Pathophysiological significance of T-type Ca2+ channels: properties and functional roles of T-type Ca2+ channels in cardiac pacemaking

Pacemaker Channels and the Chronotropic Response in Health and Disease

Loss or dysregulation of the normally precise control of heart rate via the autonomic nervous system plays a critical role during the development and progression of cardiovascular disease-including ischemic heart disease, heart failure, and arrhythmias. While the clinical significance of regulating changes in heart rate, known as the chronotropic effect, is undeniable, the mechanisms controlling these changes remain not fully understood. Heart rate acceleration and deceleration are mediated by increasing or decreasing the spontaneous firing rate of pacemaker cells in the sinoatrial node. During the transition from rest to activity, sympathetic neurons stimulate these cells by activating β-adrenergic receptors and increasing intracellular cyclic adenosine monophosphate. The same signal transduction pathway is targeted by positive chronotropic drugs such as norepinephrine and dobutamine, which are used in the treatment of cardiogenic shock and severe heart failure. The cyclic adenosine monophosphate-sensitive hyperpolarization-activated current (If) in pacemaker cells is passed by hyperpolarization-activated cyclic nucleotide-gated cation channels and is critical for generating the autonomous heartbeat. In addition, this current has been suggested to play a central role in the chronotropic effect. Recent studies demonstrate that cyclic adenosine monophosphate-dependent regulation of HCN4 (hyperpolarization-activated cyclic nucleotide-gated cation channel isoform 4) acts to stabilize the heart rate, particularly during rapid rate transitions induced by the autonomic nervous system. The mechanism is based on creating a balance between firing and recently discovered nonfiring pacemaker cells in the sinoatrial node. In this way, hyperpolarization-activated cyclic nucleotide-gated cation channels may protect the heart from sinoatrial node dysfunction, secondary arrhythmia of the atria, and potentially fatal tachyarrhythmia of the ventricles. Here, we review the latest findings on sinoatrial node automaticity and discuss the physiological and pathophysiological role of HCN pacemaker channels in the chronotropic response and beyond.

Pacemaker activity and ion channels in the sinoatrial node cells: MicroRNAs and arrhythmia

The primary pacemaking activity of the heart is determined by a spontaneous action potential (AP) within sinoatrial node (SAN) cells. This unique AP generation relies on two mechanisms: membrane clocks and calcium clocks. Nonhomologous arrhythmias are caused by several functional and structural changes in the myocardium. MicroRNAs (miRNAs) are essential regulators of gene expression in cardiomyocytes. These miRNAs play a vital role in regulating the stability of cardiac conduction and in the remodeling process that leads to arrhythmias. Although it remains unclear how miRNAs regulate the expression and function of ion channels in the heart, these regulatory mechanisms may support the development of emerging therapies. This study discusses the spread and generation of AP in the SAN as well as the regulation of miRNAs and individual ion channels. Arrhythmogenicity studies on ion channels will provide a research basis for miRNA modulation as a new therapeutic target.

T-Type Calcium Channels: A Mixed Blessing

The role of T-type calcium channels is well established in excitable cells, where they preside over action potential generation, automaticity, and firing. They also contribute to intracellular calcium signaling, cell cycle progression, and cell fate; and, in this sense, they emerge as key regulators also in non-excitable cells. In particular, their expression may be considered a prognostic factor in cancer. Almost all cancer cells express T-type calcium channels to the point that it has been considered a pharmacological target; but, as the drugs used to reduce their expression are not completely selective, several complications develop, especially within the heart. T-type calcium channels are also involved in a specific side effect of several anticancer agents, that act on microtubule transport, increase the expression of the channel, and, thus, the excitability of sensory neurons, and make the patient more sensitive to pain. This review puts into context the relevance of T-type calcium channels in cancer and in chemotherapy side effects, considering also the cardiotoxicity induced by new classes of antineoplastic molecules.

Functional Heterogeneity of Cell Populations Increases Robustness of Pacemaker Function in a Numerical Model of the Sinoatrial Node Tissue

Each heartbeat is initiated by specialized pacemaker cells operating within the sinoatrial node (SAN). While individual cells within SAN tissue exhibit substantial heterogeneity of their electrophysiological parameters and Ca cycling, the role of this heterogeneity for cardiac pacemaker function remains mainly unknown. Here we investigated the problem numerically in a 25 × 25 square grid of connected coupled-clock Maltsev-Lakatta cell models. The tissue models were populated by cells with different degree of heterogeneity of the two key model parameters, maximum L-type Ca current conductance (g_CaL) and sarcoplasmic reticulum Ca pumping rate (P_up). Our simulations showed that in the areas of P_up-g_CaL parametric space at the edge of the system stability, where action potential (AP) firing is absent or dysrhythmic in SAN tissue models populated with identical cells, rhythmic AP firing can be rescued by populating the tissues with heterogeneous cells. This robust SAN function is synergistic with respect to heterogeneity in g_CaL and P_up and can be further strengthened by clustering of cells with similar properties. The effect of cell heterogeneity is not due to a simple summation of activity of intrinsically firing cells naturally present in heterogeneous SAN; rather AP firing cells locally and critically interact with non-firing/dormant cells. When firing cells prevail, they recruit many dormant cells to fire, strongly enhancing overall SAN function; and vice versa, prevailing dormant cells suppress AP firing in cells with intrinsic automaticity and halt SAN function. The transitions between firing and non-firing states of the system are sharp, resembling phase transitions in statistical physics. Furthermore, robust function of heterogeneous SAN tissue requires weak cell coupling, a known property of the central area of SAN where cardiac impulse emerges; stronger cell coupling reduces AP firing rate and ultimately halts SAN automaticity at the edge of stability.

Mechanisms of Sinoatrial Node Dysfunction in Heart Failure With Preserved Ejection Fraction

BACKGROUND

The ability to increase heart rate during exercise and other stressors is a key homeostatic feature of the sinoatrial node (SAN). When the physiological heart rate response is blunted, chronotropic incompetence limits exercise capacity, a common problem in patients with heart failure with preserved ejection fraction (HFpEF). Despite its clinical relevance, the mechanisms of chronotropic incompetence remain unknown.

METHODS

Dahl salt-sensitive rats fed a high-salt diet and C57Bl6 mice fed a high-fat diet and an inhibitor of constitutive nitric oxide synthase (Nω-nitro-L-arginine methyl ester [L-NAME]; 2-hit) were used as models of HFpEF. Myocardial infarction was created to induce HF with reduced ejection fraction. Rats and mice fed with a normal diet or those that had a sham surgery served as respective controls. A comprehensive characterization of SAN function and chronotropic response was conducted by in vivo, ex vivo, and single-cell electrophysiologic studies. RNA sequencing of SAN was performed to identify transcriptomic changes. Computational modeling of biophysically-detailed human HFpEF SAN was created.

RESULTS

Rats with phenotypically-verified HFpEF exhibited limited chronotropic response associated with intrinsic SAN dysfunction, including impaired β-adrenergic responsiveness and an alternating leading pacemaker within the SAN. Prolonged SAN recovery time and reduced SAN sensitivity to isoproterenol were confirmed in the 2-hit mouse model. Adenosine challenge unmasked conduction blocks within the SAN, which were associated with structural remodeling. Chronotropic incompetence and SAN dysfunction were also found in rats with HF with reduced ejection fraction. Single-cell studies and transcriptomic profiling revealed HFpEF-related alterations in both the "membrane clock" (ion channels) and the "Ca2+ clock" (spontaneous Ca2+ release events). The physiologic impairments were reproduced in silico by empirically-constrained quantitative modeling of human SAN function.

