Evaluation of Contractile Activity of Stem Cell-Derived Human Cardiomyocytes using Impedance Analysis

Abstract

Single stem cell-derived human cardiomyocyte (SC-hCM) electrophysiolo-gy is a valuable tool to assess cardiac risk associated with drugs (Peng 2010, Ma 2011). However, impedance-based measurement of SC-hCM contractile activity (Roche xCELLigence RTCA Cardio system) adds a new dimension to cardiac risk assessment as events downstream of excitation-contraction coupling can now be investigated in a higher throughput assay. SC-hCMs show spontaneous contractile activity resulting in transient changes in impedance (impedance twitches). We performed a series of pharmacological studies to investigate the mechanisms underlying this transient behavior in iCELL cardiomyocytes (Cellular Dynamics International). Here we show that impedance twitches are dependent on calcium entry since calcium channel blockers (verapamil, nifedipine) and agonists (FPL64176) decrease and increase twitch amplitude, respectively. Twitch frequency is slowed by the HCN channel blocker ZD7288 (33±8% @ 300 nM), a phenomenon associated with prolongation of relaxation by 26±7% (n=5, p<0.05), but independent of changes in amplitude. β-adrenergic stimulation increases twitch frequency and amplitude, and shortens the time to peak and twitch relaxation time, consistent with results from other cardiac preparations. Impedance twitch activity reflects contractile activity since it is completely obliterated by the myosin II inhibitor blebbistatin at concentrations that do not affect the shape or frequency of the spontaneous action potentials. These results suggest that several aspects of the excitation-contraction coupling mechanism observed in adult cardiomyocytes are intact in SC-hCMs, including the dependence of the twitch contraction on Ca2+ entry and the regulation of contraction by frequency and β-adrenergic stimulation.