MEAs offer the potential for moderate

MEAs offer the potential for moderate-throughput characterization of the electrophysiological and pharmacological properties of iPSC-derived CMs. Analysis of the pharmacological effects of various antiarrhythmic agents provides additional insight into the underlying electrophysiology of these CMs. For example, the slowing of conduction velocity by LIDO indicates an essential role for Na+ Mifepristone in impulse propagation, an important feature of mature CMs. Moreover, the sensitivity of diltiazem to slow the spontaneous cycle length is consistent with the known role for L-type Ca2+ channels in modulating pacemaker activity. Interestingly, we found no specific alterations in the measured electrophysiological parameters in response to increasing concentrations of amiodarone. Although considered a class III antiarrhythmic agent, amiodarone’s effects are complex and vary with respect to acute and chronic administration. There is considerable variation in the degree of APD modulation induced by acute exposure to amiodarone, with reports of shortening, prolongation, and no effect on APD (Kodama et al., 1997). Acute amiodarone blocks inward Na+ and Ca2+ currents as well as delayed rectifier K+ currents, and some of the effects are rate dependent (Kato et al., 1988; Kodama et al., 1996). We speculate that the multiple channel and receptor-blocking effects of amiodarone result in competing effects that ultimately cause no net change in our measured MEA parameters. Finally, ARI prolongation and the induction of triggered depolarizations by dofetilide underscore the importance of IKr in repolarization of CMs. The pharmacological effects of antiarrhythmic agents in PBMC-derived iPSC CMs described here are similar to those reported for CMs differentiated from fibroblast-derived iPSCs (Braam et al., 2013; Liang et al., 2013; Navarrete et al., 2013). These observations further support the notion that epigenetic memory based on the somatic cell source may not have important functional consequences for iPSC-derived CMs, with respect to basic electrophysiology. Whether epigenetic memory plays an important role in other physiologic or metabolic processes remains to be determined. In addition, we found that individual iPSC lines derived from the same patient displayed similar electrophysiological features and pharmacological responses. These findings have important implications for determining how many cell lines are necessary for proper characterization of disease-specific iPSC-based models of cardiovascular disease. MEA-based analyses are an important tool to characterize patient- or disease-specific responses to pharmacological interventions and provide a platform for in vitro preclinical screening for QT interval-prolonging drugs. As such, proper measurement of repolarization is critical for the accurate determination of risk assessment for drug development and a mechanistic understanding of arrhythmia models. The extracellular field potential is determined by the transmembrane currents that shape the AP of individual cells within the vicinity of the electrode. However, the currently accepted standard measures of repolarization do not precisely relate to local repolarization at the level of the transmembrane AP. For example, most published stem cell-derived myocyte studies report the time at the “end” of the repolarization wave (field potential duration), as if one were measuring the end of a T wave on a surface electrocardiogram (ECG) (Itzhaki et al., 2011, 2012; Lahti et al., 2012; Liang et al., 2013; Malan et al., 2011; Matsa et al., 2011; Moretti et al., 2010; Navarrete et al., 2013; Yazawa et al., 2011). Alternatively, the time to peak of the repolarization wave is reported (Zwi et al., 2009). Although both measurements track repolarization in a general sense, neither reflects an accurate correlate of local APD. Moreover, despite the lack of any data supporting its applicability, the Bazett correction factor is often applied to the field potential duration measurement to correct for cycle length changes. By contrast, the ARI is an established measure of local repolarization that has been extensively validated in animal models and human studies, using simultaneously acquired intracellular recordings and monophasic APD recordings (Ajijola et al., 2013; Chinushi et al., 2001; Compton et al., 1996; Fuller et al., 2000a, 2000b; Haws and Lux, 1990; Nosten et al., 1993; Vaseghi et al., 2013; Yue et al., 2004). The ARI directly correlates with the duration between the times of steepest transmembrane AP upstroke and downstroke (Chinushi et al., 2001; Fuller et al., 2000b; Haws and Lux, 1990). The steepest portion of phase 3 AP repolarization reflects the timing of APD50 and is expressed in local electrograms (field potentials) as the time of steepest upstroke of the T wave. Thus, ARI measurements have a direct correlate with transmembrane APs (i.e., APD50) in cells local to the recording electrode, as opposed to field potential duration, a global assessment that reflects the time at which all cells across the electrode array have repolarized. Based on our work here and published elsewhere (Ajijola et al., 2013; Chinushi et al., 2001; Compton et al., 1996; Fuller et al., 2000a, 2000b; Haws and Lux, 1990; Nosten et al., 1993; Vaseghi et al., 2013; Yue et al., 2004), we propose the ARI as the gold standard measure of repolarization to most accurately define in vitro repolarization in MEA analyses of iPSC CMs.