Abstract
BACKGROUND: The implantation of electronic devices has become the preferred treatment for symptomatic bradyarrhythmias. However, there are many shortcomings in electronic pacemakers. The usage of molecular biology principle to develop biological pacemaker has become a topic of discussion in research. When sinoatrial node is inhibited, pacemaker effect runs by transfecting hyperpolarization-activated cyclic nucleotide-gated (HCN) channel gene of If current, overexpressing HCN, and increasing inward current in diastolic phase of the heart. Construction of biological pacemakers by gene therapy and cell therapy may become an optimal substitute of electronic pacemakers in the near future.
OBJECTIVE: To sum up the research advancement in application of HCN channel gene to the development of biological pacemaker.
RETRIEVAL STRATEGY: The relevant articles published between January 1979 and June 2007 were searched for in Pubmed database by researcher of this article with the key words of "hyperpolarization-activated cyclic nucleotide-gated channel, biological pacemaker" in English. 157 articles were selected and reviewed by the inclusive criteria of: ① articles closely related with the application of HCN to the development of biological pacemaker; ②the late articles and articles in authority journals in the same field. Exclusive criterion: repetitive studies.
LITERATURE EVALUATION: The main sources of literatures were randomized clinical trial (RCT) on biological pacemaker by HCN. Among 36 selected articles, 10 were reviews, and others were elementary experimental studies.
DATA SYNTHESIS: ①Of all four HCN isoforms, HCN1, HCN2, and HCN4 are the main isoforms in the heart. HCN3 only expresses in embryonic pacemaker cells in a low level. HCN2 highly expressed in low pacing regions (ventricular muscle), whereas HCN4 highly expressed in high pacing regions. Moreover, HCN2 are the main isoforms in the ventricle. Expression ratio of HCN2 to HCN4 is 5:1 in neonate rats and 13:1 in adult rats. ②Defects in HCN channels may underlie sick-sinus syndrome. ③Up to now, HCN genes of If current contribute importantly to the generation of the regular pacemaker potential.
CONCLUSION: Gene therapy and cell therapy have become an optimal approach to improve biological pacemakers. Application of HCN channel gene in development of biological pacemaker may hold great promise in the treatment of chronic arhythmia.

INTRODUCTION

Cardiac function depends on the appropriate timing and synchronization of the mechanical contraction in various regions of the heart as well as on achieving the appropriate heart rate. These properties are ensured through the hierarchical organization and electrical specialization of the cardiac conduction system, which are governed by the differential expression of cardiac ion channels in each component. The electrical impulses originate in a group of pacemaking cells in the sinoatrial (SA) node. These cells possess specific ionic channel current combinations that enable them to spontaneously depolarize at a constant rate subject to neurohormonal influences [1]. Abnormalities of impulse generation, propagation or the duration and configuration of individual cardiac action potentials form the basis of disorders of cardiac rhythm, which may result in abnormally low heart rate, circulatory failure, even death.
The implantation of electronic devices has become the preferred treatment for symptomatic bradyarrhythmias, including atrioventricular blocks and sinus node dysfunctions with excellent success and minimal morbidity. The shortcomings of electronic pacemakers include limited battery life, need for lead implantation into heart, which in most cases can no longer be removed, and lack of response to autonomic and physiologic demands on the heart. Furthermore, cardiac pacing in the pediatric age still needs improvements, as the devices cannot follow the somatic growth of the little patients and have to be changed over the years. Nonetheless, the ideal therapy for these disorders may be the development of a biological solution allowing reconstitution of the physiological electrical activity of the cardiac conduction system with the same plasticity and adaptability to the human body and to the physiology of the cardiovascular system. At present, molecular approaches to the development of a biological pacemaker are a conceptually attractive alternate treatment modality for bradyarrhythmias. In this field, both gene and stem cell therapies represent new and promising strategies for the development of a biological pacemaker [2].

OBJECTIVE

This paper will focus on the possible therapeutic application of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel gene to cardiac electrophysiology.

RETRIEVAL STRATEGY

The relevant article published between January 1979 and December 2006 were searched for in
Pubmed database by researcher of this article with the key words of "hyperpolarization-activated cyclic nucleotide- gated channel, biological pacemaker" in English.