CONCLUSIONS

Chronotropic incompetence and SAN dysfunction were seen in both models of HF. We identified that intrinsic abnormalities of SAN structure and function underlie the chronotropic response in HFpEF.

A comparative review on heart ion channels, action potentials and electrocardiogram in rodents and human: extrapolation of experimental insights to clinic

Electrocardiogram (ECG) is a non-invasive valuable diagnostic tool that is used in clinics for investigation and monitoring of heart electrical rhythm/conduction, ischemia/injury of heart, electrolyte disturbances and agents/drugs induced cardiac toxicity. Nowadays using animal models to study heart diseases such as electrical and mechanical disturbance is common. In addition, given to ethical consideration and availability, the use of small rodents has been a top priority for cardiovascular researchers. However, extrapolation of experimental findings from the lab to the clinic needs sufficient basic knowledge of similarities and differences between heart action potential and ECG of rodents and humans in normal and disease conditions. This review compares types of human action potentials, the dominant ion currents during action potential phases, alteration in ion channels activities in channelopathies-induced arrhythmias and the ECG appearance of mouse, rat, guinea pig, rabbit and human. Also, it briefly discusses the responsiveness and alterations in ECG following some interventions such as cardiac injury and arrhythmia induction. Overall, it provides a roadmap for researchers in selecting the best animal model/species whose studies results can be translated into clinical practice. In addition, this study will also be useful to biologists, physiologists, pharmacologists, veterinarians and physicians working in the fields of comparative physiology, pharmacology, toxicology and diseases.

Reciprocal interaction between IK1 and If in biological pacemakers: A simulation study

Pacemaking dysfunction (PD) may result in heart rhythm disorders, syncope or even death. Current treatment of PD using implanted electronic pacemakers has some limitations, such as finite battery life and the risk of repeated surgery. As such, the biological pacemaker has been proposed as a potential alternative to the electronic pacemaker for PD treatment. Experimentally and computationally, it has been shown that bio-engineered pacemaker cells can be generated from non-rhythmic ventricular myocytes (VMs) by knocking out genes related to the inward rectifier potassium channel current (IK1) or by overexpressing hyperpolarization-activated cyclic nucleotide gated channel genes responsible for the "funny" current (If). However, it is unclear if a bio-engineered pacemaker based on the modification of IK1- and If-related channels simultaneously would enhance the ability and stability of bio-engineered pacemaking action potentials. In this study, the possible mechanism(s) responsible for VMs to generate spontaneous pacemaking activity by regulating IK1 and If density were investigated by a computational approach. Our results showed that there was a reciprocal interaction between IK1 and If in ventricular pacemaker model. The effect of IK1 depression on generating ventricular pacemaker was mono-phasic while that of If augmentation was bi-phasic. A moderate increase of If promoted pacemaking activity but excessive increase of If resulted in a slowdown in the pacemaking rate and even an unstable pacemaking state. The dedicated interplay between IK1 and If in generating stable pacemaking and dysrhythmias was evaluated. Finally, a theoretical analysis in the IK1/If parameter space for generating pacemaking action potentials in different states was provided. In conclusion, to the best of our knowledge, this study provides a wide theoretical insight into understandings for generating stable and robust pacemaker cells from non-pacemaking VMs by the interplay of IK1 and If, which may be helpful in designing engineered biological pacemakers for application purposes.

The effect of forskolin on membrane clock and calcium clock in the hypoxic/reoxygenation of sinoatrial node cells and its mechanism

BACKGROUND

In this study, we investigated the effect of forskolin (FSK, a selective adenylate cyclase agonist) on the automatic diastolic depolarization of sinus node cells (SNC) with hypoxia/reoxygenation (H/R) injury.

METHODS

The SNC of the newborn rat was randomly assigned into the control group, the H/R (H/R injury) group, or the H/R + FSK (H/R injury + FSK treatment) group. Patch-clamp was performed to record the action potential and electrophysiological changes. The cellular distribution of intracellular calcium concentration was analyzed by fluorescence staining.

RESULTS

Compared with the control cells, spontaneous pulsation frequency (SPF) and diastolic depolarization rate (DDR) of H/R cells were reduced from 244.3 ± 10.6 times/min and 108.7 ± 7.8 mV/s to 130.5 ± 7.6 times/min and 53.4 ± 6.5 mV/s, respectively. FSK significantly increased SPF and DDR of H/R cells to 208.3 ± 8.3 times/min and 93.2 ± 8.9 mV/s (n = 15, both p < 0.01), respectively. H/R reduced the current densities of I_f, I_Ca,T and inward I_NCX, which were significantly increased by 10 μM FSK treatment (n = 15, p < 0.01). Furthermore, reduced expression of HCN4 and NCX1.1 channel protein were significantly increased by FSK. Inhibitor studies showed that both SQ22536 (a selective adenylate cyclase inhibitor) and H89 (a selective protein kinases A [PKA] inhibitor) blocked the effects of FSK on SPF and DDR.

CONCLUSIONS

H/R causes pacemaker dysfunction in newborn rat sinoatrial node cells leading to divergence of the DD and the slow of spontaneous APs, which change can be dramatically reversed by FSK through increasing I_NCX and I_f current in H/R injury.

Inhibitory Effects of Cyclopiazonic Acid on the Pacemaker Current in Sinoatrial Nodal Cells

OBJECTIVE

The spontaneous action potential of isolated sinoatrial node (SAN) cells is regulated by a coupled-clock system of two clocks: the calcium clock and membrane clock. However, it remains unclear whether calcium clock inhibitors have a direct effect on the membrane clock. The purpose of this study was to investigate the direct effect of cyclopiazonic acid (CPA), a selective calcium clock inhibitor, on the function of the membrane clock of SAN cells.

METHODS

at SAN cells were isolated by trypsinization and identified based on morphology and electrophysiology. I_f and HCN currents were recorded via patch clamp technique. The expression of the HCN channel protein was determined by Western blotting analysis.

RESULTS

The diastolic depolarization rate of spontaneous action potentials and the current densities of I_f were reduced by exposure to 10 μM CPA. The inhibitory effect of CPA was concentration-dependent with an IC₅₀ value of 16.3 μM and a Hill coefficient of 0.98. The effect of CPA on I_f current was also time-dependent, and the I_f current amplitude was partially restored after washout. Furthermore, the steady-state activation curve of the I_f current was shifted to a negative potential, indicating that channel activation slowed down. Finally, the protein expression of HCN4 in HEK293 cells was markedly downregulated by CPA.

CONCLUSIONS

These results indicate that the direct inhibition effect of CPA on the I_f current in SAN cells is both concentration- and time-dependent. The underlying mechanisms may involve slowing down steady-state activation and the downregulation of pacemaker channel protein expression.

Autoimmune Calcium Channelopathies and Cardiac Electrical Abnormalities

Patients with autoimmune diseases are at increased risk for developing cardiovascular diseases, and abnormal electrocardiographic findings are common. Voltage-gated calcium channels play a major role in the cardiovascular system and regulate cardiac excitability and contractility. Particularly, by virtue of their localization and expression in the heart, calcium channels modulate pace making at the sinus node, conduction at the atrioventricular node and cardiac repolarization in the working myocardium. Consequently, emerging evidence suggests that calcium channels are targets to autoantibodies in autoimmune diseases. Autoimmune-associated cardiac calcium channelopathies have been recognized in both sinus node dysfunction atrioventricular block in patients positive for anti-Ro/La antibodies, and ventricular arrhythmias in patients with dilated cardiomyopathy. In this review, we discuss mechanisms of autoimmune-associated calcium channelopathies and their relationship with the development of cardiac electrical abnormalities.

