Since cardiac transplantation is limited by the small availability of donor organs, regeneration of the diseased myocardium by cell transplantation is an attractive therapeutic modality. To determine the compatibility of human embryonic stem cellderived cardiomyocytes (hESC-CMs) (7 to 55 days old) with the myocardium, we investigated their functional properties regarding intracellular Ca ] i transient and contraction, indicating that at this developmental stage, contraction depends on transsarcolemmal Ca 2؉ influx rather than on sarcoplasmic reticulum Ca 2؉ release. Third, in agreement with the notion that a voltage-dependent Ca 2؉ current is present in hESC-CMs and contributes to the mechanical function, verapamil completely blocked contraction. Fourth, whereas hESC-CMs expressed SERCA2 and NCX at levels comparable to those of the adult porcine myocardium, calsequestrin and phospholamban were not expressed. Our study shows for the first time that functional properties related to intracellular Ca 2؉ handling of hESC-CMs differ markedly from the adult myocardium, probably due to immature sarcoplasmic reticulum capacity. STEM CELLS 2006;24:236 -245

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Regeneration of the diseased myocardium by cardiac cell transplantation is an attractive therapeutic modality. Yet, because the transplanted cardiomyocytes should functionally integrate within the diseased myocardium, it is preferable that their properties resemble those of the host. To determine the functional adaptability of human embryonic stem cell-derived cardiomyocytes (hESC-CM) to the host myocardium, the authors investigated the excitation-contraction (E-C) coupling and the responsiveness to common physiological stimuli. The main findings are: (1) hESC-CM readily respond to electrical pacing and generate corresponding [Ca(2+)](i) transients (measured by fura-2 fluorescence) and contractions (measured by video edge detector). (2) In contrast to the mature myocardium, hESC-CM display negative force-frequency relations. (3) The hESC-CM contraction is dependent on [Ca(2+)](o) and blocked by verapamil. (4) Surprisingly, ryanodine, the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin, and caffeine do not affect the [Ca(2+)](i) transient or contraction. Collectively, these results indicate that at the developmental stage of 45 to 60 days, the contraction is largely dependent on [Ca(2+)](o) rather than on sarcoplasmic reticulum (SR) Ca(2+) stores. The results show for the first time that the E-C coupling properties of hESC-CM differ from the adult myocardium, probably due to immature SR function. Based on these findings, genetic manipulation of hESC-CM toward the adult myocardium should be considered.

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The reported prevalence of AEF among Canadian AF ablators is 0.07 %. AEF was associated with high mortality. The use of GA and non-brushing movements during posterior wall ablation were two factors associated with AEF.

Gap junctions produce low resistance pathways between cardiomyocytes and are major determinants of electrical conduction in the heart. Altered distribution and function of connexin 43 (Cx43), the major gap junctional protein in the ventricles, can slow conduction, and thus contribute to arrhythmogenesis in experimental models such as ischemic rat heart and pacing-induced atrial fibrillation. The mechanisms underlying reduced gap junctional density and conductance during ischemia may involve decreased Cx43 synthesis, increased degradation and/or Cx43 migration into areas which do not contribute to intercellular communication. To test more rigorously the hypothesis that hypoxia resulting from ischemia causes Cx43 internalization, we infected neonatal rat ventricular myocytes (NRVM) with a Cx43-GFP chimera, which enabled us to investigate by means of confocal microscopy the effect of hypoxia (1% O 2 for 5 h) on Cx43 distribution in live cardiomyocytes. Importantly, this protocol permitted each culture to serve as its own control. To this end we used life confocal microscopy analysis to determine in the same pair of myocytes the effects of hypoxia on Cx43-GFP distribution at the gap junctional (GJ) and non-GJ areas. In support of this hypothesis, we found that compared to normoxia, 5 h of hypoxia reduced the Cx43-GFP signal at the GJ areas (defined as the border area) and caused a corresponding increase in the Cx43-GFP signal at the non-border areas, thus providing an additional explanation for the intercellular uncoupling during ischemic conditions.