Functional roles of Cav1.3, Cav3.1 and HCN channels in automaticity of mouse atrioventricular cells: Insights into the atrioventricular pacemaker mechanism (original) (raw)
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Circulation Research, 2006
The generation of the mammalian heartbeat is a complex and vital function requiring multiple and coordinated ionic channel activities. The functional role of low-voltage activated (LVA) T-type calcium channels in the pacemaker activity of the sinoatrial node (SAN) is, to date, unresolved. Here we show that disruption of the gene coding for Ca v 3.1/␣ 1G T-type calcium channels (cacna1g) abolishes T-type calcium current (I Ca,T ) in isolated cells from the SAN and the atrioventricular node without affecting the L-type Ca 2ϩ current (I Ca,L ). By using telemetric electrocardiograms on unrestrained mice and intracardiac recordings, we find that cacna1g inactivation causes bradycardia and delays atrioventricular conduction without affecting the excitability of the right atrium. Consistently, no I Ca,T was detected in right atrium myocytes in both wild-type and Ca v 3.1 Ϫ/Ϫ mice. Furthermore, inactivation of cacna1g significantly slowed the intrinsic in vivo heart rate, prolonged the SAN recovery time, and slowed pacemaker activity of individual SAN cells through a reduction of the slope of the diastolic depolarization. Our results demonstrate that Ca v 3.1/T-type Ca 2ϩ channels contribute to SAN pacemaker activity and atrioventricular conduction.
Basic Research in Cardiology, 2005
Voltage-gated calcium channels are key components in cardiac electrophysiology. We demonstrate that Ca v 2.3 is expressed in mouse and human heart and that mice lacking the Ca v 2.3 voltage-gated calcium channel exhibit severe alterations in cardiac function. Amplified cDNA fragments from murine heart and single cardiomyocytes reveal the expression of three different Ca v 2.3 splice variants. The ablation of Ca v 2.3 was found to be accompanied by a compensatory upregulation of the Ca v 3.1 T-type calcium channel, while other voltage-gated calcium channels remained unaffected. Telemetric ECG recordings from Ca v 2.3 deficient mice displayed subsidiary escape rhythm, altered atrial activation patterns, atrioventricular conduction disturbances and alteration in QRS-morphology. Furthermore, time domain analysis of heart rate variability (HRV) in Ca v 2.3(-|-) mice exhibited a significant increase in heart rate as well as in the coefficient of variance (CV) compared to control mice. Administration of atropin/propranolol revealed that increased heart rate was due to enhanced sympathetic tonus and that partial decrease of CV in Ca v 2.3(-|-) mice after autonomic block was in accordance with a complete abolishment of 2 nd degree atrioventricular block. However, escape rhythms, atrial activation disturbances and QRS-dysmorphology remained unaffected, indicating that these are intrinsic cardiac features in Ca v 2.3(-|-) mice. We conclude that the expression of Ca v 2.3 is essential for normal impulse generation and conduction in murine heart.
Circulation Research, 2006
The generation of the mammalian heartbeat is a complex and vital function requiring multiple and coordinated ionic channel activities. The functional role of low-voltage activated (LVA) T-type calcium channels in the pacemaker activity of the sinoatrial node (SAN) is, to date, unresolved. Here we show that disruption of the gene coding for Ca v 3.1/α 1G T-type calcium channels ( cacna1g ) abolishes T-type calcium current ( I Ca,T ) in isolated cells from the SAN and the atrioventricular node without affecting the L-type Ca 2+ current ( I Ca,L ). By using telemetric electrocardiograms on unrestrained mice and intracardiac recordings, we find that cacna1g inactivation causes bradycardia and delays atrioventricular conduction without affecting the excitability of the right atrium. Consistently, no I Ca,T was detected in right atrium myocytes in both wild-type and Ca v 3.1 −/− mice. Furthermore, inactivation of cacna1g significantly slowed the intrinsic in vivo heart rate, prolonged the...
