Abstract
The inward rectifier potassium current (IK1) is generally thought to suppress cardiac automaticity by hyperpolarizing membrane potential (MP). We recently observed that IK1 could promote the spontaneously-firing automaticity induced by upregulation of pacemaker funny current (If) in adult ventricular cardiomyocytes (CMs). However, the intriguing ability of IK1 to activate If and thereby promote automaticity has not been explored. In this study, we combined mathematical and experimental assays and found that only IK1 and If, at a proper-ratio of densities, were sufficient to generate rhythmic MP-oscillations even in unexcitable cells (i.e. HEK293T cells and undifferentiated mouse embryonic stem cells [ESCs]). We termed this effect IK1-induced If activation. Consistent with previous findings, our electrophysiological recordings observed that around 50% of mouse (m) and human (h) ESC-differentiated CMs could spontaneously fire action potentials (APs). We found that spontaneously-firing ESC-CMs displayed more hyperpolarized maximum diastolic potential and more outward IK1 current than quiescent-yet-excitable m/hESC-CMs. Rather than classical depolarization pacing, quiescent mESC-CMs were able to fire APs spontaneously with an electrode-injected small outward-current that hyperpolarizes MP. The automaticity to spontaneously fire APs was also promoted in quiescent hESC-CMs by an IK1-specific agonist zacopride. In addition, we found that the number of spontaneously-firing m/hESC-CMs was significantly decreased when If was acutely upregulated by Ad-CGI-HCN infection. Our study reveals a novel role of IK1 promoting the development of cardiac automaticity in m/hESC-CMs through a mechanism of IK1-induced If activation and demonstrates a synergistic interaction between IK1 and If that regulates cardiac automaticity.