Bradycardia and Slowing of the Atrioventricular Conduction in Mice Lacking Ca V 3.1/α 1G T-Type Calcium Channels

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 CaV3.1/alpha1G 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 Ca2+ 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 CaV3.1(-/-) 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 CaV3.1/T-type Ca2+ channels contribute to SAN pacemaker activity and atrioventricular conduction.

If current inhibition: cellular basis and physiology

The slow diastolic depolarization phase in cardiac pacemaker cells is the electrical basis of cardiac automaticity. The hyperpolarization-activated current (I(f)) is one of the key mechanisms underlying diastolic depolarization. Particularly, I(f) is unique in being activated on membrane hyperpolarization following the repolarization phase of the action potential. I(f) has adapted biophysical properties and voltage-dependent gating to initiate pacemaker activity. I(f) possibly constitutes the first voltage-dependent trigger of the diastolic depolarization. For these reasons, I(f) is a natural pharmacological target for controlling heart rate in cardiovascular disease. In this view, I(f) inhibitors have been developed in the past, yet the only molecule to have reached the clinical development is ivabradine. At the cellular level, the remarkable success of ivabradine is to be ascribed to its relatively high affinity for f-channels. Furthermore, ivabradine is the most I(f)-specific inhibitor known to date, since moderate inhibition of other voltage-dependent ionic currents involved in automaticity can be observed only at very high concentrations of ivabradine, more than one order of magnitude from that inhibiting I(f). Finally, the mechanism of block of f-channels by ivabradine has particularly favorable properties in light of controlling heart rate under variable physiological conditions. In this article, we will discuss how I(f) inhibition by ivabradine can lead to reduction of heart rate. To this aim, we will comment on the role of I(f) in cardiac automaticity and on the mechanism of action of ivabradine on f-channels. Some aspects of the cardiac pacemaker mechanism that improve the degree of security of ivabradine will also be highlighted.

Chronic heart rate reduction remodels ion channel transcripts in the mouse sinoatrial node but not in the ventricle

We investigated the effects of chronic and moderate heart rate (HR) reduction on ion channel expression in the mouse sinoatrial node (SAN) and ventricle. Ten-week-old male C57BL/6 mice were treated twice daily with either vehicle or ivabradine at 5 mg/kg given orally during 3 wk. The effects of HR reduction on cardiac electrical activity were investigated in anesthetized mice with serial ECGs and in freely moving mice with telemetric recordings. With the use of high-throughput real-time RT-PCR, the expression of 68 ion channel subunits was evaluated in the SAN and ventricle at the end of the treatment period. In conscious mice, ivabradine induced a mean 16% HR reduction over a 24-h period that was sustained over the 3-wk administration. Other ECG parameters were not modified. Two-way hierarchical clustering analysis of gene expression revealed a separation of ventricles from SANs but no discrimination between treated and untreated ventricles, indicating that HR reduction per se induced limited remodeling in this tissue. In contrast, SAN samples clustered in two groups depending on the treatment. In the SAN from ivabradine-treated mice, the expression of nine ion channel subunits, including Navbeta1 (-25%), Cav3.1 (-29%), Kir6.1 (-28%), Kvbeta2 (-41%), and Kvbeta3 (-30%), was significantly decreased. Eight genes were significantly upregulated, including K+ channel alpha-subunits (Kv1.1, +30%; Kir2.1, +29%; Kir3.1, +41%), hyperpolarization-activated cation channels (HCN2, +24%; HCN4, +52%), and connexin 43 (+26%). We conclude that reducing HR induces a complex remodeling of ion channel expression in the SAN but has little impact on ion channel transcripts in the ventricle.