Retrieval method
Inclusive criteria: ① having close relations with the application of HCN to the development of biological pacemaker; ② the late articles and articles in authority journals were chosen in the same field.
Exclusion criteria: repetitive studies.
Literature category and analysis of data: The main sources of literatures are randomized clinical trial (RCT) on biological pacemaker by HCN. Among the 36 selected articles, 10 are reviews [2-11], and others are elementary experimental studies [1,12-36].

ROUTINE TECHNIQUE EVALUATION

Evaluation of efficiency：All the empirical methods can achieve the prospective purpose in the literature.
Evaluation of safety：The correlated empirical methods and materials had no obvious harm to experimenters.
Evaluation of ethics：The experiments involved in literature corresponded to ethics criterion.

GENERAL EVALUATION

Characteristics of HCN channel and If current
Action potential of pacemaker cells is unique in that they have a slow depolarizing phase, rendering them spontaneously active [3]. HCN channels generate hyperpolarization-activated cation currents, which contribute to genesis of pacemaker activity. Hyperpolarization- activated cation currents, originally termed If for "funny" in the heart and Ih for "hyperpolarization" or Iq for "queer" in neurons, were initially described in sinoatrial node myocytes [4-5,12-14], in hippocampal pyramidal cells, and in photoreceptor cells [15-18] during the late 1970s and early 1980s.
In the heart, If was firstly identified in the sinoatrial node (SAN) and initially thought to be restricted to the cardiac conduction system. If is mostly apparent in the SAN, but also observed in non-pacing cardiac tissue of the ventricles and atria. If channel is activated on membrane hyperpolarization rather than on depolarization [19]. The threshold of voltage for activation of If is around -40 to -50 mV in SAN cells and is extremely negative in adult ventricular myocytes (as negative as -145 mV). When HCN2 is overexpressed in adult ventricular myocytes, large inward currents can be induced [20].
If channel has four-fold selectivity for K+ than Na+, and yields the reversal potential between -25 mV and -40 mV. The typical features of If current include activation by hyperpolarized membrane potential, conduction of Na+ and K+, but not of Li+ and divalent cations, modulation by cyclic adenosine monophospate (cAMP) and blockade by cesium (Cs+) [6]. HCN generated current also has the above features. Normal resting potentials activate the channel and induce a net inward current mainly carried by Na+, which may lead to depolarization of the membrane and subsequent action potential generation. Four different HCN genes have been identified [21]. HCN1 is the most rapidly acting channel, HCN4 the slowest with HCN2 and HCN3 possessing intermediate kinetics [7]. If channels have a very small single channel conductance. Even under conditions of elevated [K+]o, channel conductance is only 1 pS [8]. Reduction of [K+ ]o to lower than normal extracellular levels can result in a dramatic decrease in Ik magnitude due to the unusual feature that the channe1 conductance is highly sensitive to [K+ ]o levels [9].
Another characteristic of If is its sensitivity to relatively low concentrations of extracellular Cs+ (1-2 mmol/L) and its insensitivity to extracellular Ba2+ (1-2 mmol/L). Activation of If in cardiomyocytes is quite slow with several seconds to reach a steady state. The amplitude of the tail currents at a fixed holding potential is usually analyzed for a voltage-dependent activation curve of If. The activation curve is fitted by a Boltzmann function to obtain the V1/2 of activation. The values of the V1/2 among different cardiac cells can very widely from -60 mV (SAN cells) to -120 mV (adult ventricular cells). This variation may result from a composition ration of the HCN isoforms, because the HCN2 isoform is associated with a negative activation threshold in ventricular myocytes [10,22].
An important feature of If is the regulation by cyclic nucleotides. Elevation of intracellular cAMP levels enhances the amplitude of If and shifts the voltage-dependent activation to positive potentials (usually 10 mV). On the other hand, reduction of intracellular cAMP decreases the amplitude of If and shifts the activation curve to more negative potentials. Interestingly, the regulation of Ih by cAMP is via its binding on the site (CBND) of the C-terminal and does not require protein phosphorylation. Cyclic GMP binds to the same site with much less affinity and has less effect on If as cAMP. Adrenergic or muscarinic agonists can accelerate or slow down the heart rate via modulation of intracellular cAMP levels [23-24].

