Induction of cardiac alternans in human iPS-derived cardiomyocytes through β-adrenergic receptor stimulation

Cardiac alternans (C-ALT) is a phenomenon of alternating strong and weak contractions in the heart and is considered a risk factor for the development of heart failure and arrhythmias. However, no model has been reported that can induce C-ALT in vitro using human cells, and the developmental mechanism of C-ALT has not been studied using human cells. In this study, we successfully induced C-ALT in vitro using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). By stimulating β-adrenergic receptor with isoproterenol on hiPSC-CMs cultured in atmospheric condition (with ~0.04% CO₂), contractility and calcium transient were observed to alternately increase and decrease with each beat. In contrast, C-ALT was not induced in hiPSC-CMs cultured at 5% CO₂ concentration. Since previous studies have linked C-ALT to problems with calcium regulation in the sarcoplasmic reticulum (SR), we exposed hiPSC-CMs to compounds that alter SR Ca²⁺ loading and analyzed their contractile responses. The results showed that exposure to verapamil, thapsigargin, and ryanodine either suppressed or eliminated C-ALT. In contrast, omecamtiv mecarbil and blebbistatin, which alter contractility without SR Ca²⁺ loading, did not induce or suppress C-ALT. These results suggest that C-ALT in hiPSC-CMs induced by isoproterenol may be due to abnormal regulation of the ryanodine receptor's opening and closing caused by excessive Ca²⁺ load in the SR from β-adrenergic receptor stimulation.