Functional role of L-type Cav1.3 Ca2+ channels in cardiac pacemaker activity
Proceedings of the National Academy of Sciences, 2003
The spontaneous activity of pacemaker cells in the sino-atrial node (SAN) controls the heart rhythm and rate under physiological conditions. Pacemaker activity in SAN cells is due to the presence of the diastolic depolarization, a slow depolarization phase that drives the membrane voltage from the end of an action potential to the threshold of a new action potential. SAN cells express a wide array of ionic channels, but we have limited knowledge about their functional role in pacemaker activity and we still do not know which channels play a prominent role in the generation of the diastolic depolarization. It is thus important to provide genetic evidence linking the activity of genes coding for ionic channels to specific alterations of pacemaker activity of SAN cells. Here, we show that target inactivation of the gene coding for ␣1D (Cav1.3) Ca 2؉ channels in the mouse not only significantly slows pacemaker activity but also promotes spontaneous arrhythmia in SAN pacemaker cells. These alterations of pacemaker activity are linked to abolition of the major component of the L-type current (I Ca,L) activating at negative voltages. Pharmacological analysis of I Ca,L demonstrates that Cav1.3 gene inactivation specifically abolishes I Ca,L in the voltage range corresponding to the diastolic depolarization. Taken together, our data demonstrate that Ca v1.3 channels play a major role in the generation of cardiac pacemaker activity by contributing to diastolic depolarization in SAN pacemaker cells.
Journal of the American Heart Association, 2018
Background-Attenuated cardiac vagal activity is associated with ventricular arrhythmogenesis and related mortality in patients with chronic heart failure. Our recent study has shown that expression of N-type Ca 2+ channel a-subunits (Ca v 2.2-a) and N-type Ca 2+ currents are reduced in intracardiac ganglion neurons from rats with chronic heart failure. Rat intracardiac ganglia are divided into the atrioventricular ganglion (AVG) and sinoatrial ganglion. Ventricular myocardium receives projection of neuronal terminals only from the AVG. In this study we tested whether a decrease in N-type Ca 2+ channels in AVG neurons contributes to ventricular arrhythmogenesis. Methods and Results-Lentiviral Ca v 2.2-a shRNA (2 lL, 2910 7 pfu/mL) or scrambled shRNA was in vivo transfected into rat AVG neurons. Nontransfected sham rats served as controls. Using real-time single-cell polymerase chain reaction and reverse-phase protein array, we found that in vivo transfection of Ca v 2.2-a shRNA decreased expression of Ca v 2.2-a mRNA and protein in rat AVG neurons. Whole-cell patch-clamp data showed that Ca v 2.2-a shRNA reduced N-type Ca 2+ currents and cell excitability in AVG neurons. The data from telemetry electrocardiographic recording demonstrated that 83% (5 out of 6) of conscious rats with Ca v 2.2a shRNA transfection had premature ventricular contractions (P<0.05 versus 0% of nontransfected sham rats or scrambled shRNAtransfected rats). Additionally, an index of susceptibility to ventricular arrhythmias, inducibility of ventricular arrhythmias evoked by programmed electrical stimulation, was higher in rats with Ca v 2.2-a shRNA transfection compared with nontransfected sham rats and scrambled shRNA-transfected rats. Conclusions-A decrease in N-type Ca 2+ channels in AVG neurons attenuates vagal control of ventricular myocardium, thereby initiating ventricular arrhythmias.
T-type channels in the sino-atrial and atrioventricular pacemaker mechanism
Pflügers Archiv - European Journal of Physiology, 2014
Cardiac automaticity is a fundamental physiological function in vertebrates. Heart rate is under the control of several neurotransmitters and hormones and is permanently adjusted by the autonomic nervous system to match the physiological demand of the organism. Several classes of ion channels and proteins involved in intracellular Ca 2+ handling contribute to pacemaker activity. Voltage-dependent T-type Ca 2+ channels are an integral part of the complex mechanism underlying pacemaking. T-type channels also contribute to impulse conduction in mice and humans. Strikingly, T-type channel isoforms are coexpressed in the cardiac conduction system with other ion channels that play a major role in pacemaking such as f-(HCN4) and L-type Ca v 1.3 channels. Pharmacologic inhibition of T-type channels reduces the spontaneous activity of isolated pacemaker myocytes of the sino-atrial node, the dominant heart rhythmogenic centre. Target inactivation of T-type Ca v 3.1 channels abolishes I Ca,T in both sino-atrial and atrioventricular myocytes and reduces the daily heart rate of freely moving mice. Ca v 3.1 channels contribute also to automaticity of the atrioventricular node and to ventricular escape rhythms, thereby stressing the importance of these channels in automaticity of the whole cardiac conduction system. Accordingly, loss-of-function of Ca v 3.1 channels contributes to severe form of congenital bradycardia and atrioventricular block in paediatric patients.