Expression of HCN channels in the heart
The mammalian genome encodes four HCN genes termed HCN1 through 4. All four HCN isoforms have been found in the heart. While the expression of these isoforms varies somewhat among species, cardiac tissue, and developmental stage analyzed, HCN1, HCN2, and HCN4 are the main isoforms in the heart. HCN 3 expression is low in embryonic pacemaker cells and is very low in the mouse heart. The normal pacemaking region (sinoatrial node) of the heart shows the significant amount of HCN4 and exhibits both the largest and the most positively activating pacemaker current. The most predominant isoform of rabbit SAN is HCN4, accounting for 81% of the total HCN mRNA. Significant levels of HCN1 are present in the rabbit SAN (18% of total HCN mRNA). In contrast, in the mouse SAN, the rest of the HCN transcript is made up of HCN2, and only very low levels of HCN1 are found. In the rabbit ventric1e only HCN2 was detected. Rat ventricle exhibits greater If and higher HCN levels than rabbit. In rats, HCN2 is the predominant ventricular isoform, especially in adult animals. Although HCN2 is the predominant isoform in both the rat and the rabbit ventricles, the level of HCN2 expression is much higher in the rats than in the rabbits. HCN2 is the most abundant HCN channel isoform in the whole rabbit heart [25-27].
Although the biophysical properties of heterologously expressed HCN isoforms can not fully account for the observed variation in native If, there is some correlation between which isoforms are expressed in a specific region and the voltage dependence of the native pacemaker current; regions with the most negative activation (e.g., ventricle) tend to express HCN2 predominantly. In contrast, regions with more positive voltage ranges of activation (e.g. SAN) mainly express HCN4. In addition, HCN2 is the dominant ventricular isoform throughout development. The relative expression ratio of HCN2: HCN4 increases from 5:1 in the neonatal rat ventric1e to 13:1 in the adult rat ventric1e. At the mean time, the voltage dependence of native If is becoming more negative. In addition, decreasing percentages of the isoform of HCN4 may also account for more negative If thresholds. The percentage of HCN4 is 81% in rabbit SAN, 18% in neonatal rat ventricle, and 7% in adult rat ventricle [27].

HCN 2 Knockout mice
Mice lacking HCN2 channels show a pronounced cardiac phenotype [28-29]. Mutant animals display a cardiac arrhythmia characterized by varying RR intervals, but normal PQ and QT intervals and individual waveforms, indicating that the arrhythmia is due to dysfunction of the sinus node. RR intervals of mutant mice do not fluctuate randomly. Periods of normal activity ranging from seconds to minutes are followed by periods with intermitting slower beats. It should be noted that the shortest RR intervals of mutants are not shorter than those of wild type. Similarly, the heart rate during activity and in response to isoproterenol is not different between HCN2-deficient and wild-type mice. These results indicate that HCN2 is not required for sympathetic stimulation of heart rate. Current-clamp recordings displayed that the membrane potential of HCN2-deficient sinoatrial node cells is more hyperpolarized than wild-type cells. The maximum diastolic potential of HCN2 knockout sinoatrial cells was shifted by -5 mV. These results show that HCN2 is activated in wild-type cells and sets the membrane potential to a more depolarized level. Hence, the predominant role of HCN2 in these cells may be the stabilization of a normal diastolic membrane potential. The absence of this functional resetting in HCN2-deficient mice provides a plausible explanation for their cardiac arrhythmia. In summary, HCN2 may provide a safety mechanism for the correct setting of the maximum diastolic potential. Without this channel, the membrane potential is more prone to fluctuations in the hyperpolarizing direction, resulting in irregular generation of action potentials.

HCN4 Knockout mice
HCN4-deficient mice were generated by deleting exon 4 of the HCN4 gene [30]. Homozygous knockouts as well as mice lacking HCN4 specifically in the heart died between embryonic days 9.5 and 11.5. In the cardiomyocytes of these embryos, If is reduced by about 80%. The hearts of knockout animals showed no structural defect. However, the contraction rate of isolated HCN4-deficient hearts was reduced by about 40% compared with the contraction of wild type hearts. In contrast to wild-type hearts, the contraction rate of HCN4-deficient hearts was not increased by the addition of cAMP. Importantly, both wild-type and knockout hearts contracted regularly without obvious arrhythmias. Action potential recordings revealed the presence of cardiac cells with a "primitive" action potential [31] in both wild-type and HCN4 knockout cells. However, cardiac cells with "mature" pacemaker potentials, observed in wild-type embryos starting at day 9, were never detected in HCN4-deficient embryos. Taken together, these results indicate that HCN4 contributes importantly to the generation of the regular pacemaker potential and the determination of the basal heart rate.

