Cesium and the pacemaker current

The Cardiac Pacemaker Story-Fundamental Role of the Na+/Ca2+ Exchanger in Spontaneous Automaticity

The electrophysiological mechanism of the sinus node automaticity was previously considered exclusively regulated by the so-called "funny current". However, parallel investigations increasingly emphasized the importance of the Ca²⁺-homeostasis and Na⁺/Ca²⁺ exchanger (NCX). Recently, increasing experimental evidence, as well as insight through mechanistic in silico modeling demonstrates the crucial role of the exchanger in sinus node pacemaking. NCX had a key role in the exciting story of discovery of sinus node pacemaking mechanisms, which recently settled with a consensus on the coupled-clock mechanism after decades of debate. This review focuses on the role of the Na⁺/Ca²⁺ exchanger from the early results and concepts to recent advances and attempts to give a balanced summary of the characteristics of the local, spontaneous, and rhythmic Ca²⁺ releases, the molecular control of the NCX and its role in the fight-or-flight response. Transgenic animal models and pharmacological manipulation of intracellular Ca²⁺ concentration and/or NCX demonstrate the pivotal function of the exchanger in sinus node automaticity. We also highlight where specific hypotheses regarding NCX function have been derived from computational modeling and require experimental validation. Nonselectivity of NCX inhibitors and the complex interplay of processes involved in Ca²⁺ handling render the design and interpretation of these experiments challenging.

Novel Na+/Ca2+ Exchanger Inhibitor ORM-10962 Supports Coupled Function of Funny-Current and Na+/Ca2+ Exchanger in Pacemaking of Rabbit Sinus Node Tissue

Background and Purpose

The exact mechanism of spontaneous pacemaking is not fully understood. Recent results suggest tight cooperation between intracellular Ca²⁺ handling and sarcolemmal ion channels. An important player of this crosstalk is the Na⁺/Ca²⁺ exchanger (NCX), however, direct pharmacological evidence was unavailable so far because of the lack of a selective inhibitor. We investigated the role of the NCX current in pacemaking and analyzed the functional consequences of the I_f-NCX coupling by applying the novel selective NCX inhibitor ORM-10962 on the sinus node (SAN).

Experimental Approach

Currents were measured by patch-clamp, Ca²⁺-transients were monitored by fluorescent optical method in rabbit SAN cells. Action potentials (AP) were recorded from rabbit SAN tissue preparations. Mechanistic computational data were obtained using the Yaniv et al. SAN model.

Key Results

ORM-10962 (ORM) marginally reduced the SAN pacemaking cycle length with a marked increase in the diastolic Ca²⁺ level as well as the transient amplitude. The bradycardic effect of NCX inhibition was augmented when the funny-current (I_f) was previously inhibited and vice versa, the effect of I_f was augmented when the Ca²⁺ handling was suppressed.

Conclusion and Implications

We confirmed the contribution of the NCX current to cardiac pacemaking using a novel NCX inhibitor. Our experimental and modeling data support a close cooperation between I_f and NCX providing an important functional consequence: these currents together establish a strong depolarization capacity providing important safety factor for stable pacemaking. Thus, after individual inhibition of I_f or NCX, excessive bradycardia or instability cannot be expected because each of these currents may compensate for the reduction of the other providing safe and rhythmic SAN pacemaking.

Genetically-engineered mesenchymal stem cells transfected with human HCN1 gene to create cardiac pacemaker cells

OBJECTIVE

To test the proof-of-principle that genetically-engineered mesenchymal stem cells (MSCs) transfected with the human hyperpolarization-activated cyclic nucleotide-gated channel 1 (hHCN1) gene can be modified to become cardiac pacemaker cells.

METHODS

MSCs were transfected with the hHCN1 gene using lentiviral-based transfection. The expressed pacemaker current (I(f)) in hHCN1-transfected MSCs was recorded using whole-cell patch-clamp analysis. The effect of the hHCN1-transfected MSCs on cardiomyocyte excitability was determined by coculturing the MSCs with neonatal rabbit ventricular myocytes (NRVM). The spontaneous action potentials of the NRVM were recorded by whole-cell current-clamp analysis.

