Mechanism of automaticity in cardiomyocytes derived from human induced pluripotent stem cells

The creation of cardiomyocytes derived from human induced pluripotent stem cells (hiPS-CMs) has spawned broad excitement borne out of the prospects to diagnose and treat cardiovascular diseases based on personalized medicine. A common feature of hiPS-CMs is their spontaneous contractions but the mechanism(s) remain uncertain. Methods. Intrinsic activity was investigated by the voltage-clamp technique, optical mapping of action potentials (APs) and intracellular Ca²⁺ (Ca_i) transients (Ca_iT) at subcellular-resolution and pharmacological interventions. Results. The frequency of spontaneous Ca_iT (sCa_iT) in monolayers of hiPS-CMs was not altered by ivabradine, an inhibitor of the pacemaker current, I_f despite high levels of HCN transcripts (1-4). HiPS-CMs had negligible I_f and I_K1 (inwardly-rectifying K⁺-current) and a minimum diastolic potential of -59.1±3.3mV (n=18). APs upstrokes were preceded by a depolarizing-foot coincident with a rise of Ca_i. Subcellular Ca_i wavelets varied in amplitude, propagated and died-off; larger Ca_i-waves triggered cellular sCaTs and APs. SCa_iTs increased in frequency with [Ca²⁺]_out (0.05-to-1.8mM), isoproterenol (1μM) or caffeine (100μM) (n≥5, p<0.05). HiPS-CMs became quiescent with ryanodine receptor stabilizers (K201=2μM); tetracaine; Na-Ca exchange (NCX) inhibition (SEA0400=2μM); higher [K⁺]_out (5→8mM), and thiol-reducing agents but could still be electrically stimulated to elicit Ca_iTs. Cell-cell coupling of hiPS-CM in monolayers was evident from connexin-43 expression and Ca_iT propagation. SCa_iTs from an ensemble of dispersed hiPS-CMs were out-of-phase but became synchronous through the outgrowth of inter-connecting microtubules. Conclusions. Automaticity in hiPS-CMs originates from a Ca²⁺-clock mechanism involving Ca²⁺ cycling across the sarcoplasmic reticulum linked to NCX to trigger APs.