Fig. 2.

Schematic model of Ca²⁺ handling and AP generation in early stage ESC-CMs and mature cardiomyocytes. In early stage ESC-CMs, T-tubules are not developed and RyRs are poorly coupled to VDCC. Instead, IP₃R is well coupled to NCX and the IP₃R-mediated Ca²⁺ release predominantly drives spontaneous Ca²⁺ oscillations which subsequently cause spontaneous beating even under high extracellular K⁺ condition.³¹ Trans-sarcolemmal pathways such as TRPC3 and VDCC also significantly contribute to IP₃R-mediated Ca²⁺ release. Ca²⁺ removal depends on both NCX and SERCA in the SR. NCX is believed to translate the spontaneous Ca²⁺ oscillations into membrane potential fluctuations.³² If the fluctuation of membrane depolarization reaches a threshold for VDCC, an AP is generated. Ca²⁺ oscillation is essential for maturation of T-tubule and RyR-VDCC coupling.³⁴ Therefore, the RyR-mediated Ca²⁺ release is dominant in mature cardiomyocytes.

AP, action potential; ESC-CMs, embryonic stem cell-derived cardiomyocytes; IP₃, inositol-1,4,5-trisphosphate; IP₃R, inositol-1,4,5-trisphosphate receptor; NCX, Na⁺/Ca²⁺ exchanger; RyRs, ryanodine receptor channels; SERCA, sarcoplasmic reticulum Ca²⁺-ATPase; SR, sarcoplasmic reticulum; T-tubule, transverse tubule; VDCC, voltage-dependent L-type Ca²⁺ channel.