HCN mutations and sinus node dysfunction
As soon as the molecular basis of If was elucidated, it was suggested that primary defects in HCN channels might underlie certain cardiac arrhythmias, in particular familial sinus rhythm diseases [28]. It was also proposed in these two studies that defects in HCN channel function might contribute to acquired disorders of heartbeat generation. Indeed, a mutation of HCN4, not HCN2 was linked to the cause of idiopathic SND in a SND patient [32]. A heterozygous 1-bp deletion (1 631delC) in exon 5 of the human HCN4 gene was detected in a patient with idiopathic SND. The mutant HCN4 protein (HCN4-573X) had a truncated C-terminus and lacked the cyclic nucleotide-binding domain. Expression of HCN4-573X cDNA in COS-7 cells exhibited normal intracelular traficking and membrane integration of the mutant channel and also showed If-like currents. As expected, the mutated channel cannot be stimulated by cAMP. Coexpression with the wild-type of HCN4 indicated a dominant-negative effect of HCN4-573X channels that did not respond to a rise in intracellular cAMP. More recently, HCN4 mutation was reported to be associated with life-threatening cardiac arrhythmias in another patient [33]. A missense mutation, D553N, was found in a patient with SND. This particular mutation caused defective HCN4 channel traficking to the cell membrane. A functional study in vitro of the D553N mutation showed a decrease in If due to HCN4 traficking defect in a dominant-negative manner. However, no genetic linkage data are available to provide further evidence that the mutation is indeed pathogenic. The role of HCN4 for pacemaking in the adult heart is not yet settled. Adult mice lacking HCN4 channels are not available yet, because HCN4-deficient mice die in utero. Nonetheless, results from HCN4 knockout embryos indicate that loss of the channel leads to severe bradycardia and chronotropic incompetence. Mutations in human HCN channel genes other than HCN4 have not been reported yet.
HCNs and biological pacemaker
Pacemaker cells differ from common cardiomyocytes for the presence of a spontaneous depolarization process, which progressively reduces the membrane potential during the diastolic phase of the cardiac cycle. When the reduction reaches a critical threshold value, the sodium channels open and the action potential ensues. The spontaneous diastolic depolarization of pacemaker cells is due to the expression of four genes (HCN1-4), which code for four specific proteins, providing the presence of an inward current named If. Therefore, the main difference between pacemaker cells and other cardiomyocytes depends only upon which genes are fully expressed.
Local overexpression of pacemaker currents in the heart in vivo is one of novel approaches for developing biological pacemakers. Recently, several studies demonstrated that overexpression of If in myocytes via gene delivery or implantation of stem cells with overexpression of If successfully induced automaticity of a biological pacemaker in arrhythmic animal models.
Research by Qu et al [11] showed that HCN2 over expression provides an If-based pacemaker current sufficient to drive the heart when injected into a localized region of atrium. Adenoviral constructs of mouse HCN2 and green fluorescent protein (GFP) or GFP alone were injected into LA, terminal studies performed 3-4 days later, myocytes examined for native and expressed pacemaker current (If). Spontaneous LA rhythms occurred after vagal stimulation induced sinus arrest in 4 of 4 HCN2 + GFP dogs and 0 of 3 GFP dogs (P < 0.05). Electrophysiological mapping identified the source of the rhythm to the injection site at the left atrium. Whole cell electrophysiological recordings from transfected myocytes demonstrated the presence of a relatively high-magnitude pacemaker current.
In another research, Plotnikov et al [34] studied the effect of administration of the HCN2 gene to the left bundle branch system of dogs. An adenoviral construct incorporating HCN2 and GFP as a marker was injected via catheter under fluoroscopic control into the posterior division of the LBB. Controls were injected with an adenoviral construct of GFP alone or saline. During vagal stimulation, HCN2 injected dogs showed rhythms originating from the left ventricle, the rate of which was significantly more rapid than controls. The presence of a pacemaker channel was verified by immunohistochemical assay of the HCN2 channe1. Biophysical analysis at physiological potentials showed a marked increase in pacemaker current in the isolated Purkinje myocytes with transfection of the channel.
Potapova et al[35] tested the ability of human mesenchymal stem cells (hMSCs) to deliver a biological pacemaker to the heart. Genetically modified hMSCs were engineered to express functional HCN2 channels in vitro and in vivo, mimicking overexpression of HCN2 genes in cardiac myocytes, and represent a noval delivery system for pacemaker genes into the heart or other electrical syncytia. hMSCs transfected with mouse HCN2 expressed high levels of Cs+ -sensitive If current. The expressed current responded to isoproterenol with a positive shift in activation. Acetylcholine had no direct effect, but in the presence of isoproterenol, shifted activation negatively. HCN2- transfected hMSCs significantly increased beating rate in vitro when plated onto a localized region of a coverslip and overlaid with neonatal rat ventricular myocytes. Implantation of HCN2-transfected hMSCs left ventricular wall in to the subepicardial canine situ developed spontaneous rhythms of left-sided origin during sinus arrest via vagal stimulation. Moreover, when injected in a canine left ventricle, these transgenic mesenchymal cells were able to establish gap junctions with host cardiomyocytes.
More recently, using various electrophysiological and mapping techniques, Tse et al [36] examined the effects of in situ focal expression of HCN1 the S3-S4 linker of which has been shortened to favor channel opening, on impulse generation and conduction. Single left ventricular cardiomyocytes isolated from guinea pig hearts preinjected with the recombinant adenovirus Ad-CMV-GFP-IRES- HCN1 in vivo uniquely exhibited automaticity with a normal firing rate [(237±12) beats/min]. High-resolution ex vivo optical mapping of Ad-CGI-HCN1 injected Langendorff- perfused hearts revealed the generation of spontaneous action potentials from the transduced region in the left ventricle. To evaluate the efficacy of their approach for reliable atrial pacing, they generated a porcine model of sick-sinus syndrome by guiding radiofrequency ablation of the native SA node, followed by implantation of a dual-chamber electronic pacemaker to prevent bradycardia-induced hemodynamic collapse. Interestingly, focal transduction of Ad-CGI-HCN1 in the left atrium of animals with sick-sinus syndrome reproducibly induced a stable, catecholamine-responsive in vivo "bioartificial node" that exhibited a physiological heart rate and was capable of reliably pacing the myocardium, substantially reducing electronic pacing.

