Background—: Hypertrophic cardiomyopathy (HCM), the most common mendelian heart disorder, remains an orphan of disease-specific pharmacological treatment because of the limited understanding of cellular mechanisms underlying arrhythmogenicity and diastolic dysfunction. Methods and Results—: We assessed the electromechanical profile of cardiomyocytes from 26 HCM patients undergoing myectomy compared with those from nonfailing nonhypertrophic surgical patients by performing patch-clamp and intracellular Ca ²⁺ (Ca ²⁺ _i ) studies. Compared with controls, HCM cardiomyocytes showed prolonged action potential related to increased late Na ⁺ ( I _NaL ) and Ca ²⁺ ( I _CaL ) currents and decreased repolarizing K ⁺ currents, increased occurrence of cellular arrhythmias, prolonged Ca ²⁺ _i transients, and higher diastolic Ca ²⁺ _i . Such changes were related to enhanced Ca ²⁺ /calmodulin kinase II (CaMKII) activity and increased phosphorylation of its targets. Ranolazine at therapeutic concentrations partially reversed the HCM-related cellular abnormalities via I _NaL inhibition, with negligible effects in controls. By shortening the action potential duration in HCM cardiomyocytes, ranolazine reduced the occurrence of early and delayed afterdepolarizations. Finally, as a result of the faster kinetics of Ca ²⁺ _i transients and the lower diastolic Ca ²⁺ _i , ranolazine accelerated the contraction-relaxation cycle of HCM trabeculae, ameliorating diastolic function. Conclusions—: We highlighted a specific set of functional changes in human HCM myocardium that stem from a complex remodeling process involving alterations of CaMKII-dependent signaling, rather than being a direct consequence of the causal sarcomeric mutations. Among the several ion channel and Ca ²⁺ _i handling proteins changes identified, an enhanced I _NaL seems to be a major contributor to the electrophysiological and Ca ²⁺ _i dynamic abnormalities of ventricular myocytes and trabeculae from patients with HCM, suggesting potential therapeutic implications of I _NaL inhibition.

Cardiomyocytes derived from human embryonic stem cells constitute a promising cell source for the regeneration of damaged hearts. The assessment of their in vitro functional properties is mandatory to envisage appropriate cardiac cell-based therapies. In this study, we characterized human embryonic stem cell-derived cardiomyocytes over a 3-month period, using patch-clamp or intracellular recordings to assess their functional maturation and reverse transcriptase-polymerase chain reaction to evaluate the expression of ion channel-encoding subunits. Ito1 and IK1, the transient outward and inward rectifier potassium currents, were present in cardiomyocytes only, whereas the rapid delayed rectifier potassium current (IKr), pacemaker current (If), and L-type calcium current (ICa,L) could be recorded both in undifferentiated human embryonic stem cells and in cardiomyocytes. Most of the currents underwent developmental maturation in cardiomyocytes, as assessed by modifications in current density (Ito1, IK1, and ICa,L) and properties (If). Ion-channel mRNAs were always present when the current was recorded. Intracellular recordings in spontaneously beating clusters of cardiomyocytes revealed changes in action potential parameters and in response to pharmacological tools according to time of differentiation. In summary, human embryonic stem cell-derived cardiomyocytes mature over time during in vitro differentiation, approaching an adult phenotype.Disclosure of potential conflicts of interest is found at the end of this article.

HL‐1 cells are adult mouse atrial myocytes induced to proliferate indefinitely by SV40 large T antigen. These cells beat spontaneously when confluent and express several adult cardiac cell markers including the outward delayed rectifier K⁺ channel. Here, we examined the presence of a hyperpolarization‐activated I_f current in HL‐1 cells using the whole‐cell patch‐clamp technique on isolated cells enzymatically dissociated from the culture at confluence. Cell membrane capacitance (C_m) ranged from 5 to 53 pF. I_f was detected in about 30 % of the cells and its occurrence was independent of the stage of the culture. I_f maximal slope conductance was 89.7 ± 0.4 pS pF⁻¹ (n= 10). I_f current in HL‐1 cells showed typical characteristics of native cardiac I_f current: activation threshold between −50 and −60 mV, half‐maximal activation potential of −83.1 ± 0.7 mV (n= 50), reversal potential at −20.8 ± 1.5 mV (n= 10), time‐dependent activation by hyperpolarization and blockade by 4 mm Cs⁺. In half of the cells tested, activation of adenylyl cyclase by the forskolin analogue L858051 (20 μm) induced both a ≈6 mV positive shift of the half‐activation potential and a ≈37 % increase in the fully activated I_f current. RT‐PCR analysis of the hyperpolarization‐activated, cyclic nucleotide‐gated channels (HCN) expressed in HL‐1 cells demonstrated major contributions of HCN1 and HCN2 channel isoforms to I_f current. Cytosolic Ca²⁺ oscillations in spontaneously beating HL‐1 cells were measured in Fluo‐3 AM‐loaded cells using a fast‐scanning confocal microscope. The oscillation frequency ranged from 1.3 to 5 Hz and the spontaneous activity was stopped in the presence of 4 mm Cs⁺. Action potentials from HL‐1 cells had a triangular shape, with an overshoot at +15 mV and a maximal diastolic potential of −69 mV, i.e. more negative than the threshold potential for I_f activation. In conclusion, HL‐1 cells display a hyperpolarization‐activated I_f current which might contribute to the spontaneous contractile activity of these cells.