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

Background and objectives. 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 (CaiT) at subcellular-resolution and pharmacological interventions. Results. The frequency of spontaneous Calf (sCa(i)T) in monolayers of hiPS-CMs was not altered by ivabradine, an inhibitor of the pacemaker current, If 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 +/- 3.3 mV (n = 18). APs upstrokes were preceded by a depolarizing-foot coincident with a rise of Ca-i. Subcellular Ca-1 wavelets varied in amplitude, propagated and died-off; larger Ca-i-waves triggered cellular sCaTs and APs. SCaiTs increased in frequency with [Ca2+](out) (0.05-to-1.8 mM), isoproterenol (1 mu M) or caffeine (100 mu M) (n >= 5, p < 0.05). HiPS-CMs became quiescent with ryanodine receptor stabilizers (K201 = 2 mu M); tetracaine; Na-Ca exchange (NCX) inhibition (SEA0400 = 2 mu M); higher [K+](out) (5 -> 8 mM), and thiol-reducing agents but could still be electrically stimulated to elicit CaiTs. Cell-cell coupling of hiPS-CM in monolayers was evident from connexin-43 expression and CaiT propagation. SCaiTs 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. (C) 2015 Elsevier Ltd. All rights reserved.