Asparagine-linked glycosylation modifies voltage-dependent gating properties of CaV3.1-T-type Ca2+ channel

T-type channels are low-voltage-activated channels that play a role in the cardiovascular system particularly for pacemaker activity. Glycosylation is one of the most prevalent post-translational modifications in protein. Among various glycosylation types, the most common one is asparagine-linked (N-linked) glycosylation. The aim of this study was to elucidate the roles of N-linked glycosylation for the gating properties of the CaV3.1-T-type Ca2+ channel. N-linked glycosylation synthesis inhibitor tunicamycin causes a reduction of CaV3.1-T-type Ca2+ channel current (CaV3.1-ICa.T) when applied for 12 h or longer. Tunicamycin (24 h) significantly shifted the activation curve to the depolarization potentials, whereas the steady-state inactivation curve was unaffected. Use-dependent inactivation of CaV3.1-ICa.T was accelerated, and recovery from inactivation was prolonged by tunicamycin (24 h). CaV3.1-ICa.T was insensitive to a glycosidase PNGase F when the channels were expressed on the plasma membrane. These findings suggest that N-glycosylation contributes not only to the cell surface expression of the CaV3.1-T-type Ca2+ channel but to the regulation of the gating properties of the channel when the channel proteins were processed during the folding and trafficking steps in the cell.

MicroRNA-204 Is Necessary for Aldosterone-Stimulated T-Type Calcium Channel Expression in Cardiomyocytes

Activation of the mineralocorticoid receptor (MR) in the heart is considered to be a cardiovascular risk factor. MR activation leads to heart hypertrophy and arrhythmia. In ventricular cardiomyocytes, aldosterone induces a profound remodeling of ion channel expression, in particular, an increase in the expression and activity of T-type voltage-gated calcium channels (T-channels). The molecular mechanisms immediately downstream from MR activation, which lead to the increased expression of T-channels and, consecutively, to an acceleration of spontaneous cell contractions in vitro, remain poorly investigated. Here, we investigated the putative role of a specific microRNA in linking MR activation to the regulation of T-channel expression and cardiomyocyte beating frequency. A screening assay identified microRNA 204 (miR-204) as one of the major upregulated microRNAs after aldosterone stimulation of isolated neonatal rat cardiomyocytes. Aldosterone significantly increased the level of miR-204, an effect blocked by the MR antagonist spironolactone. When miR-204 was overexpressed in isolated cardiomyocytes, their spontaneous beating frequency was significantly increased after 24 h, like upon aldosterone stimulation, and messenger RNAs coding T-channels (CaV3.1 and CaV3.2) were increased. Concomitantly, T-type calcium currents were significantly increased upon miR-204 overexpression. Specifically repressing the expression of miR-204 abolished the aldosterone-induced increase of CaV3.1 and CaV3.2 mRNAs, as well as T-type calcium currents. Finally, aldosterone and miR-204 overexpression were found to reduce REST-NRSF, a known transcriptional repressor of CaV3.2 T-type calcium channels. Our study thus strongly suggests that miR-204 expression stimulated by aldosterone promotes the expression of T-channels in isolated rat ventricular cardiomyocytes, and therefore, increases the frequency of the cell spontaneous contractions, presumably through the inhibition of REST-NRSF protein.

Cardiac voltage gated calcium channels and their regulation by β-adrenergic signaling

Voltage-gated calcium channels (VGCCs) are the predominant source of calcium influx in the heart leading to calcium-induced calcium release and ultimately excitation-contraction coupling. In the heart, VGCCs are modulated by the β-adrenergic signaling. Signaling through β-adrenergic receptors (βARs) and modulation of VGCCs by β-adrenergic signaling in the heart are critical signaling and changes to these have been significantly implicated in heart failure. However, data related to calcium channel dysfunction in heart failure is divergent and contradictory ranging from reduced function to no change in the calcium current. Many recent studies have highlighted the importance of functional and spatial microdomains in the heart and that may be the key to answer several puzzling questions. In this review, we have briefly discussed the types of VGCCs found in heart tissues, their structure, and significance in the normal and pathological condition of the heart. More importantly, we have reviewed the modulation of VGCCs by βARs in normal and pathological conditions incorporating functional and structural aspects. There are different types of βARs, each having their own significance in the functioning of the heart. Finally, we emphasize the importance of location of proteins as it relates to their function and modulation by co-signaling molecules. Its implication on the studies of heart failure is speculated.

Mitochondrial pyruvate dehydrogenase phosphatase 1 regulates the early differentiation of cardiomyocytes from mouse embryonic stem cells

Mitochondria are crucial for maintaining the properties of embryonic stem cells (ESCs) and for regulating their subsequent differentiation into diverse cell lineages, including cardiomyocytes. However, mitochondrial regulators that manage the rate of differentiation or cell fate have been rarely identified. This study aimed to determine the potential mitochondrial factor that controls the differentiation of ESCs into cardiac myocytes. We induced cardiomyocyte differentiation from mouse ESCs (mESCs) and performed microarray assays to assess messenger RNA (mRNA) expression changes at differentiation day 8 (D8) compared with undifferentiated mESCs (D0). Among the differentially expressed genes, Pdp1 expression was significantly decreased (27-fold) on D8 compared to D0, which was accompanied by suppressed mitochondrial indices, including ATP levels, membrane potential, ROS and mitochondrial Ca²⁺. Notably, Pdp1 overexpression significantly enhanced the mitochondrial indices and pyruvate dehydrogenase activity and reduced the expression of cardiac differentiation marker mRNA and the cardiac differentiation rate compared to a mock control. In confirmation of this, a knockdown of the Pdp1 gene promoted the expression of cardiac differentiation marker mRNA and the cardiac differentiation rate. In conclusion, our results suggest that mitochondrial PDP1 is a potential regulator that controls cardiac differentiation at an early differentiation stage in ESCs.

Endostatin is protective against monocrotaline-induced right heart disease through the inhibition of T-type Ca(2+) channel

Endostatin (ES), a C-terminal fragment of collagen XVIIIα1, has a potent anti-angiogenic effect. ES prevents tumor proliferation through inhibiting T-type Ca(2+) channel. T-type Ca(2+) channel is re-expressed during heart diseases including monocrotaline (MCT)-induced right heart failure. The present study aimed to clarify the effects of ES on T-type Ca(2+) channel and pathogenesis of MCT-induced right ventricular disease. MCT or saline was injected intraperitoneally to rats. After cardiomyocytes were isolated from right ventricles (RVs), T-type Ca(2+) channel current (I CaT) was measured by a patch-clamp method. After ES small interfering RNA (siRNA) or control siRNA (20 μg) was administrated for 1 week via the right jugular vein 1 week after MCT injection, echocardiography and histological analysis were done. I CaT was significantly increased in RV from MCT-injected rats, and ES significantly inhibited it. The survival rate of ES siRNA-administrated MCT rats (MCT ES si group) was decreased. In echocardiography, although ES siRNA did not affect pulmonary arterial pressure, RV systolic function was impaired in MCT ES si group compared with control siRNA-administrated MCT rats (MCT cont si group). In the histological analysis of RV, ES expression was increased in MCT cont si group, and ES siRNA inhibited it. Furthermore, although MCT cont si group showed only cardiomyocyte hypertrophy, MCT ES si group showed notable enlargement of intercellular spaces. The present study for the first time revealed that ES inhibits T-type Ca(2+) channel activity in RV from MCT-injected rats. ES gene knockdown deteriorates MCT-induced right heart disease. ES is thus cardioprotective possibly through inhibiting T-type Ca(2+) channel activity.