Cell Biochemistry and Function, 2012
Voltage-gated Ca 2+ channels regulate cardiac automaticity, rhythmicity and excitation-contraction coupling. Whereas L-type (Ca v 1Á2, Ca v 1Á3) and T-type (Ca v 3Á1, Ca v 3Á2) channels are widely accepted for their functional relevance in the heart, the role of Ca v 2Á3 Ca 2+ channels expressing R-type currents remains to be elucidated. We have investigated heart rate dynamics in control and Ca v 2Á3-deficient mice using implantable electrocardiogram radiotelemetry and pharmacological injection experiments. Autonomic block revealed that the intrinsic heart rate does not differ between both genotypes. Systemic administration of isoproterenol resulted in a significant reduction in interbeat interval in both genotypes. It remained unaffected after administering propranolol in Ca v 2Á3(À|À) mice. Heart rate from isolated hearts as well as atrioventricular conduction for both genotypes differed significantly. Additionally, we identified and analysed the developmental expression of two splice variants, i.e. Ca v 2Á3c and Ca v 2Á3e. Using patch clamp technology, R-type currents could be detected in isolated prenatal cardiomyocytes and be related to R-type Ca 2+ channels. Our results indicate that on the systemic level, the pharmacologically inducible heart rate range and heart rate reserve are impaired in Ca v 2Á3 (À|À) mice. In addition, experiments on Langendorff perfused hearts elucidate differences in basic properties between both genotypes. Thus, Ca v 2Á3 does not only contribute to the cardiac autonomous nervous system but also to intrinsic rhythm propagation.
The Journal of Physiology, 2006
Ventricular arrhythmogenesis in long QT 3 syndrome (LQT3) involves both triggered activity and re-entrant excitation arising from delayed ventricular repolarization. Effects of specific L-type Ca 2+ channel antagonism were explored in a gain-of-function murine LQT3 model produced by a ∆KPQ 1505-1507 deletion in the SCN5A gene. Monophasic action potentials (MAPs) were recorded from epicardial and endocardial surfaces of intact, Langendorff-perfused Scn5a+/∆ hearts. In untreated Scn5a+/∆ hearts, epicardial action potential duration at 90% repolarization (APD 90 ) was 60.0 ± 0.9 ms compared with 46.9 ± 1.6 ms in untreated wild-type (WT) hearts (P < 0.05; n = 5). The corresponding endocardial APD 90 values were 52.0 ± 0.7 ms and 53.7 ± 1.6 ms in Scn5a+/∆ and WT hearts, respectively (P > 0.05; n = 5). Epicardial early afterdepolarizations (EADs), often accompanied by spontaneous ventricular tachycardia (VT), occurred in 100% of MAPs from Scn5a+/∆ but not in any WT hearts (n = 10). However, EAD occurrence was reduced to 62 ± 7.1%, 44 ± 9.7%, 10 ± 10% and 0% of MAPs following perfusion with 10 nM, 100 nM, 300 nM and 1 µM nifedipine, respectively (P < 0.05; n = 5), giving an effective IC 50 concentration of 79.3 nM. Programmed electrical stimulation (PES) induced VT in all five Scn5a+/∆ hearts (n = 5) but not in any WT hearts (n = 5). However, repeat PES induced VT in 3, 2, 2 and 0 out of 5 Scn5a+/∆ hearts following perfusion with 10 nM, 100 nM, 300 nM and 1 µM nifedipine, respectively. Patch clamp studies in isolated ventricular myocytes from Scn5a+/∆ and WT hearts confirmed that nifedipine (300 nM) completely suppressed the inward Ca 2+ current but had no effect on inward Na + currents. No significant effects were seen on epicardial APD 90 , endocardial APD 90 or ventricular effective refractory period in Scn5a+/∆ and WT hearts following perfusion with nifedipine at 1 nM, 10 nM, 100 nM, 300 nM and 1 µM nifedipine concentrations. We conclude that L-type Ca 2+ channel antagonism thus exerts specific anti-arrhythmic effects in Scn5a+/∆ hearts through suppression of EADs.
The Journal of Physiology, 2012
• In the sinoatrial node (SAN), Ca v 1 voltage-gated Ca 2+ channels mediate L-type currents that are essential for normal cardiac pacemaking. • Both Ca v 1.2 and Ca v 1.3 Ca 2+ channels are expressed in the SAN but how their distinct properties affect cardiac pacemaking is unknown. • Here, we show that unlike Ca v 1.2, Ca v 1.3 undergoes voltage-dependent facilitation and colocalizes with ryanodine receptors in sarcomeric structures. • By mathematical modelling, these properties of Ca v 1.3 can improve recovery of pacemaking after pauses and stabilize SAN pacemaking during excessively slow heart rates. • We conclude that voltage-dependent facilitation and colocalization with ryanodine receptors distinguish Ca v 1.3 from Ca v 1.2 channels in the SAN and contribute to the major impact of Ca v 1.3 on pacemaking.