Voltage-dependent calcium channels and cardiac pacemaker activity: from ionic currents to genes

The spontaneous activity of pacemaker cells in the sino-atrial node controls the heart rhythm and rate under physiological conditions. Compared to working myocardial cells, pacemaker cells express a specific array of ionic channels. The functional importance of different ionic channels in the generation and regulation of cardiac automaticity is currently subject of an extensive research effort and has long been controversial. Among families of ionic channels, Ca(2+) channels have been proposed to substantially contribute to pacemaking. Indeed, Ca(2+) channels are robustly expressed in pacemaker cells, and influence the cell beating rate. Furthermore, they are regulated by the activity of the autonomic nervous system in both a positive and negative way. In this manuscript, we will first discuss how the concept of the involvement of Ca(2+) channels in cardiac pacemaking has been proposed and then subsequently developed by the recent advent in the domain of cardiac physiology of gene-targeting techniques. Secondly, we will indicate how the specific profile of Ca(2+) channels expression in pacemaker tissue can help design drugs which selectively regulate the heart rhythm in the absence of concomitant negative inotropism. Finally, we will indicate how the new possibility to assign a specific gene activity to a given ionic channel involved in cardiac pacemaking could implement the current postgenomic research effort in the construction of the cardiac Physiome.

Specific pattern of ionic channel gene expression associated with pacemaker activity in the mouse heart

Even though sequencing of the mammalian genome has led to the discovery of a large number of ionic channel genes, identification of the molecular determinants of cellular electrical properties in different regions of the heart has been rarely obtained. We developed a high-throughput approach capable of simultaneously assessing the expression pattern of ionic channel repertoires from different regions of the mouse heart. By using large-scale real-time RT-PCR, we have profiled 71 channels and related genes in the sinoatrial node (SAN), atrioventricular node (AVN), the atria (A) and ventricles (V). Hearts from 30 adult male C57BL/6 mice were microdissected and RNA was isolated from six pools of five mice each. TaqMan data were analysed using the threshold cycle (C(t)) relative quantification method. Cross-contamination of each region was checked with expression of the atrial and ventricular myosin light chains. Two-way hierarchical clustering analysis of the 71 genes successfully classified the six pools from the four distinct regions. In comparison with the A, the SAN and AVN were characterized by higher expression of Nav beta 1, Nav beta 3, Cav1.3, Cav3.1 and Cav alpha 2 delta 2, and lower expression of Kv4.2, Cx40, Cx43 and Kir3.1. In addition, the SAN was characterized by higher expression of HCN1 and HCN4, and lower expression of RYR2, Kir6.2, Cav beta 2 and Cav gamma 4. The AVN was characterized by higher expression of Nav1.1, Nav1.7, Kv1.6, Kvbeta1, MinK and Cav gamma 7. Other gene expression profiles discriminate between the ventricular and the atrial myocardium. The present study provides the first genome-scale regional ionic channel expression profile in the mouse heart.

Dissecting the functional role of different isoforms of the L-type Ca2+ channel

There currently exist a great number of different mouse lines in which the activity of a particular gene of interest has been inactivated or enhanced. However, it is also possible to insert specific mutations in a gene so that the pharmacological sensitivity of the gene product is altered. An example of such an approach shows how the abolition of the sensitivity of an L-type Ca(2+) channel isoform to dihydropyridines allows the investigation of the physiological role of these channels in different tissues.

A rapidly activating delayed rectifier K+ current regulates pacemaker activity in adult mouse sinoatrial node cells

We have investigated the physiological role of the “rapidly activating” delayed rectifier K+ current ( IKr) in pacemaker activity in isolated sinoatrial node (SAN) myocytes and the expression of mouse ether-a-go-go (mERG) genes in the adult mouse SAN. In isolated, voltage-clamped SAN cells, outward currents evoked by depolarizing steps (greater than –40 mV) were strongly inhibited by the class III methanesulfonanilide compound E-4031 (1–2.5 μM), and the deactivation “tail” currents that occurred during repolarization to a membrane potential of –45 mV were completely blocked. E-4031-sensitive currents ( IKr) reached a maximum at a membrane potential of –10 mV and showed pronounced inward rectification at more-positive membrane potentials. Activation of IKr occurred at –40 to 0 mV, with half-activation at about –24 mV. The contribution of IKr to action potential repolarization and diastolic depolarization was estimated by determining the E-4031-sensitive current evoked during voltage clamp with a simulated mouse SAN action potential. IKr reached its peak value (∼0.6 pA/pF) near –25 mV, close to the midpoint of the repolarization phase of the simulated action potential, and deactivated almost completely during the diastolic interval. E-4031 (1 μM) slowed the spontaneous pacing rate of Langendorff-perfused, isolated adult mouse hearts by an average of 36.5% ( n = 5). Expression of mRNA corresponding to three isoforms coded by the mouse ERG1 gene (mERG1), mERG1a, mERG1a′, and mERG1b, was consistently found in the SAN. Our data provide the first detailed characterization of IKr in adult mouse SAN cells, demonstrate that this current plays an important role in pacemaker activity, and indicate that multiple isoforms of mERG1 can contribute to native SAN IKr.

