Sino-atrial node (SAN) automaticity is an essential mechanism of heart rate generation that is still not completely understood. Recent studies highlighted the importance of intracellular Ca²⁺ ([Ca²⁺]_i) dynamics during SAN pacemaker activity. Nevertheless, the functional role of voltage-dependent L-type Ca²⁺ channels in controlling SAN [Ca²⁺]_i release is largely unexplored. Since Ca_v1.3 is the predominant L-type Ca²⁺ channel isoform in SAN cells, we studied [Ca²⁺]_i dynamics in isolated cells and ex vivo SAN preparations explanted from wild-type (WT) and Ca_v1.3 knockout (KO) mice (Ca_v1.3^−/−). We found that Ca_v1.3 deficiency strongly impaired [Ca²⁺]_i dynamics, reducing the frequency of local [Ca²⁺]_i release events and preventing their synchronization. This impairment inhibited the generation of Ca²⁺ transients and delayed spontaneous activity. We also used action potentials recorded in WT SAN cells as voltage-clamp commands for Ca_v1.3^−/− cells. Although these experiments showed abolished Ca²⁺ entry through L-type Ca²⁺ channels in the diastolic depolarization range of KO SAN cells, their sarcoplasmic reticulum Ca²⁺ load remained normal. β-Adrenergic stimulation enhanced pacemaking of both genotypes, though, Ca_v1.3^−/− SAN cells remained slower than WT. Conversely, we rescued pacemaker activity in Ca_v1.3^−/− SAN cells and intact tissues through caffeine-mediated stimulation of Ca²⁺-induced Ca²⁺ release. Ca_v1.3 channels play a critical role in the regulation of [Ca²⁺]_i dynamics, providing an unanticipated mechanism for triggering local [Ca²⁺]_i releases and thereby controlling pacemaker activity. Our study also provides an additional pathophysiological mechanism for congenital SAN dysfunction and heart block linked to Ca_v1.3 loss of function in humans.

Genetic deficiency of dystrophin leads to disability and premature death in Duchenne muscular dystrophy (DMD), affecting the heart as well as skeletal muscle. Here, we report that clinical-stage cardiac progenitor cells, known as cardiosphere-derived cells (CDCs), improve cardiac and skeletal myopathy in the mdx mouse model of DMD. Injection of CDCs into the hearts of mdx mice augments cardiac function, ambulatory capacity, and survival. Exosomes secreted by human CDCs reproduce the benefits of CDCs in mdx mice and in human induced pluripotent stem cell-derived Duchenne cardiomyocytes. Surprisingly, CDCs and their exosomes also transiently restored partial expression of full-length dystrophin in mdx mice. The findings further motivate the testing of CDCs in Duchenne patients, while identifying exosomes as next-generation therapeutic candidates.

Pacemaker activity of automatic cardiac myocytes controls the heartbeat in everyday life. Cardiac automaticity is under the control of several neurotransmitters and hormones and is constantly regulated by the autonomic nervous system to match the physiological needs of the organism. Several classes of ion channels and proteins involved in intracellular Ca2+ dynamics contribute to pacemaker activity. The functional role of voltage-gated calcium channels (VGCCs) in heart automaticity and impulse conduction has been matter of debate for 30 years. However, growing evidence shows that VGCCs are important regulators of the pacemaker mechanisms and play also a major role in atrio-ventricular impulse conduction. Incidentally, studies performed in genetically modified mice lacking L-type Cav1.3 (Cav1.3-/-) or T-type Cav3.1 (Cav3.1-/-) channels show that genetic inactivation of these channels strongly impacts pacemaking. In cardiac pacemaker cells, VGCCs activate at negative voltages at the beginning of the diastolic depolarization and importantly contribute to this phase by supplying inward current. Loss-of-function of these channels also impairs atrio-ventricular conduction. Furthermore, inactivation of Cav1.3 channels promotes also atrial fibrillation and flutter in knockout mice suggesting that these channels can play a role in stabilizing atrial rhythm. Genomic analysis demonstrated that Cav1.3 and Cav3.1 channels are widely expressed in pacemaker tissue of mice, rabbits and humans. Importantly, human diseases of pacemaker activity such as congenital bradycardia and heart block have been attributed to loss-of-function of Cav1.3 and Cav3.1 channels. In this article, we will review the current knowledge on the role of VGCCs in the generation and regulation of heart rate and rhythm. We will discuss also how loss of Ca2+ entry through VGCCs could influence intracellular Ca2+ handling and promote atrial arrhythmias.

