Previous studies investigating human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) have proposed the distinction of heart chamber-specific (atrial, ventricular, pacemaker) electrophysiological phenotypes based on action potential (AP) morphology. This suggestion has been based on data acquired using techniques that allow measurements from only a small number of cells and at low seeding densities. It has also been observed that density of culture affects the properties of iPSC-CMs. Here we systematically analyze AP morphology from iPSC-CMs at two seeding densities: 60,000 cells/well (confluent monolayer) and 15,000 cells/well (sparsely-seeded) using a noninvasive optical method. The confluent cells (n = 360) demonstrate a series of AP morphologies on a normally distributed spectrum with no evidence for specific subpopulations. The AP morphologies of sparsely seeded cells (n = 32) displayed a significantly different distribution, but even in this case there is no clear evidence of chamber-specificity. Reduction in gap junction conductance using carbenoxolone only minimally affected APD distribution in confluent cells. These data suggest that iPSC-CMs possess a sui generis AP morphology, and when observed in different seeding densities may encompass any shape including those resembling chamber-specific subtypes. These results may be explained by different functional maturation due to culture conditions.

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