Functional role of L-type Cav1.3 Ca2+ channels in cardiac pacemaker activity

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 alpha(1D) (Ca(v)1.3) 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 Ca(v)1.3 gene inactivation specifically abolishes I(Ca,L) in the voltage range corresponding to the diastolic depolarization. Taken together, our data demonstrate that Ca(v)1.3 channels play a major role in the generation of cardiac pacemaker activity by contributing to diastolic depolarization in SAN pacemaker cells.

Properties of the hyperpolarization-activated current (I(f)) in isolated mouse sino-atrial cells

OBJECTIVE

We have investigated the properties of the hyperpolarization-activated (I(f)) current in pacemaker cells from the mouse sino-atrial node (SAN).

METHODS

The I(f) current was studied in cells isolated enzymatically from the SAN region of adult C57BL6/J mice. The whole-cell variation of the patch-clamp technique was employed to investigate the basic properties of I(f).

RESULTS

In mouse SAN cells, the I(f) current density at -120 mV was 18+/-2 pA/pF (n=23). I(f) was not detected in cells showing atrial-like morphology that were also found in SAN preparations (n=7). I(f) was blocked by 5 mM Cs(+), was inhibited by application of 5 microM acetylcholine, and was increased by 10 microM noradrenaline. The I(f) current reversal potential was -31+/-2 mV under physiological concentration of Na(+) and K(+) ions. Lowering the extracellular Na(+) concentration reduced I(f) amplitude, while increased when the extracellular K(+) concentration was augmented. I(f) voltage for half activation was -87+/-1 mV (n=6).

CONCLUSIONS

We conclude that the native I(f) current in mouse SAN cells shows functional properties that are similar to I(f) described in rabbit SAN tissue. This study opens the possibility of investigating the involvement of I(f) in the regulation of heart rate in genetically modified mice.

Inhibition of T-type and L-type calcium channels by mibefradil: physiologic and pharmacologic bases of cardiovascular effects

Ca2+ channel antagonists of the dihydropyridine, benzothiazepine, and phenylalkylamine classes have selective effects on L-type versus T-type Ca2+ channels. In contrast, mibefradil was reported to be more selective for T-type channels. We used the whole-cell patch-clamp technique to investigate the effects of mibefradil on T-type and L-type Ca2+ currents (I(CaT) and I(CaL)) recorded at physiologic extracellular Ca2+ in different cardiac cell types. At a stimulation rate of 0.1 Hz, mibefradil blocked I(CaT) evoked from negative holding potentials (HPs) (-100 mV to -80 mV) with an IC50 of 0.1 microM in rat atrial cells. This concentration had no effect on I(CaL) in rat ventricular cells (IC50: approximately3 microM). However, block of I(CaL) was enhanced when the HP was depolarized to -50 mV (IC50: approximately 0.1 microM). Besides a resting block, mibefradil displayed voltage- and use-dependent effects on both I(CaT) and I(CaL). In addition, inhibition was enhanced by increasing the duration of the step-depolarizations. Similar effects were observed in human atrial and rabbit sinoatrial cells. In conclusion, mibefradil combines the voltage- and use-dependent effects of dihydropyridines and benzothiazepines on I(CaL). Inhibition of I(CaL), which has probably been underestimated before, may contribute to most of the cardiovascular effects of mibefradil.