Generation of cardiac pacemaker cells by programming and differentiation

A number of diseases are caused by faulty function of the cardiac pacemaker and described as "sick sinus syndrome". The medical treatment of sick sinus syndrome with electrical pacemaker implants in the diseased heart includes risks. These problems may be overcome via "biological pacemaker" derived from different adult cardiac cells or pluripotent stem cells. The generation of cardiac pacemaker cells requires the understanding of the pacing automaticity. Two characteristic phenomena the "membrane-clock" and the "Ca(2+)-clock" are responsible for the modulation of the pacemaker activity. Processes in the "membrane-clock" generating the spontaneous pacemaker firing are based on the voltage-sensitive membrane ion channel activity starting with slow diastolic depolarization and discharging in the action potential. The influence of the intracellular Ca(2+) modulating the pacemaker activity is characterized by the "Ca(2+)-clock". The generation of pacemaker cells started with the reprogramming of adult cardiac cells by targeted induction of one pacemaker function like HCN1-4 overexpression and enclosed in an activation of single pacemaker specific transcription factors. Reprogramming of adult cardiac cells with the transcription factor Tbx18 created cardiac cells with characteristic features of cardiac pacemaker cells. Another key transcription factor is Tbx3 specifically expressed in the cardiac conduction system including the sinoatrial node and sufficient for the induction of the cardiac pacemaker gene program. For a successful cell therapeutic practice, the generated cells should have all regulating mechanisms of cardiac pacemaker cells. Otherwise, the generated pacemaker cells serve only as investigating model for the fundamental research or as drug testing model for new antiarrhythmics. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Cardiac ion channels

Ion channels are critical for all aspects of cardiac function, including rhythmicity and contractility. Consequently, ion channels are key targets for therapeutics aimed at cardiac pathophysiologies such as atrial fibrillation or angina. At the same time, off-target interactions of drugs with cardiac ion channels can be the cause of unwanted side effects. This manuscript aims to review the physiology and pharmacology of key cardiac ion channels. The intent is to highlight recent developments for therapeutic development, as well as elucidate potential mechanisms for drug-induced cardiac side effects, rather than present an in-depth review of each channel subtype.

Eps15 Homology Domain-containing Protein 3 Regulates Cardiac T-type Ca2+ Channel Targeting and Function in the Atria

Proper trafficking of membrane-bound ion channels and transporters is requisite for normal cardiac function. Endosome-based protein trafficking of membrane-bound ion channels and transporters in the heart is poorly understood, particularly in vivo. In fact, for select cardiac cell types such as atrial myocytes, virtually nothing is known regarding endosomal transport. We previously linked the C-terminal Eps15 homology domain-containing protein 3 (EHD3) with endosome-based protein trafficking in ventricular cardiomyocytes. Here we sought to define the roles and membrane protein targets for EHD3 in atria. We identify the voltage-gated T-type Ca(2+) channels (CaV3.1, CaV3.2) as substrates for EHD3-dependent trafficking in atria. Mice selectively lacking EHD3 in heart display reduced expression and targeting of both Cav3.1 and CaV3.2 in the atria. Furthermore, functional experiments identify a significant loss of T-type-mediated Ca(2+) current in EHD3-deficient atrial myocytes. Moreover, EHD3 associates with both CaV3.1 and CaV3.2 in co-immunoprecipitation experiments. T-type Ca(2+) channel function is critical for proper electrical conduction through the atria. Consistent with these roles, EHD3-deficient mice demonstrate heart rate variability, sinus pause, and atrioventricular conduction block. In summary, our findings identify CaV3.1 and CaV3.2 as substrates for EHD3-dependent protein trafficking in heart, provide in vivo data on endosome-based trafficking pathways in atria, and implicate EHD3 as a key player in the regulation of atrial myocyte excitability and cardiac conduction.

Distinct roles of L- and T-type voltage-dependent Ca2+ channels in regulation of lymphatic vessel contractile activity

Lymph drainage maintains tissue fluid homeostasis and facilitates immune response. It is promoted by phasic contractions of collecting lymphatic vessels through which lymph is propelled back into the blood circulation. This rhythmic contractile activity (i.e. lymphatic pumping) increases in rate with increase in luminal pressure and relies on activation of nifedipine-sensitive voltage-dependent Ca(2+) channels (VDCCs). Despite their importance, these channels have not been characterized in lymphatic vessels. We used pressure- and wire-myography as well as intracellular microelectrode electrophysiology to characterize the pharmacological and electrophysiological properties of L-type and T-type VDCCs in rat mesenteric lymphatic vessels and evaluated their particular role in the regulation of lymphatic pumping by stretch. We complemented our study with PCR and confocal immunofluorescence imaging to investigate the expression and localization of these channels in lymphatic vessels. Our data suggest a delineating role of VDCCs in stretch-induced lymphatic vessel contractions, as the stretch-induced increase in force of lymphatic vessel contractions was significantly attenuated in the presence of L-type VDCC blockers nifedipine and diltiazem, while the stretch-induced increase in contraction frequency was significantly decreased by the T-type VDCC blockers mibefradil and nickel. The latter effect was correlated with a hyperpolarization. We propose that activation of T-type VDCCs depolarizes membrane potential, regulating the frequency of lymphatic contractions via opening of L-type VDCCs, which drive the strength of contractions.

Computational modeling of inhibition of voltage-gated Ca channels: identification of different effects on uterine and cardiac action potentials

The uterus and heart share the important physiological feature whereby contractile activation of the muscle tissue is regulated by the generation of periodic, spontaneous electrical action potentials (APs). Preterm birth arising from premature uterine contractions is a major complication of pregnancy and there remains a need to pursue avenues of research that facilitate the use of drugs, tocolytics, to limit these inappropriate contractions without deleterious actions on cardiac electrical excitation. A novel approach is to make use of mathematical models of uterine and cardiac APs, which incorporate many ionic currents contributing to the AP forms, and test the cell-specific responses to interventions. We have used three such models-of uterine smooth muscle cells (USMC), cardiac sinoatrial node cells (SAN), and ventricular cells-to investigate the relative effects of reducing two important voltage-gated Ca currents-the L-type (ICaL) and T-type (ICaT) Ca currents. Reduction of ICaL (10%) alone, or ICaT (40%) alone, blunted USMC APs with little effect on ventricular APs and only mild effects on SAN activity. Larger reductions in either current further attenuated the USMC APs but with also greater effects on SAN APs. Encouragingly, a combination of ICaL and ICaT reduction did blunt USMC APs as intended with little detriment to APs of either cardiac cell type. Subsequent overlapping maps of ICaL and ICaT inhibition profiles from each model revealed a range of combined reductions of ICaL and ICaT over which an appreciable diminution of USMC APs could be achieved with no deleterious action on cardiac SAN or ventricular APs. This novel approach illustrates the potential for computational biology to inform us of possible uterine and cardiac cell-specific mechanisms. Incorporating such computational approaches in future studies directed at designing new, or repurposing existing, tocolytics will be beneficial for establishing a desired uterine specificity of action.

