SK4 Ca²⁺ activated K⁺ channel is a critical player in cardiac pacemaker derived from human embryonic stem cells

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 "Ca²⁺ clock," where cyclical release of Ca²⁺ from Ca²⁺ stores depolarizes the membrane during diastole via activation of the Na⁺-Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ 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 Ca²⁺-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.