Cyclosporin A increases basal intracellular calcium and calcium responses to endothelin and vasopressin in human coronary myocytes

Cyclosporin A (CsA) is a widely used immunosuppressive agent with severe side effects including hypertension. Here, we investigated the effects of CsA on intracellular free calcium ([Ca(2+)](i)) and the mechanisms involved in vasoconstriction in cultured human coronary myocytes. We used the Fura-2 technique for Ca(2+) imaging. Acute application of CsA at therapeutic concentrations (0.1-10 micromol/l) had no effect. Chronic exposure to CsA (1 micromol/l) for 24 h induced a small (20 nmol/l) but highly significant increase of basal [Ca(2+)](i) and enhanced the occurrence of spontaneous Ca(2+) oscillations. Endothelin- and vasopressin-induced rises of [Ca(2+)](i) were also enhanced. The demonstration that CsA increases basal [Ca(2+)](i) in addition to its impact on agonist receptor stimulation is of major importance for new therapeutic approaches.

Facilitation of the L-type calcium current in rabbit sino-atrial cells: effect on cardiac automaticity

OBJECTIVE

The L-type Ca(2+) current (I(Ca,L)) contributes to the generation and modulation of the pacemaker action potential (AP). We investigated facilitation of I(Ca,L) in sino-atrial cells.

METHODS

Facilitation was studied in regularly-beating cells isolated enzymatically from young albino rabbits (0.8-1 kg). We used the whole-cell patch-clamp technique to vary the frequency of the test depolarizations evoked at -10 mV or the conditioning diastolic membrane potential prior to the test pulse.

RESULTS

High frequencies (range 0.2-3.5 Hz) slowed the decay kinetics of I(Ca,L) evoked from a holding potential (HP) of -80 mV in 68% of cells resulting in a larger Ca(2+) influx during the test pulse. The amount of facilitation increased progressively between 0.2 and 3.0 Hz. When the frequency was changed from 0.1 to 1 Hz, the averaged increase in the time integral of I(Ca,L) was 27+/-7% (n=22). Application of conditioning voltages between -80 and -50 mV induced similar facilitation of I(Ca,L) in 73% of cells. The maximal increase of Ca(2+) entry occurred between -60 and -50 mV, and was on average 38+/-14% for conditioning prepulses of 5 s in duration (n=15). Numerical simulations of the pacemaker activity showed that facilitation of I(Ca,L) promotes stability of sino-atrial rate by enhancing Ca(2+) entry, thus establishing a negative feedback control against excessive heart rate slowing.

CONCLUSION

Facilitation of I(Ca,L) is present in rabbit sino-atrial cells. The underlying mechanism reflects modulation of I(Ca,L) decay kinetics by diastolic membrane potential and frequency of depolarization. This phenomenon may provide an important regulatory mechanism of sino-atrial automaticity.

Evidence for tetrodotoxin-sensitive sodium currents in primary cultured myocytes from human, pig and rabbit arteries

Primary cultured human coronary myocytes express a tetrodotoxin-sensitive sodium current (I(Na)). Here, we have investigated whether I(Na) is expressed in vascular smooth muscles cells (VSMCs) isolated from other large arteries, and other mammals. VSMCs were enzymatically dissociated, kept in primary culture, and macroscopic I(Na) was recorded using the whole-cell patch-clamp technique. We found that I(Na) is expressed in VSMCs grown from human aortic (90%; n=50) and pulmonary (44%; n=19) arteries, and in the human aortic myocyte cell line HAVSMC (94%; n=27). I(Na) was also detected in pig coronary (60%; n=33), and rabbit aortic (47%; n=15), but not in rat aortic VSMCs (n=20). These different I(Na) had similar voltage thresholds for activation (approximately equal to -50 mV), and were highly sensitive to extracellularly applied tetrodotoxin. We conclude that I(Na) is expressed in VSMCs grown from various types of large arteries in humans, pig and rabbit.