Ca2+ channel inhibitor NNC 55-0396 inhibits voltage-dependent K+ channels in rabbit coronary arterial smooth muscle cells

We demonstrated the inhibitory effect of NNC 55-0396, a T-type Ca(2+) channel inhibitor, on voltage-dependent K(+) (K(V)) channels in freshly isolated rabbit coronary arterial smooth muscle cells. NNC 55-0396 decreased the amplitude of K(V) currents in a concentration-dependent manner, with an IC(50) of 0.080 μM and a Hill coefficient of 0.76.NNC 55-0396 did not affect steady-state activation and inactivation curves, indicating that the compound does not affect the voltage sensitivity of K(V) channel gating. Both the K(V) currents and the inhibitory effect of NNC 55-0396 on K(V) channels were not altered by depletion of extracellular Ca(2+) or intracellular ATP, suggesting that the inhibitory effect of NNC 55-0396 is independent of Ca(2+)-channel activity and phosphorylation-dependent signaling cascades. From these results, we concluded that NNC 55-0396 dosedependently inhibits K(V) currents, independently of Ca(2+)-channel activity and intracellular signaling cascades.

T-type channels become highly permeable to sodium ions using an alternative extracellular turret region (S5-P) outside the selectivity filter

T-type (Cav3) channels are categorized as calcium channels, but invertebrate ones can be highly sodium-selective channels. We illustrate that the snail LCav3 T-type channel becomes highly sodium-permeable through exon splicing of an extracellular turret and descending helix in domain II of the four-domain Cav3 channel. Highly sodium-permeable T-type channels are generated without altering the invariant ring of charged residues in the selectivity filter that governs calcium selectivity in calcium channels. The highly sodium-permeant T-type channel expresses in the brain and is the only splice isoform expressed in the snail heart. This unique splicing of turret residues offers T-type channels a capacity to serve as a pacemaking sodium current in the primitive heart and brain in lieu of Nav1-type sodium channels and to substitute for voltage-gated sodium channels lacking in many invertebrates. T-type channels would also contribute substantially to sodium leak conductances at rest in invertebrates because of their large window currents.

HCN channelopathy and cardiac electrophysiologic dysfunction in genetic and acquired rat epilepsy models

OBJECTIVE

Evidence from animal and human studies indicates that epilepsy can affect cardiac function, although the molecular basis of this remains poorly understood. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels generate pacemaker activity and modulate cellular excitability in the brain and heart, with altered expression and function associated with epilepsy and cardiomyopathies. Whether HCN expression is altered in the heart in association with epilepsy has not been investigated previously. We studied cardiac electrophysiologic properties and HCN channel subunit expression in rat models of genetic generalized epilepsy (Genetic Absence Epilepsy Rats from Strasbourg, GAERS) and acquired temporal lobe epilepsy (post-status epilepticus SE). We hypothesized that the development of epilepsy is associated with altered cardiac electrophysiologic function and altered cardiac HCN channel expression.

METHODS

Electrocardiography studies were recorded in vivo in rats and in vitro in isolated hearts. Cardiac HCN channel messenger RNA (mRNA) and protein expression were measured using quantitative PCR and Western blotting respectively.

RESULTS

Cardiac electrophysiology was significantly altered in adult GAERS, with slower heart rate, shorter QRS duration, longer QTc interval, and greater standard deviation of RR intervals compared to control rats. In the post-SE model, we observed similar interictal changes in several of these parameters, and we also observed consistent and striking bradycardia associated with the onset of ictal activity. Molecular analysis demonstrated significant reductions in cardiac HCN2 mRNA and protein expression in both models, providing a molecular correlate of these electrophysiologic abnormalities.

SIGNIFICANCE

These results demonstrate that ion channelopathies and cardiac dysfunction can develop as a secondary consequence of chronic epilepsy, which may have relevance for the pathophysiology of cardiac dysfunction in patients with epilepsy.

Cardiac functions of voltage-gated Ca(2+) channels: role of the pharmacoresistant type (E-/R-Type) in cardiac modulation and putative implication in sudden unexpected death in epilepsy (SUDEP)

Voltage-gated Ca(2+) channels (VGCCs) are ubiquitous in excitable cells. These channels play key roles in many physiological events like cardiac regulation/pacemaker activity due to intracellular Ca(2+) transients. In the myocardium, the Cav1 subfamily (L-type: Cav1.2 and Cav1.3) is the main contributor to excitation-contraction coupling and/or pacemaking, whereas the Cav3 subfamily (T-type: Cav3.1 and Cav3.2) is important in rhythmically firing of the cardiac nodal cells. No established cardiac function has been attributed to the Cav2 family (E-/R-type: Cav2.3) despite accumulating evidence of cardiac dysregulation observed upon deletion of the Cav2.3 gene, the only member of this family so far detected in cardiomyocytes. In this review, we summarize the pathophysiological changes observed after ablation of the E-/R-type VGCC and propose a cardiac mechanism of action for this channel. Also, considering the role played by this channel in epilepsy and its reported sensitivity to antiepileptic drugs, a putative involvement of this channel in the cardiac mechanism of sudden unexpected death in epilepsy is also discussed.

The mechanism of increased postnatal heart rate and sinoatrial node pacemaker activity in mice

Heart rate (HR) of mammalian species changes postnatally, i.e., HR of large animals including humans decreases, while HR in small animals such as mice and rats increases. To clarify cellular mechanisms underlying the postnatal HR changes, we performed in vivo HR measurement and electrophysiological analysis on sinoatrial node (SAN) cells in mice. The in vivo HR was ~320 beats min(-1) (bpm) immediately after birth, and increased with age to ~690 bpm at postnatal day 14. Under blockage of autonomic nervous systems, HR remained constant until postnatal day 5 and then increased day by day. The spontaneous beating rate of SAN preparation showed a similar postnatal change. The density of the L-type Ca(2+) current (LCC) was smaller in neonatal SAN cells than in adult cells, accompanied by a positive shift of voltage-dependent activation. Thus, the postnatal increase in HR is caused by both the increased sympathetic influence and the intrinsic activity of SAN cells. The different conductance and kinetics of LCC may be involved in the postnatal increase in pacemaker activity.

Pharmacoresistant Cav 2·3 (E-type/R-type) voltage-gated calcium channels influence heart rate dynamics and may contribute to cardiac impulse conduction

Voltage-gated Ca(2+) channels regulate cardiac automaticity, rhythmicity and excitation-contraction coupling. Whereas L-type (Cav 1·2, Cav 1·3) and T-type (Cav 3·1, Cav 3·2) channels are widely accepted for their functional relevance in the heart, the role of Cav 2·3 Ca(2+) channels expressing R-type currents remains to be elucidated. We have investigated heart rate dynamics in control and Cav 2·3-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 Cav 2·3(-|-) 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. Cav 2·3c and Cav 2·3e. 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 Cav 2·3 (-|-) mice. In addition, experiments on Langendorff perfused hearts elucidate differences in basic properties between both genotypes. Thus, Cav 2·3 does not only contribute to the cardiac autonomous nervous system but also to intrinsic rhythm propagation.