Limitations of approaches to the development of biological pacemakers
Ideally, a new biological pacemaker should mimic physiological activation of the heart with a physiological rate. To accomplish this, a biological pacemaker would be delivered to a site that would activate the conductive system of the ventricles. Favorable benefits of biological pacemakers may include long-term cure rather than palliative treatment. Moreover, the heart rate generating from an appropriately engineered biological pacemaker may respond more naturally to humoral and neuronal stimuli. The ultimate goal of developing biological pacemakers would be to supplement or replace electronic pacemakers.
The pioneering studies described in the previous sections established the feasibility of gene delivery to modify the excitable properties of the myocardial tissue but also raised several issues that may limit the clinical utility of this approach. A major issue is duration of efficacy of biological pacemakers. The duration of pacemaker function in approaches using viruses depend on how long the viruses and resulting protein constructs survive in the host. To ensure long-term function the appropriate delivery system in which the construct is effective for long periods must be identified. What will be the longevity and stability of next generation of pacemaker genes?
The other limitations include those that are inherent to other gene therapy strategies such as the possible expression of the transgene in nontarget organs, the potential to trigger autoimmunity, potential for neoplasia, potential toxic effect of the vector or transgene, and host immune response. Besides these pitfalls, the use of gene therapy for the treatment of cardiac arrhythmias may be hampered by a number of specific limitations. First, the limited knowledge of the molecular mechanisms underlying many of the cardiac arrhythmias and the spatial and temporal complexity of ion channel expression in various regions of the hearts may preclude the utilization of a single ion channel transgene. A second major hurdle relates to the inability to adequately control several other key parameters such as the level of transgene expression within the cells, the number of transfected myocytes, their transmural distribution, and their regional distribution within the heart. Consequentially, in vivo myocardial expression using currently available viral vectors is not predictable, is relatively short-lived, is inhomogenous, may lead to increased dispersion of different electrophysiological properties, and may actually facilitate the generation of arrhythmias. In that respect, the degree of coupling between the myocytes may have an important effect on this arrhythmogenic risk, because the electrotonic currents generated between the cells would tend to reduce the inhomogeneities of repolarization generated by the heterogenous gene expression between neighboring cells.