Characterisation of alpha 1A Ba2+, Sr2+ and Ca2+ currents recorded with the ancillary beta 1-4 subunits

Xenopus oocytes have been injected with different combinations of expression plasmids carrying the rat brain alpha 1A and different beta (beta 1-4) Ca2+ channel subunit cDNAs. Whole-cell Ba2+ and Ca2+ currents were recorded up to seven days after injection. Intra-oocyte injection of BAPTA allowed us to record uncontaminated Ba2+, Sr2+ currents. The alpha 1A calcium channel showed relative current amplitudes according to the sequence: IBa2+ > ISr2+ > ICa2+. The ratio ICa2+/IBa2+ was significantly larger when compared to the class C L-type Ca2+ channel (alpha 1C). However, currents flowing through alpha 1A and alpha (1C) subunits saturate for similar Ba2+ concentrations and display the anomalous mole fraction effect in the presence of mixtures of Ba2+ and Ca2+ ions in the external medium. In oocytes expressing the alpha 1A Ca2+ channel subunit, switching from extracellular Ba2+ to Ca2+ also induced a depolarising shift of current-to-voltage relation and the steady-state inactivation curve, and increased the time-to-peak of the current. Inactivation kinetics were poorly affected. Changes in gating and voltage-dependence of activation, but not in the voltage-dependent inactivation, were independent from the coexpressed beta subunit (except with the beta 4 subunit). Our data constitute strong evidence for the existence of differences in intra-pore Ca2+ binding sites between the alpha 1C and alpha 1A subunits, and emphasise the influence of the charge carrier on the modulation of alpha 1A properties by the beta subunits.

Modulation of the alpha 1A Ca2+ channel by beta subunits at physiological Ca2+ concentration

The class A Ca2+ channel alpha 1 subunit (alpha 1A) was expressed in Xenopus oocytes alone or in combination with the beta 1b, beta 2a, beta 3, or beta 4 subunit. Analysis of voltage-dependent activation and inactivation in the presence of 1.8 mM external Ca2+ showed an hyperpolarising shift of both relations when compared to similar recordings performed in the presence of 40 mM Ba2+. These shifts, which differed for activation and inactivation, were strongly modulated by the nature of the coexpressed beta subunit. On the other hand, for each combination, the kinetics of inactivation were similar in 1.8 mM Ca2+ and 40 mM Ba2+ (for example co-expression of the beta 2a subunit reduced inactivation using either 40 mM Ba2+ or 1.8 mM Ca2+). Thus, modulation of channel properties by the beta subunit is different in physiological Ca2+ or high Ba2+ concentrations. These results must be taken into consideration to extrapolate the role of the beta subunit in native cells.

Change in membrane permeability induced by protegrin 1: implication of disulphide bridges for pore formation

Protegrin 1 (PG-1) is a naturally occurring cationic antimicrobial peptide that is 18 residues long, has an aminated carboxy terminus and contains two disulphide bridges. Here, we investigated the antimicrobial activity of PG-1 and three linear analogues. Then, the membrane permeabilisation induced by these peptides was studied upon Xenopus laevis oocytes by electrophysiological methods. From the results obtained, we concluded that protegrin is able to form anion channels. Moreover, it seems clear that the presence of disulphide bridges is a prerequisite for the pore formation at the membrane level and not for the antimicrobial activity.

Coexpression of the beta2 subunit does not induce voltage-dependent facilitation of the class C L-type Ca channel

Voltage-dependent facilitation of L-type Ca2+ channels is an important regulatory mechanism by which excitable cells modulate Ca2+ entry during a train of action potentials. Expression of the alpha1 and beta subunits of the alpha1C Ca2+ channel is necessary and sufficient to reproduce this kind of facilitation in Xenopus oocytes. Here we show that, by expressing the alpha1C together with different beta subunits in oocytes, the beta1, beta3 and beta4, but not the beta2 subunits are permissive for Ca2+ channel facilitation. The poor facilitation observed in rat ventricular cells, together with the presence of the beta2 subunit mRNA, suggest that beta2 may be the beta subunit associated with functional cardiac L-type Ca2+ channels.