Syncytium cell growth increases IK1 contribution in human iPS-cardiomyocytes

Human induced pluripotent stem-cell-derived cardiomyocytes (hiPS-CMs) enable cardiotoxicity testing and personalized medicine. However, their maturity is of concern, including relatively depolarized resting membrane potential and more spontaneous activity compared to adult cardiomyocytes, implicating low or lacking inward-rectifier potassium current (Ik1). Here, protein quantification confirms Ik1 expression in hiPS-CM syncytia, albeit several times lower than in adult heart tissue. We find that hiPS-CM cell culture density influences Ik1 expression and the associated electrophysiology phenotype. All-optical cardiac electrophysiology and pharmacological treatments reveal reduction of spontaneous and irregular activity in denser cultures. Blocking Ik1 with BaCl2 increased spontaneous frequency and blunted action potential upstrokes during pacing in a dose-dependent manner only in the highest-density cultures, in line with Ik1s role in regulating the resting membrane potential. Our results emphasize the importance of syncytial growth of hiPS-CM for more physiologically-relevant phenotype and the power of all-optical electrophysiology to study cardiomyocytes in their multicellular setting.