CONCLUSION

Cardiac arrhythmias represent a major cause of worldwide morbidity and mortality. Although marked progress has been achieved in several of the pharmacological and nonpharmacological therapeutic modalities for these rhythm disorders in recent years, there is still a need for the development of new therapeutic paradigms that are more targeted and associated with fewer side effects. Improvement in the understanding of the mechanisms underlying many of these arrhythmias and the development of molecular and cellular tools suggest a future role for gene and cell therapies for the treatment of these different rhythm disorders. Nevertheless, bridging the gap between the proof-of-concept and the clinical application will require important methodological developments as well as extensive animal experimentation.

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Contributors: The first author composed and designed the review, and analyzed relevant data. After four revisions, all the authors drafted the review. The first author is in charge of this article.
Competing interests: None declared.
Ethical approval: No ethical differences.
What is already known on this topic: Characteristics of HCN channel and If current, distribution of HCN channel in the heart, and defects of HCN channel can lead to sick-sinus syndrome.
What this study adds: Based on former studies, animal experiment was performed in this study. HCH gene was transfected into myocardial cells by various approaches. It was confirmed that expression of pacemaker channel protein and pacemaker If current was found in vitro and in vivo. Surface electrocardiogram showed ectopic rhythm in injection site.

心脏电子起搏器时代与生物起搏替代的
前沿话题*☆

周亚峰，杨向军
苏州大学附属第一医院心内科，江苏省苏州市 215006
周亚峰☆，男，1973年生，江苏省泰兴市人，汉族，苏州大学在读博士，主治医师，主要从事心血管疾病的基础和临床研究。
江苏省“135”重点人才基金项目（RC2003019）*
摘要
学术背景：植入电子起搏器是目前治疗症状性缓慢心律失常的主要方法，然而它存在许多缺点。能否利用分子生物学原理发展生物起搏器成为大家关注的热点。通过转染编码If电流的超极化激活环核苷酸门控通道基因，过度表达超极化激活环核苷酸门控通道，增加心脏舒张期内向电流，从而在窦房结被抑制时提供起搏作用，这种利用基因治疗和细胞治疗构建的生物起搏在不久的将来可能会成为电子起搏器最为理想的替代方法。
目的：总结超极化激活环核苷酸门控通道基因构建生物起搏的研究进展。
检索策略：由该论文的研究人员应用计算机检索Pubmed数据库1979-01/2007-06的相关文献，检索词“hyperpolarization-activated cyclic nucleotide-gated channel; biological pacemaker”，并限定文章语言种类为English。共检索到157篇文献，对资料进行初审，纳入标准：①与生物起搏及超极化激活环核苷酸门控通道基因密切相关。②同一领域选择近期发表或在权威杂志上发表的文章。排除标准：重复性研究。
文献评价：文献的来源主要是超极化激活环核苷酸门控通道基因的基础实验。所选用的36篇文献中，10篇为综述，其余均为临床或基础实验研究。
资料综合：①在4种异构体中超极化激活环核苷酸门控通道1,2,4是心脏中的主要部分，超极化激活环核苷酸门控通道3只在胚胎起搏细胞中有低水平表达。起搏活性小的区域（如心室肌），超极化激活环核苷酸门控通道2的表达占优势；而起搏活性高的区域超极化激活环核苷酸门控通道4的表达占优势。此外，超极化激活环核苷酸门控通道2在整个发育阶段，是心室的主要异构体，超极化激活环核苷酸门控通道2:超极化激活环核苷酸门控通道4的相当表达量在乳鼠为5:1，成年鼠为13:1。②超极化激活环核苷酸门控通道通道缺陷可导致病窦综合征。③到目前为止，转染编码If电流的超极化激活环核苷酸门控通道基因被认为是最有可能实现生物起搏的。
结论：基因治疗和细胞治疗必将成为改善生物“起搏”功能最理想的方法，以超极化激活环核苷酸门控通道基因和细胞为主的生物起搏在缓慢性心律失常治疗中必将占有一席之地。
关键词：超极化激活环核苷酸门控通道基因；生物起搏；综述文献
中图分类号: R318 文献标识码: A 文章编号: 1673-8225(2008)09-01787-06
周亚峰，杨向军.心脏电子起搏器时代与生物起搏替代的前沿话题[J].中国组织工程研究与临床康复，2008，12(9):1787-1792
[www.zglckf.com/zglckf/ejournal/upfiles/08-9/9k-1787(ps).pdf]
（Edited by Qiu Y/Wang L）