Zebrafish: a novel research tool for cardiac (patho)electrophysiology and ion channel disorders

The zebrafish is a cold-blooded tropical freshwater teleost with two-chamber heart morphology. A major advantage of the zebrafish for heart studies is that the embryo is transparent, allowing for easy assessment of heart development, heart rate analysis and phenotypic characterization. Moreover, rapid and effective gene-specific knockdown can be achieved using morpholino oligonucleotides. Lastly, zebrafish are small in size, are easy to maintain and house, grow fast, and have large offspring size, making them a cost-efficient research model. Zebrafish embryonic and adult heart rates as well as action potential (AP) shape and duration and electrocardiogram morphology closely resemble those of humans. However, whether the zebrafish is truly an attractive alternative model for human cardiac electrophysiology depends on the presence and gating properties of the various ion channels in the zebrafish heart, but studies into the latter are as yet limited. The rapid component of the delayed rectifier K⁺ current (I_Kr) remains the best characterized and validated ion current in zebrafish myocytes, and zebrafish may represent a valuable model to investigate human I_Kr channel-related disease, including long QT syndrome. Arguments against the use of zebrafish as model for human cardiac (patho)electrophysiology include its cold-bloodedness and two-chamber heart morphology, absence of t-tubuli, sarcoplamatic reticulum function, and a different profile of various depolarizing and repolarizing ion channels, including a limited Na⁺ current density. Based on the currently available literature, we propose that zebrafish may constitute a relevant research model for investigating ion channel disorders associated with abnormal repolarization, but may be less suitable for studying depolarization disorders or Ca²⁺-modulated arrhythmias.

Effect of an L- and T-type calcium channel blocker on 24-hour systolic blood pressure and heart rate in hypertensive patients

Background and Objectives

The aim of this study was to evaluate the effects of an L- and T-type calcium channel blocker (CCB) on 24-hour systolic blood pressure (24-hour SBP) and heart rate (24-hour HR) profiles in essential hypertensive patients.

Subjects and Methods

Thirty-seven consecutive patients were enrolled in this study. The 24-hour SBP and HR were recorded before and after treatment with efonidipine (L- and T-type CCB, 40 mg), after waking. Changes in 24-hour SBP and HR and the diurnal to nocturnal SBP ratio were measured. The best-fit curves of changes in SBP and HR were depicted using a periodic function.

Results

The mean 24-hour SBP and HR decreased significantly after treatment. The diurnal to nocturnal SBP ratio in dipper-type hypertension cases decreased from 16.7±6.1% to 8.3±9.8% (p<0.05), whereas in non-dipper hypertension cases, it increased from 2.3±2.9% to 7.7±5.1% (p<0.01). The antihypertensive effect was minimal at 5.0 hours after drug administration and it slowly recovered at a constant rate (2.1 mm Hg/h) over 12 hours in dipper cases. The median 24-hour changes in HR in the dipper and non-dipper cases were -2.3/min and -5.4/min, respectively. A continuous reduction in the change in HR was seen from 3.5 to 23 hours after drug administration.

Conclusion

The antihypertensive action of efonidipine was characterized by a slow recovery of the SBP decrease at a constant rate (2.1 mm Hg/h) and a non-administration time dependent reduction in 24-hour HR.

Low-threshold calcium currents contribute to locomotor-like activity in neonatal mice

In this study, we examined the contribution of a low-threshold calcium current [I(Ca(T))] to locomotor-related activity in the neonatal mouse. Specifically, the role of I(Ca(T)) was studied during chemically induced, locomotor-like activity in the isolated whole cord and in a genetically distinct population of ventromedial spinal interneurons marked by the homeobox gene Hb9. In isolated whole spinal cords, cycle frequency was decreased in the presence of low-threshold calcium channel blockers, which suggests a role for I(Ca(T)) in the network that produces rhythmic, locomotor-like activity. Additionally, we used Hb9 interneurons as a model to study the cellular responses to application of low-threshold calcium channel blockers. In transverse slice preparations from transgenic Hb9::enhanced green fluorescent protein neonatal mice, N-methyl-d-aspartate-induced membrane potential oscillations in identified Hb9 interneurons also slowed in frequency with application of nickel when fast, spike-mediated, synaptic transmission was blocked with TTX. Voltage-clamp and immunolabeling experiments confirmed expression of I(Ca(T)) and channels, respectively, in Hb9 interneurons located in the ventromedial spinal cord. Taken together, these results provide support that T-type calcium currents play an important role in network-wide rhythm generation during chemically evoked, fictive locomotor activity.

Electrophysiological mapping of embryonic mouse hearts: mechanisms for developmental pacemaker switch and internodal conduction pathway

INTRODUCTION

Understanding sinoatrial node (SAN) development could help in developing therapies for SAN dysfunction. However, electrophysiological investigation of SAN development remains difficult because mutant mice with SAN dysfunctions are frequently embryonically lethal. Most research on SAN development is therefore limited to immunocytochemical observations without comparable functional studies.

METHODS AND RESULTS

We applied a multielectrode array (MEA) recording system to study SAN development in mouse hearts acutely isolated at embryonic ages (E) 8.5-12.5 days. Physiological heart rates were routinely restored, enabling accurate functional assessment of SAN development. We found that dominant pacemaking activity originated from the left inflow tract (LIFT) region at E8.5, but switched to the right SAN by E12.5. Combining MEA recordings and pharmacological agents, we show that intracellular calcium (Ca(2+))-mediated automaticity develops early and is the major mechanism of pulse generation in the LIFT of E8.5 hearts. Later in development at E12.5, sarcolemmal ion channels develop in the SAN at a time when pacemaker channels are down-regulated in the LIFT, leading to a switch in the dominant pacemaker location. Additionally, low micromolar concentrations of tetrodotoxin (TTX), a sodium channel blocker, minimally affect pacemaker rhythm at E8.5-E12.5, but suppress atrial activation and reveal a TTX-resistant SAN-atrioventricular node (internodal) pathway that mediates internodal conduction in E12.5 hearts.

CONCLUSIONS

Using a physiological mapping method, we demonstrate that differential mechanistic development of automaticity between the left and right inflow tract regions confers the pacemaker location switch. Moreover, a TTX-resistant pathway mediates preferential internodal conduction in E12.5 mouse hearts. 

Mechanism underlying the block of human Cav3.2 T-type Ca2+ channels by benidipine, a dihydropyridine Ca2+ channel blocker

AIMS

Benidipine, a dihydropyridine Ca(2+) channel blocker, has been reported to block T-type Ca(2+) channels; however, the mechanism underlying this effect was unclear. In this study, we characterized the mechanism responsible for this blocking activity. Furthermore, the blocking activity was compared between two enantiomers of benidipine, (S, S)- and (R, R)-benidipine.

MAIN METHODS

Human Ca(v)3.2 (hCa(v)3.2) T-type Ca(2+) channels stably expressed in the human embryonic kidney cell line, HEK-293, were studied in whole-cell patch-clamp recordings and Ca(2+) mobilization assay.

KEY FINDINGS

In whole-cell patch-clamp recordings, benidipine blocked hCa(v)3.2 T-type Ca(2+) currents elicited by depolarization to a comparable extent as efonidipine. The block was dependent on stimulation frequency and holding potential, but not test potential. Benidipine significantly shifted the steady-state inactivation curve to the hyperpolarizing direction, but had no effect on the activation curve. Benidipine prolonged the recovery from inactivation of hCa(v)3.2 T-type Ca(2+) channels without any effect on the kinetics of activation, inactivation, or deactivation. In the Ca(2+) mobilization assay, benidipine was more potent than efonidipine in blocking Ca(2+) influx through hCa(v)3.2 T-type Ca(2+) channels. (S, S)-Benidipine was more potent than (R, R)-benidipine in blocking hCa(v)3.2 T-type Ca(2+) currents, but there was no difference in blocking the Ca(2+) influx.

