Cellular calcium handling and electrophysiology are modulated by chronic physiological pacing in human induced pluripotent stem cell-derived cardiomyocytes.

Abstract

Electric pacing of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) has been increasingly used to simulate cardiac arrhythmias in vitro and to enhance cardiomyocyte maturity. However, the impact of electric pacing on cellular electrophysiology and Ca²⁺-handling in differentiated hiPSC-CM is less characterized. Here we studied the effects of electric pacing for 24h or 7d at a physiological rate of 60 bpm on cellular electrophysiology and Ca²⁺-cycling in late-stage, differentiated hiPSC-CM (>90% troponin⁺, >60d post differentiation). Electric culture pacing for 7d did not influence cardiomyocyte cell size, apoptosis or generation of reactive oxygen species in differentiated hiPSC-CM compared to 24h pacing. However, epifluorescence measurements revealed that electric pacing for 7d improved systolic Ca²⁺-transient amplitude and Ca²⁺-transient upstroke, which could be explained by elevated sarcoplasmic reticulum Ca²⁺-load and SERCA activity. Diastolic Ca²⁺-leak was not changed in line-scanning confocal microscopy suggesting that the improvement in systolic Ca²⁺-release was not associated with a higher open probability of RyR2 during diastole. While bulk cytosolic Na⁺-concentration and NCX activity were not changed, patch-clamp studies revealed that chronic pacing caused a slight abbreviation of the action potential duration (APD) in hiPSC-CM. We found in whole-cell voltage-clamp measurements that chronic pacing for 7d led to a decrease in late Na⁺-current, which might explain the changes in APD. In conclusion, our results show that chronic pacing improves systolic Ca²⁺-handling and modulates the electrophysiology of late-stage, differentiated iPSC-CM. This study might help to understand the effects of electric pacing and its numerous applications in stem cell research including arrhythmia simulation.