RESULTS

A high level time- and voltage-dependent inward hyperpolarization current that was inhibited by 4 mM caesium chloride was detected in hHCN1-transfected MSCs, suggesting that the HCN1 proteins acted as I(f) channels in MSCs. The mean ± SE beating frequency in NRVMs cocultured with control MSCs transfected with the pcDNA3 plasmid control was 82 ± 8 beats/min (n = 5) compared with 129 ± 11 beats/min (n = 5) in NRVMs cocultured with hHCN1-transfected MSCs.

CONCLUSIONS

Genetically-engineered MSCs transfected with the hHCN1 gene can be modified to become cardiac pacemaker cells.

Countervailing modulation of Ih by neuropeptide Y and corticotrophin-releasing factor in basolateral amygdala as a possible mechanism for their effects on stress-related behaviors

Stress and anxiety-related behaviors controlled by the basolateral amygdala (BLA) are regulated in vivo by neuropeptide Y (NPY) and corticotrophin-releasing factor (CRF): NPY produces anxiolytic effects, whereas CRF produces anxiogenic effects. These opposing actions are likely mediated via regulation of excitatory output from the BLA to afferent targets. In these studies, we examined mechanisms underlying the effects of NPY and CRF in the BLA using whole-cell patch-clamp electrophysiology in rat brain slices. NPY, even with tetrodotoxin present, caused a dose-dependent membrane hyperpolarization in BLA pyramidal neurons. The hyperpolarization resulted in the inhibition of pyramidal cells, despite arising from a reduction in a voltage-dependent membrane conductance. The Y(1) receptor agonist, F(7)P(34) NPY, produced a similar membrane hyperpolarization, whereas the Y(1) antagonist, BIBO3304 [(R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]methyl]-N(2)-(diphenylacetyl)-argininamide trifluoroacetate], blocked the effect of NPY. The NPY-inhibited current was identified as I(h), which is active at and hyperpolarized to rest. Responses to NPY were occluded by either Cs(+) or ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride), but unaffected by the G(IRK)-preferring blockers Ba(2+) and SCH23390 [(R)-(+)-7-chloro-8-hydroxy-3-methyl-l-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride]. Application of CRF, with or without TTX present, depolarized NPY-sensitive BLA pyramidal neurons, resulting from an increase in I(h). Electrophysiological and immunocytochemical data were consistent with a major role for the HCN1 subunit. Our results indicate that NPY, via Y(1) receptors, directly inhibits BLA pyramidal neurons by suppressing a postsynaptic I(h), whereas CRF enhances resting I(h), causing an increased excitability of BLA pyramidal neurons. The opposing actions of these two peptides on the excitability of BLA output cells are consistent with the observed behavioral actions of NPY and CRF in the BLA.

Mesenchymal Stem Cells as a Gene Delivery System to Create Biological Pacemaker Cells in vitro

Pacemaker cells differ from common cardiomyocytes due to the presence of a spontaneous depolarization process during the diastolic phase of the cardiac cycle. This is due to hyperpolarization-activated cyclic nucleotide-gated ( HCN) channels, which are responsible for providing an inward current. Genetically engineered mesenchymal stem cells (MSCs) were transfected with hHCN4 genes using lentiviral transfection, and their potential use as biological pacemaker cells was investigated. In addition to expressing an anticipated high level of the hHCN4 gene, MSCs transfected with hHCN4 genes also expressed characteristic hHCN4 protein, a cardiac pacemaker-like current and were capable of increasing the spontaneous beating rate of co-cultured cardiac myocytes. Control MSCs did not exert these effects. It is hypothesized that genetically engineered MSCs transfected with hHCN4 genes by lentiviral transfection can be modified to be cardiac pacemaker cells in vitro.

Hyperpolarization-activated cyclic nucleotide-gated channels in mouse vomeronasal sensory neurons

Hyperpolarization-activated currents (Ih) are present in several neurons of the central and peripheral nervous system. However, Ih in neurons of the vomeronasal organ (VNO) is not well characterized. We studied the properties of Ih in sensory neurons from acute slices of mouse VNO. In voltage-clamp studies, Ih was identified by the characteristic kinetics of activation, voltage dependence, and blockage by Cs+ or ZD-7288, two blockers of the Ih. Forskolin, an activator of adenylyl cyclase, shifted the activation curve for Ih to less negative potentials. A comparison of Ih properties in VNO neurons with those of heterologously expressed hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, together with RT-PCR experiments in VNO, indicate that Ih is caused by HCN2 and/or HCN4 subunits. In current-clamp recordings, blocking Ih with ZD-7288 induced a hyperpolarization of 5.1 mV, an increase in input resistance, a decrease in the sensitivity to elicit action potentials in response to small current injections, and did not modify the frequency of action potentials elicited by a large current injection. It has been shown that in VNO neurons some pheromones induce a decrease in cAMP concentration, but the physiological role of cAMP is unknown. After application of blockers of adenylyl cyclase, we measured a hyperpolarization of 5.1 mV in 11 of 14 neurons, suggesting that basal levels of cAMP could modulate the resting potential. In conclusion, these results show that mouse VNO neurons express HCN2 and/or HCN4 subunits and that Ih contributes to setting the resting membrane potential and to increase excitability at stimulus threshold.

