ca ; transient outward current, I_to ; slow delayed rectifier potassium current, I_Ks ; rapid delayed rectifier potassium current, I_Kr ; inward rectifier potassium current, I_K1 ) to identify current influence on AP response to exercise. Simulated APD_50-90 closely resembled experimental findings. Rate-dependent increases in I_Ks (6%-101%), I_Kr (141%-1339%), and I_Ca (0%-15%) and reductions in I_to (11%-57%) and I_K1 (1%-9%) were observed. Meanwhile, adrenergic stimulation triggered moderate increases in all currents (23%-67%) except I_K1 . Further analyses suggest AP plateau is most sensitive to modulations in I_to and I_Ca while late repolarization is most sensitive to I_K1 , I_Ca , and I_Ks , with alterations in I_Ks predominantly stimulating the greatest magnitude of influence on late repolarization (35%-846% APD₉₀ prolongation). The modified Leeds rat model (mLR) is capable of accurately modeling APs during physiological stress. This study highlights the importance of I_Ca , I_to , I_K1, and I_Ks in controlling electrophysiological responses to exercise. This work will benefit the study of cardiac dysfunction, arrythmia, and disease, though future physiologically relevant experimental studies and model development are required.

Action potential variability in human pluripotent stem cell-derived cardiomyocytes obtained from healthy donors

Human pluripotent stem cells (PSC) have been used for disease modelling, after differentiation into the desired cell type. Electrophysiologic properties of cardiomyocytes derived from pluripotent stem cells are extensively used to model cardiac arrhythmias, in cardiomyopathies and channelopathies. This requires strict control of the multiple variables that can influence the electrical properties of these cells. In this article, we report the action potential variability of 780 cardiomyocytes derived from pluripotent stem cells obtained from six healthy donors. We analyze the overall distribution of action potential (AP) data, the distribution of action potential data per cell line, per differentiation protocol and batch. This analysis indicates that even using the same cell line and differentiation protocol, the differentiation batch still affects the results. This variability has important implications in modeling arrhythmias and imputing pathogenicity to variants encountered in patients with arrhythmic diseases. We conclude that even when using isogenic cell lines to ascertain pathogenicity to variants associated to arrythmias one should use cardiomyocytes derived from pluripotent stem cells using the same differentiation protocol and batch and pace the cells or use only cells that have very similar spontaneous beat rates. Otherwise, one may find phenotypic variability that is not attributable to pathogenic variants.

Incorrectly corrected? QT interval analysis in rats and mice

QT interval, a surrogate measure for ventricular action potential duration (APD) in the surface ECG, is widely used to identify cardiac abnormalities and drug safety. In humans, cardiac APD and QT interval are prominently affected by heart rate (HR), leading to widely accepted formulas to correct the QT interval for HR changes (QT corrected - QTc). While QTc is widely used in the clinic, the proper way to correct the QT interval in small mammals such as rats and mice is not clear. Over the years, empiric correction formulas were developed for rats and mice, which are widely used in the literature. Recent experimental findings obtained from pharmacological and direct pacing experiments in unanesthetized rodents show that the rate-adaptation properties are markedly different from those in humans and the use of existing QTc formulae can lead to major errors in data interpretation. In the present review, these experimental findings are summarized and discussed.