SIGNIFICANCE

We have characterized the blocking activity of benidipine against hCa(v)3.2 Ca(2+) channels and revealed the difference between the two enantiomers of benidipine. The blocking action of benidipine could be mediated by stabilizing hCa(v)3.2 Ca(2+) channels in an inactivated state.

T-type Ca channel blockade as a determinant of kidney protection

Voltage-dependent Ca channels are classified into several subtypes based on the isoform of their α1 subunits. Traditional Ca channels blockers (CCBs), including nifedipine and amlodipine, act predominantly on L-type Ca channels, whereas novel CCBs such as efonidipine, benidipine and azelnidipine inhibit both L-type and T-type Ca channels. Furthermore, cilnidipine blocks L-type and N-type Ca channels. These CCBs exert divergent actions on renal microvessels. L-type CCBs preferentially dilate afferent arterioles, whereas both L-/T-type and L-/N-type CCBs potently dilate afferent and efferent arterioles. The distinct actions of CCBs on the renal microcirculation are reflected by changes in glomerular capillary pressure and subsequent renal injury: L-type CCBs favor an increase in glomerular capillary pressure, whereas L-/T-type and L-/N-type CCBs alleviate glomerular hypertension. The renal protective action of L-/T-type CCBs is also mediated by non-hemodynamic mechanisms, i.e., inhibition of the inflammatory process and inhibition of Rho kinase and aldosterone secretion. Finally, a growing body of evidence indicates that T-type CCBs offer more beneficial action on proteinuria and renal survival rate than L-type CCBs in patients with chronic kidney disease (CKD). Similarly, in CKD patients treated with renin-angiotensin blockers, add-on therapy with N-type CCBs is more potent in reducing proteinuria than that with L-type CCBs, although no difference is found in the subgroup with diabetic nephropathy. Thus, the strategy for hypertension treatment with CCBs has entered a new era: treatment selection depends not only on blood pressure control but also on the subtypes of CCBs.

Caveolin-3 regulates protein kinase A modulation of the Ca(V)3.2 (alpha1H) T-type Ca2+ channels

Voltage-gated T-type Ca(2+) channel Ca(v)3.2 (α(1H)) subunit, responsible for T-type Ca(2+) current, is expressed in different tissues and participates in Ca(2+) entry, hormonal secretion, pacemaker activity, and arrhythmia. The precise subcellular localization and regulation of Ca(v)3.2 channels in native cells is unknown. Caveolae containing scaffolding protein caveolin-3 (Cav-3) localize many ion channels, signaling proteins and provide temporal and spatial regulation of intracellular Ca(2+) in different cells. We examined the localization and regulation of the Ca(v)3.2 channels in cardiomyocytes. Immunogold labeling and electron microscopy analysis demonstrated co-localization of the Ca(v)3.2 channel and Cav-3 relative to caveolae in ventricular myocytes. Co-immunoprecipitation from neonatal ventricular myocytes or transiently transfected HEK293 cells demonstrated that Ca(v)3.1 and Ca(v)3.2 channels co-immunoprecipitate with Cav-3. GST pulldown analysis confirmed that the N terminus region of Cav-3 closely interacts with Ca(v)3.2 channels. Whole cell patch clamp analysis demonstrated that co-expression of Cav-3 significantly decreased the peak Ca(v)3.2 current density in HEK293 cells, whereas co-expression of Cav-3 did not alter peak Ca(v)3.1 current density. In neonatal mouse ventricular myocytes, overexpression of Cav-3 inhibited the peak T-type calcium current (I(Ca,T)) and adenovirus (AdCa(v)3.2)-mediated increase in peak Ca(v)3.2 current, but did not affect the L-type current. The protein kinase A-dependent stimulation of I(Ca,T) by 8-Br-cAMP (membrane permeable cAMP analog) was abolished by siRNA directed against Cav-3. Our findings on functional modulation of the Ca(v)3.2 channels by Cav-3 is important for understanding the compartmentalized regulation of Ca(2+) signaling during normal and pathological processes.

Role of polyamines and cAMP-dependent mechanisms on 5alpha-dihydrotestosterone-elicited functional effects in isolated right atria of rat

Androgens produce acute vasodilation of systemic, pulmonary, and coronary arteries in several mammal preparations and increase cardiomyocyte contractility. A decrease of the spontaneous beating of sinoatrial cells has also been described. The aim of this study was to characterize the direct effect of 5alpha-dihydrotestosterone on the spontaneous chronotropism and inotropism in the same preparation as an approach to establish the effect on cardiac output and their mechanism of action. The effects were studied on isolated right atria of Wistar rats placed in an organ bath in Tyrode solution at 37 degrees C and bubbled with carbogen. In male rats, the acute administration of 5alpha-dihydrotestosterone, a nonaromatizable derivate of testosterone, elicited a positive inotropism, which was associated with a negative chronotropism. As reported in the left atria, polyamines and beta-adrenoceptors played a role in 5alpha-dihydrotestosterone-elicited positive inotropism because the effect was antagonized by alpha-difluoromethylornithine, an inhibitor of polyamine synthesis, and atenolol, a beta1-adrenoceptor blocker, but not on the negative effect on chronotropism. The androgen increased the sinoatrial node recovery time, suggesting an effect on the mechanisms of spontaneous diastolic depolarization involved in atria pacemaking. These effects of 5alpha-dihydrotestosterone are not hormonally regulated because they are similarly produced in estrogenized females and gonadectomized male and female rats. These results suggest that the androgen could acutely improve cardiac performance.

Regulation of basal and reserve cardiac pacemaker function by interactions of cAMP-mediated PKA-dependent Ca2+ cycling with surface membrane channels

Decades of intensive research of primary cardiac pacemaker, the sinoatrial node, have established potential roles of specific membrane channels in the generation of the diastolic depolarization, the major mechanism allowing sinoatrial node cells to generate spontaneous beating. During the last three decades, multiple studies made either in the isolated sinoatrial node or sinoatrial node cells have demonstrated a pivotal role of Ca(2+) and, specifically Ca(2+) release from sarcoplasmic reticulum, for spontaneous beating of cardiac pacemaker. Recently, spontaneous, rhythmic local subsarcolemmal Ca(2+) releases from ryanodine receptors during late half of the diastolic depolarization have been implicated as a vital factor in the generation of sinoatrial node cell spontaneous firing. Local Ca(2+) releases are driven by a unique combination of high basal cAMP production by adenylyl cyclases, high basal cAMP degradation by phosphodiesterases and a high level of cAMP-mediated PKA-dependent phosphorylation. These local Ca(2+) releases activate an inward Na(+)-Ca(2+) exchange current which accelerates the terminal diastolic depolarization rate and, thus, controls the spontaneous pacemaker firing. Both the basal primary pacemaker beating rate and its modulation via beta-adrenergic receptor stimulation appear to be critically dependent upon intact RyR function and local subsarcolemmal sarcoplasmic reticulum generated Ca(2+) releases. This review aspires to integrate the traditional viewpoint that has emphasized the supremacy of the ensemble of surface membrane ion channels in spontaneous firing of the primary cardiac pacemaker, and these novel perspectives of cAMP-mediated PKA-dependent Ca(2+) cycling in regulation of the heart pacemaker clock, both in the basal state and during beta-adrenergic receptor stimulation.