Mesenchymal stem cells transfected with HCN2 genes by LentiV can be modified to be cardiac pacemaker cells

Pacemaker cells differ from common cardiomyocytes for the presence of a spontaneous depolarization process during the diastolic phase of the cardiac cycle, which is due to the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel genes, providing the presence of an inward current. With this in mind, we embarked on a study to test proof-of-principle if genetically engineered mesenchymal stem cells (MSCs) transfected with HCN2 genes can be modified to be cardiac pacemaker cells. In addition to expressing an anticipated high level of hHCN2 gene, MSCs transfected with HCN2 genes by LentiV in our study also expressed characteristic hHCN2 protein, the I(f)-like current and were capable of increasing the spontaneous beating rate of cocultured cardiac myocytes. Control MSCs did not exert these effects. Thus, the electrical effects of the MSCs transfected with the hHCN2 gene were similar to the effects of overexpression of the same gene in the myocytes in vitro system. For stable genetic modification, we used the self-inactivating HIV1-based lentiviral vector (LentiV) for transgene delivery in our study, which can integrate transgene into the host genome. This unique property makes LentiV ideal for modifying MSCs, which allows persistent transgene expression. With these findings, we hypothesize that genetically engineered MSCs transfected with HCN2 genes by LentiV can be modified to be cardiac pacemaker cells.

Embryonic stem cells form an organized, functional cardiac conduction system in vitro

A functional pacemaking-conduction system is essential for maintaining normal cardiac function. However, no reproducible model system exists for studying the specialized cardiac pacemaking-conduction system in vitro. Although several molecular markers have been shown to delineate components of the cardiac conduction system in vivo, the functional characteristics of the cells expressing these markers remain unknown. The ability to accurately identify cells that function as cardiac pacemaking cells is crucial for being able to study their molecular phenotype. In differentiating murine embryonic stem cells, we demonstrate the development of an organized cardiac pacemaking-conduction system in vitro using the coexpression of the minK-lacZ transgene and the chicken GATA6 (cGATA6) enhancer. These markers identify clusters of pacemaking “nodes” that are functionally coupled with adjacent contracting regions. cGATA6-positive cell clusters spontaneously depolarize, emitting calcium signals to surrounding contracting regions. Physically separating cGATA6-positive cells from nearby contracting regions reduces the rate of spontaneous contraction or abolishes them altogether. cGATA6/ minK copositive cells isolated from embryoid cells display characteristics of specialized pacemaking-conducting cardiac myocytes with regard to morphology, action potential waveform, and expression of a hyperpolarization-activated depolarizing current. Using the cGATA6 enhancer, we have isolated cells that exhibit electrophysiological and genetic properties of cardiac pacemaking myocytes. Using molecular markers, we have generated a novel model system that can be used to study the functional properties of an organized pacemaking-conducting contracting system in vitro. Moreover, we have used a molecular marker to isolate a renewable population of cells that exhibit characteristics of cardiac pacemaking myocytes.

Pacemaker Channels

The pacemaker "funny" current (I(f)) has been the object of detailed investigations since its original description in sinoatrial node myocytes in the late 1970s; its role in underlying generation of spontaneous activity and autonomic modulation of cardiac rate has been amply demonstrated. In the late 1990s four isoforms of the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, the molecular components of native pacemaker channels, were cloned, and structure-function relation studies provided a molecular interpretation of several features of the native channels. Its role in pacemaking makes I(f) a natural target of heart rate modulating agents; several heart rate reducing molecules are known today that exert their action by specific inhibition of f-channels. Experiments aimed at determining the role of I(f) relative to other proposed pacemaker mechanisms such as SR Ca(2+) transients confirm that the I(f)-mediated rate control is a key process in pacemaker generation and autonomic control.