Dynamic Current Clamp Experiments Define the Functional Roles of IK1 and Ito,f in Human Induced Pluripotent Stem Cell Derived Cardiomyocytes

The transient outward potassium current (Ito) plays a key, albeit incompletely defined, role in cardiomyocyte physiology and pathophysiology. In light of the technical challenges of studying adult human cardiomyocytes, this study examines the use of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) as a system which potentially preserves the native cellular milieu of human cardiomyocytes. ISPC-CMs express a robust Ito with slow recovery kinetics and fail to express the rapidly recovering Ito,f which is implicated in human disease. Overexpression of the accessory subunit KChIP2 (which is not expressed in iPSC-CMs) resulted in restoration of a rapid component of recovery. To define the functional role of Ito, dynamic current clamp was used to introduce computationally modeled currents into iPSC-CMs while recording action potentials. However, iPSC-CMs exhibit action potentials with multiple immature physiological properties, including slow upstroke velocity, heterogeneous action potential waveforms, and the absence of a phase 1 notch, thus potentially limiting the utility of these cells as a model of adult cardiomyocytes. Importantly, the introduction of modeled inwardly rectified current (IK1) ameliorated these immature properties by restoring a hyperpolarized resting membrane potential. In this context of normalized action potential morphologies, dynamic current clamp experiments introducing Ito,f demonstrated that there is significant cell-to-cell heterogeneity and that the functional effect of Ito,f is highly sensitive to the action potential plateau voltage in each cell.