The atrioventricular node controls cardiac impulse conduction and generates pacemaker activity in case of failure of the sino-atrial node. Understanding the mechanisms of atrioventricular automaticity is important for managing human pathologies of heart rate and conduction. However, the physiology of atrioventricular automaticity is still poorly understood. We have investigated the role of three key ion channel-mediated pacemaker mechanisms namely, Cav1.3, Cav3.1 and HCN channels in automaticity of atrioventricular node cells (AVNCs). We studied atrioventricular conduction and pacemaking of AVNCs in wild-type mice and mice lacking Cav3.1 (Cav3.1-/-), Cav1.3 (Cav1.3-/-), channels or both (Cav1.3-/-/Cav3.1-/-). The role of HCN channels in the modulation of atrioventricular cells pacemaking was studied by conditional expression of dominant-negative HCN4 channels lacking cAMP sensitivity. Inactivation of Cav3.1 channels impaired AVNCs pacemaker activity by favoring sporadic block of automaticity leading to ...

In sinoatrial node (SAN) cells, electrogenic sodium–calcium exchange (NCX) is the dominant calcium (Ca) efflux mechanism. However, the role of NCX in the generation of SAN automaticity is controversial. To investigate the contribution of NCX to pacemaking in the SAN, we performed optical voltage mapping and high-speed 2D laser scanning confocal microscopy (LSCM) of Ca dynamics in an ex vivo intact SAN/atrial tissue preparation from atrial-specific NCX knockout (KO) mice. These mice lack P waves on electrocardiograms, and isolated NCX KO SAN cells are quiescent. Voltage mapping revealed disorganized and arrhythmic depolarizations within the NCX KO SAN that failed to propagate into the atria. LSCM revealed intermittent bursts of Ca transients. Bursts were accompanied by rising diastolic Ca, culminating in long pauses dominated by Ca waves. The L-type Ca channel agonist BayK8644 reduced the rate of Ca transients and inhibited burst generation in the NCX KO SAN whereas the Ca buffer 1,2-Bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (acetoxymethyl ester) (BAPTA AM) did the opposite. These results suggest that cellular Ca accumulation hinders spontaneous depolarization in the NCX KO SAN, possibly by inhibiting L-type Ca currents. The funny current (I_f) blocker ivabradine also suppressed NCX KO SAN automaticity. We conclude that pacemaker activity is present in the NCX KO SAN, generated by a mechanism that depends upon I_f. However, the absence of NCX-mediated depolarization in combination with impaired Ca efflux results in intermittent bursts of pacemaker activity, reminiscent of human sinus node dysfunction and “tachy-brady” syndrome.

Background—: Catecholaminergic polymorphic ventricular tachycardia is characterized by stress-triggered syncope and sudden death. Patients with catecholaminergic polymorphic ventricular tachycardia manifest sinoatrial node (SAN) dysfunction, the mechanisms of which remain unexplored. Methods and Results—: We investigated SAN [Ca ²⁺ ] _i handling in mice carrying the catecholaminergic polymorphic ventricular tachycardia–linked mutation of ryanodine receptor (RyR2 ^R4496C ) and their wild-type (WT) littermates. In vivo telemetric recordings showed impaired SAN automaticity in RyR2 ^R4496C mice after isoproterenol injection, analogous to what was observed in catecholaminergic polymorphic ventricular tachycardia patients after exercise. Pacemaker activity was explored by measuring spontaneous [Ca ²⁺ ] _i transients in SAN cells within the intact SAN by confocal microscopy. RyR2 ^R4496C SAN presented significantly slower pacemaker activity and impaired chronotropic response under β-adrenergic stimulation, accompanied by the appearance of pauses (in spontaneous [Ca ²⁺ ] _i transients and action potentials) in 75% of the cases. Ca ²⁺ spark frequency was increased by 2-fold in RyR2 ^R4496C SAN. Whole-cell patch-clamp experiments performed on isolated RyR2 ^R4496C SAN cells showed that L-type Ca ²⁺ current ( I _Ca,L ) density was reduced by ≈50%, an effect blunted by internal Ca ²⁺ buffering. Isoproterenol dramatically increased the frequency of Ca ²⁺ sparks and waves by ≈5 and ≈10-fold, respectively. Interestingly, the sarcoplasmic reticulum Ca ²⁺ content was significantly reduced in RyR2 ^R4496C SAN cells in the presence of isoproterenol, which may contribute to stopping the “Ca ²⁺ clock” rhythm generation, originating SAN pauses. Conclusion—: The increased activity of RyR2 ^R4496C in SAN leads to an unanticipated decrease in SAN automaticity by a Ca ²⁺ -dependent decrease of I _Ca,L and sarcoplasmic reticulum Ca ²⁺ depletion during diastole, identifying subcellular pathophysiological alterations contributing to the SAN dysfunction in catecholaminergic polymorphic ventricular tachycardia patients.