Biophysical and Molecular Analysis of the Sodium Current in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Background: Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have been used for safety pharmacology and to investigate genetic diseases affecting cardiac ion channels. We examined INa in hiPSC-CMs and determined its contribution to action potentials (APs) recorded from monolayers of hiPSC-CMs.Methods: Commercially available hiPSC-CMs were plated at high density to form monolayers or low density to yield single cells. AP recordings from monolayers were made using high resistance electrodes at 36°C. Whole cell patch clamp was used to record INa in single hiPSC-CMs at room temperature.Results: AP recordings showed spontaneous activity with a maximum diastolic potential (MDP)= −69.2±1.4 mV and upstroke velocity=41.9±6.7 V/s. Application of tetrodotoxin resulted in a slowing of the AP rate but had little effect on AP upstroke or duration. In single hiPSC-CMs, a large INa was recorded when external Na+ was reduced to 40 mM (73.6±6.18 pA/pF). Recovery of INa (hp= 120 mV) was very fast; at hp= 80 mV, recovery of INa was slower and the size of peak INa was greatly reduced (27.0±3.38 pA/pF). Molecular analysis showed that SCN5A was the predominate Na+ channel subtype in both adult and iPSC-CMs. In adddition, we found that iPSC-CMs express both the fetal (exon 6A) and adult (exon 6) isoforms of SCN5A. Action potential clamp experiments showed that application of a ventricular or Purkinje cell waveform to the same hiPSC-CM elicited a large INa while application of a SA node waveform elicited no INa.Conclusion: A large INa is present in hiPSC-CM but its contribution to the AP upstroke is minimal. The depolarized MDP coupled with the presence of phase 4 depolarization results in a take-off potential of −60.6±1.7 mV which inactivates the majority of Na+ channels.