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.

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.

“Pacemaker” f-channels mediating the hyperpolarization-activated nonselective cation current I _f are directly regulated by cAMP. Accordingly, the activity of f-channels increases when cellular cAMP levels are elevated (e.g., during sympathetic stimulation) and decreases when they are reduced (e.g., during vagal stimulation). Although these biophysical properties seem to make f-channels ideal molecular targets for heart rate regulation by the autonomic nervous system, the exact contribution of the major I _f -mediating cardiac isoforms HCN2 and HCN4 to sinoatrial node (SAN) function remains highly controversial. To directly investigate the role of cAMP-dependent regulation of hyperpolarization activated cyclic nucleotide activated (HCN) channels in SAN activity, we generated mice with heart-specific and inducible expression of a human HCN4 mutation (573X) that abolishes the cAMP-dependent regulation of HCN channels. We found that hHCN4–573X expression causes elimination of the cAMP sensitivity of I _f and decreases the maximum firing rates of SAN pacemaker cells. In conscious mice, hHCN4–573X expression leads to a marked reduction in heart rate at rest and during exercise. Despite the complete loss of cAMP sensitivity of I _f , the relative extent of SAN cell frequency and heart rate regulation are preserved. Our data demonstrate that cAMP-mediated regulation of I _f determines basal and maximal heart rates but does not play an indispensable role in heart rate adaptation during physical activity. Our data also reveal the pathophysiologic mechanism of hHCN4–573X–linked SAN dysfunction in humans.

Significance: T ransient R eceptor P otential M elastatin 7 (TRPM7) is a divalent-permeant channel-kinase of unknown function expressed in human atrial myocytes and fibroblasts and recently implicated in atrial arrhythmias. We show that TRPM7 is highly expressed in embryonic myocardium and sinoatrial node (SAN). Trpm7 disruption in vitro, in cultured embryonic cardiomyocytes, and in vivo in zebrafish and in mice impairs cardiac automaticity. We show that this occurs via reductions in Hcn4 mRNA and the pacemaker current, I _f , in SAN. We conclude that TRPM7 influences diastolic membrane depolarization and automaticity in SAN via regulation of Hcn4 expression.

Background—: Transient receptor potential ( TRP ) channels are a superfamily of broadly expressed ion channels with diverse physiological roles. TRPC1, TRPC3 , and TRPC6 are believed to contribute to cardiac hypertrophy in mouse models. Human mutations in TRPM4 have been linked to progressive familial heart block. TRPM7 is a divalent-permeant channel and kinase of unknown function, recently implicated in the pathogenesis of atrial fibrillation; however, its function in ventricular myocardium remains unexplored. Methods and Results—: We generated multiple cardiac-targeted knockout mice to test the hypothesis that TRPM7 is required for normal ventricular function. Early cardiac Trpm7 deletion (before embryonic day 9; TnT / Isl1-C re) results in congestive heart failure and death by embryonic day 11.5 as a result of hypoproliferation of the compact myocardium. Remarkably, Trpm7 deletion late in cardiogenesis (about embryonic day 13; αMHC-Cre ) produces viable mice with normal adult ventricular size, function, and myocardial transcriptional profile. Trpm7 deletion at an intermediate time point results in 50% of mice developing cardiomyopathy associated with heart block, impaired repolarization, and ventricular arrhythmias. Microarray analysis reveals elevations in transcripts of hypertrophy/remodeling genes and reductions in genes important for suppressing hypertrophy ( Hdac9) and for ventricular repolarization ( Kcnd2 ) and conduction ( Hcn4 ). These transcriptional changes are accompanied by action potential prolongation and reductions in transient outward current ( I _to ; Kcnd2 ). Similarly, the pacemaker current ( I _f ; Hcn4 ) is suppressed in atrioventricular nodal cells, accounting for the observed heart block. Conclusions—: Trpm7 is dispensable in adult ventricular myocardium under basal conditions but is critical for myocardial proliferation during early cardiogenesis. Loss of Trpm7 at an intermediate developmental time point alters the myocardial transcriptional profile in adulthood, impairing ventricular function, conduction, and repolarization.

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 ...