Development of newer calcium channel antagonists: therapeutic potential of efonidipine in preventing electrical remodelling during atrial fibrillation

Calcium channel antagonists are most frequently prescribed for the treatment of hypertension and the majority specifically inhibit the L-type Ca2+ channel. In order to prevent reflex sympathetic over activity caused by L-type calcium channel antagonists (calcium channel blockers [CCBs]), increasing attention has focused on the blockade of the T-type Ca2+ channel. The T-type Ca2+ channel is found in the kidney and can also appear in the ventricle of the heart when in failure. Therefore, the T-type Ca2+ channel is a possible new target for the treatment of nephropathy and heart failure. In clinical trials, the efficacy and safety of T-type CCBs in hypertension and chronic renal disease have been reported. It is well known that the T-type Ca2+ channel is present in the adult atrium and plays a role in the cardiac pacemaker, but recent experimental studies suggest that this current also promotes electrical remodelling of the atrium. Using efonidipine, a dual L- and T-type CCB, it has been demonstrated that atrial electrical remodelling can be diminished in dogs. Furthermore, the T-type Ca2+ channel has recently been found in the pulmonary veins, contributing to the pulmonary vein pacemaker activity and triggered activity. A variety of drugs having T-type CCB effects have been shown to be effective in the management of atrial fibrillation, suggesting that this channel may be a novel therapeutic target.

Is sodium current present in human sinoatrial node cells?

Pacemaker activity of the sinoatrial node has been studied extensively in various animal species, but is virtually unexplored in man. As such, it is unknown whether the fast sodium current (I_Na) plays a role in the pacemaker activity of the human sinoatrial node. Recently, we had the unique opportunity to perform patch-clamp experiments on single pacemaker cells isolated from a human sinoatrial node. In 2 out of the 3 cells measured, we observed large inward currents with characteristics of I_Na. Although we were unable to analyze the current in detail, our findings provide strong evidence that I_Na is present in human sinoatrial node pacemaker cells, and that this I_Na is functionally available at potentials negative to -60 mV.

17 beta-estradiol modulates expression of low-voltage-activated Ca(V)3.2 T-type calcium channel via extracellularly regulated kinase pathway in cardiomyocytes

T-type Ca(2+) channel current (I(Ca,T)) plays an important role for spontaneous pacemaker activity and is involved in the progression of structural heart diseases. Estrogens are of importance for the regulation of growth and differentiation and function in a wide array of target tissues, including those in the cardiovascular system. The aim of this study was to elucidate the short-term and long-term effects of 17beta-estradiol (E(2)) on I(Ca,T) in cardiomyocytes. We employed in vivo and in vitro techniques to clarify E(2)-mediated modulation of heart rate (HR) in ovariectomized rats and I(Ca,T) in cardiomyocytes. Ovariectomy increased HR and E(2) supplement reduced HR in ovariectomized rats. Slowing of E(2)-induced HR was consistent with the deceleration of automaticity in E(2)-treated neonatal cardiomyocytes. Short-term application of E(2) did not have significant effects on I(Ca,T), whereas in cardiomyocytes treated with 10 nm E(2) for 24 h, estrogen receptor-independent down-regulation of peak I(Ca,T) and declination of Ca(V)3.2 mRNA were observed. Expression of a cardiac-specific transcription factor Csx/Nkx2.5 was also suppressed by E(2) treatment for 24 h. On the other hand, expression of Ca(V)3.1 mRNA was unaltered by E(2) treatment in this study. An ERK-1/2, 5 inhibitor, PD-98059, abolished the effects of E(2) on I(Ca,T) and Ca(V)3.2 mRNA as well as Csx/Nkx2.5 mRNA. These findings indicate that E(2) decreases Ca(V)3.2 I(Ca,T) through activation of ERK-1/2, 5, which is mediated by the suppression of Csx/Nkx2.5-dependent transcription, suggesting a genomic effect of E(2) as a negative chronotropic factor in the heart.

Molecular and electrical remodeling of L- and T-type Ca(2+) channels in rat right atrium with monocrotaline-induced pulmonary hypertension

BACKGROUND

Atrial arrhythmia is often encountered in chronic pulmonary disease with pulmonary hypertension (PH), but few studies have investigated the electrical remodeling of atrial Ca(2+) channels under PH.

METHODS AND RESULTS

Wistar rats were injected with monocrotaline (MCT), resulting in PH with right atrial and ventricular hypertrophy. The L-type Ca(2+) channel current density was significantly decreased in right atrial cells of MCT-treated rats, accompanied by a significant reduction in mRNA expression of the CaV1.2 (alpha(1C)) subunit and accessory beta(2) subunit. Conversely, the low voltage-activated Ca(2+) current was more marked in the right atrial cells of MCT-treated rats than in those of control rats. The current-voltage relationship and the time course of inactivation closely resembled those of T-type Ca(2+) channels, although the current was only slightly inhibited by 10-100 micromol/L Ni(2+). No significant differences were observed in the mRNA expression levels of CaV3.1 (alpha(1G)) and CaV3.2 (alpha(1H)) or the protein level of the CaV3.1 subunit. In left atrial cells, the electrophysiological molecular properties of Ca(2+) channels were unaffected by MCT treatment.

CONCLUSIONS

PH causes right atrial hypertrophy, associated with alteration of the electrophysiological molecular properties of Ca(2+) channels.

Acute effect of antidiabetic 1,4-dihydropyridine compound cerebrocrast on cardiac function and glucose metabolism in the isolated, perfused normal rat heart

Diabetes mellitus (DM) is an important cardiovascular risk factor and is associated with abnormalities in endothelial and vascular smooth muscle cell function, evoked by chronic hyperglycemia and hyperlipidemia. Chronic insulin deficiency or resistance is marked by decreases in the intensity of glucose transport, glucose phosphorylation, and glucose oxidation, plus decreases in ATP levels in cardiac myocytes. It is important to search for new agents that promote glucose consumption in the heart and partially inhibit extensive fatty acid beta-oxidation observed in diabetic, ischemia. When the oxygen supply for myocardium is decreased, the heart accumulates potentially toxic intermediates of fatty acid beta-oxidation, that is, long-chain acylcarnitine and long-chain acyl-CoA metabolites. Exogenous glucose and heart glycogen become an important compensatory source of energy. Therefore we studied the effect of the antidiabetic 1,4-dihydropyridine compound cerebrocrast at concentrations from 10(-10) M to 10(-7) M on isolated rat hearts using the method of Langendorff, on physiological parameters and energy metabolism. Cerebrocrast at concentrations from 10(-10) M to 10(-7) M has a negative inotropic effect on the rat heart. It inhibits L-type Ca(2+)channels thereby diminishing the cellular Ca(2+) supply, reducing contractile activity, and oxygen consumption, that normally favors enhanced glucose uptake, metabolism, and production of high-energy phosphates (ATP content) in myocardium. Cerebrocrast decreases heart rate and left ventricular (LV) systolic pressure; at concentrations of 10(-10) M and 10(-9) M it evokes short-term vasodilatation of coronary arteries. Increase of ATP content in the myocytes induced by cerebrocrast has a ubiquitous role. It can preserve the integrity of the cell plasma membranes, maintain normal cellular function, and inhibit release of lactate dehydrogenase (LDH) from cells that is associated with diabetes and heart ischemia. Administration of cerebrocrast together with insulin shows that both compounds only slightly enhance glucose uptake in myocardium, but significantly normalize the rate of contraction and relaxation ( +/- dp/dt). The effect of insulin on coronary flow is more pronounced by administration of insulin together with cerebrocrast at a concentration of 10(-7) M. Cerebrocrast may promote a shift of glucose consumption from aerobic to anerobic conditions (through the negative inotropic properties), and may be very significant in prevention of cardiac ischemic episodes.