SK4 Ca 2+ activated K + channel is a critical player in cardiac pacemaker derived from human embryonic stem cells

The contractions of the heart are initiated and coordinated by pacemaker tissues, responsible for cardiac automaticity. Although the cardiac pacemaker was discovered more than a hundred years ago, the pacemaker mechanisms remain controversial. We used human embryonic stem cell-derived cardiomyocytes to study the embryonic cardiac automaticity of the human heart. We identified a previously unrecognized Ca 2+ -activated K + channel (SK4), which appears to play a pivotal role in cardiac automaticity. Our results suggest that SK4 Ca 2+ -activated K + channels represent an important target for the management of cardiac rhythm disorders and open challenging horizons for developing biological pacemakers. Proper expression and function of the cardiac pacemaker is a critical feature of heart physiology. Two main mechanisms have been proposed: ( i ) the “voltage-clock,” where the hyperpolarization-activated funny current I f causes diastolic depolarization that triggers action potential cycling; and ( ii ) the “Ca 2+ clock,” where cyclical release of Ca 2+ from Ca 2+ stores depolarizes the membrane during diastole via activation of the Na + –Ca 2+ exchanger. Nonetheless, these mechanisms remain controversial. Here, we used human embryonic stem cell-derived cardiomyocytes (hESC-CMs) to study their autonomous beating mechanisms. Combined current- and voltage-clamp recordings from the same cell showed the so-called “voltage and Ca 2+ clock” pacemaker mechanisms to operate in a mutually exclusive fashion in different cell populations, but also to coexist in other cells. Blocking the “voltage or Ca 2+ clock” produced a similar depolarization of the maximal diastolic potential (MDP) that culminated by cessation of action potentials, suggesting that they converge to a common pacemaker component. Using patch-clamp recording, real-time PCR, Western blotting, and immunocytochemistry, we identified a previously unrecognized Ca 2+ -activated intermediate K + conductance (IK Ca , KCa3.1, or SK4) in young and old stage-derived hESC-CMs. IK Ca inhibition produced MDP depolarization and pacemaker suppression. By shaping the MDP driving force and exquisitely balancing inward currents during diastolic depolarization, IK Ca appears to play a crucial role in human embryonic cardiac automaticity.