The interest in mouse electrophysiology is expanding fast, despite the marked differences between mouse and man. Different methods have become available to analyse the electrical activity in the mouse heart in vivo. The mouse genome can be altered with relative ease, which allows the molecular dissection of the various components that contribute to de- and repolarisation of the cardiomyocyte and the initiation and propagation of cardiac arrhythmias. Mouse ischaemia reperfusion models have been used recently to study preventive measures against ischaemic myocyte damage. In the present review, the electrophysiological measurements performed in mice to date are discussed and complemented with results from a mouse ischaemia reperfusion model.
Heart rate (variability)
In 1968 Goldbarg et al.  published their findings on the mouse electrocardiogram (ECG). This paper was preceded by the reports from Agduhr and Stenstrom  in 1929 using the string galvanometer, and a key paper by Richards et al.  in 1953, who analysed the effect of cooling, potassium and hypoxia on T-wave behaviour in mice. The descriptive study by Goldbarg et al.  was apparently well thought of and outlined, as funding was obtained from two private funds, the National Institutes of Health, and the National Heart Institute. Clearly, already in the midsixties researchers and reviewers appreciated the potential value of the mouse for cardiovascular research. Were they clear-sighted or did they realise how important genetics would be for cardiovascular disease? The answer can be found in two publications in Science, one in 1962 showing mouse strain based differences in cholesterol levels , and one in 1966, when valvular changes mimicking human disease were described . The group of Goldbarg et al.  applied state of the art ECG technology to mouse hearts from different inbred strains by using orthogonal axes (X,Y,Z) and two unipolar leads (midchest and back; these leads were also used to record Z). They searched extensively for the electrical markers of repolarisation, and reported on the difficulty to define the QT-segment in mice. The QT-interval could be measured best from the Z-lead recordings, but varied between 83±24 and 96±12 (16%) in different strains (C57Bl and SEC, respectively). The QT-interval correction for differences in heart rate by applying Bazett's formula only further accentuated the genotype-based variation. Although our possibilities to manipulate the mouse genotype have changed dramatically during the last 30 years, we are still struggling to measure and analyse the cardiac phenotype correctly. Therefore, the work performed in the sixties in Cleveland, OH, is still relevant and important. The low frequency with which this paper has been cited over the past 30 years is therefore surprising. Probably this has to be explained by the delay of the progress in molecular genetics.
2 Mouse ECG and electrophysiology in 1999
Different methods are now available to analyse the electrical activity in the mouse heart, and even mouse pacemakers with special leads and pacing devices are being manufactured. The human like 12-lead ECG can be obtained, and electrophysiological studies can be performed in open chest models supported by artificial respiration . In this in vivo situation, conduction velocities have been recorded . The transoesophageal approach has been taken to induce supraventricular arrhythmias in mice with a closed chest model . In addition, chronic instrumentation has been applied by implanting radio frequency transmitters . All these techniques have been used in different gene-targeted mouse models . Direct comparison of data between studies is hampered by the variations in strain and anaesthesia applied, however the data are reproducible within each of these studies. To further illustrate this diversity, Table 1 contains a quantitative comparison of ECG data from older and more recent studies performed on genetically nonmodified (wild-type) mice.
Several methods have been developed to analyse cardiac ischaemia in mouse. Myocardial infarction (MI) and complications like arrhythmias are still the main cause of death in Western society, whereas the role of reperfusion injury remains to be determined. In the rat, temporary occlusion of the left coronary artery has been extensively used to induce ischaemia and reperfusion (IR) injury. The IR rat model has turned out to be extremely versatile for studies on the pathophysiology of IR induced cardiac arrhythmias . The mouse model allows mechanistic and intervention studies of IR injury and MI in genetically engineered animals . The IR injury model in mouse was developed and characterised by Michael et al. . After temporary occlusion of the left coronary artery for 30 or 60 min followed by 120 min of reperfusion, 62–67% of the mice survived. The total mass of the infarct in relation to the area at risk was between 34 and 45%. The mouse IR model was also successfully applied in genetically engineered strains. Transgenic mice that were overexpressing rat heat shock protein 72 (HSP72) and control wild-type littermates were subjected to 30 min of LAD (left anterion descending artery) occlusion followed by 120 min of reperfusion . Overexpression of HSP72 reduced the infarct size in this in vivo model of myocardial IR. The role of apoptosis and preventive interventions in the development of myocardial damage can be studied in mice by quantifying annexin V labelling .
Electrocardiography was implemented in experiments to study the role of the angiotensin II in IR injury. Harada et al.  used a left thoracotomy approach to ligate the proximal left coronary artery in angiotensin II type 1A receptor (AT1A)-deficient mice and wild-type littermates. Arrhythmias were studied during 30 min of left coronary artery occlusion followed by 120 min of reperfusion. Ventricular arrhythmias were observed in all wild-type mice following reperfusion, but not in AT1A-deficient mice, suggesting that angiotensin II may be critically involved in the initiation of reperfusion arrhythmias.
Characteristic for the mouse ECG is, that no clear ST-segment can be distinguished, and the T-wave merges with the final part of the QRS-complex . To investigate the effect of regional myocardial ischaemia on the ST-segment and T-wave morphology, we recorded ECGs during 30 min of LAD occlusion in CD2F1 mice. Typically, R-wave enlargement and ST-segment elevation developed within 5 min after coronary artery occlusion (Fig. 1). Starting from 15 min of ischaemia, Q-waves developed, while ST-segment elevation became even more pronounced.
QT-intervals measured in vivo in genetically engineered mice. Bipolar surface electrocardiogram (lead II) obtained from a CD2F1 mouse heart in vivo. Recording before coronary occlusion (A), after 5 min (B), 15 min (C), and 30 min (D) of coronary artery occlusion. Note the R-wave enlargement (B), ST-segment elevation (B–D), and development of Q-waves (C–D).
These ECG changes are similar to those observed in a rat model of myocardial ischaemia, i.e., R-wave enlargement, development of Q-waves and ST-segment elevation as MI develops . The time-course of these ECG changes may provide a useful index for the assessment of antiischaemic and antiarrhythmic drug efficacy . A similar approach in genetically modified mice will help to determine the role of gene dosage (knock-out or overexpression) for the genesis of arrhythmias following myocardial IR injury.
4 Long QT syndrome (LQTS) mutations
LQTS is an inherited cardiac arrhythmia, characterised by a prolonged QT-interval, tachyarrhythmias (torsade de pointes) and sudden death . Recently, cardiac N+-channel and K+-channel gene mutations have been identified as the cause of LQTS [20–22]. In order to assess the precise mechanisms by which these ion channel mutations can lead to QT prolongation and arrhythmias, genetically modified mice have been generated to study cardiac de- and repolarisation.
Unfortunately, many studies are being published using gene-targeted mice while we are still in the process of unravelling the physiology of the wild-type murine action potential
Overexpression of the particularly severe form of LQT2-associated HERG mutant G628S resulted in dominant-negative suppression of IKrin vivo . Although the action potential duration was significantly prolonged in isolated single ventricular myocytes from HERG-G628S transgenic mice, no changes were measured in intact ventricular strips, or in vivo. In addition, the QT-interval recorded in anaesthetised mice was unchanged, and no ventricular arrhythmias could be induced by programmed electrical stimulation.
Mutations in KCNE1 gene can cause LQTS [24,25]. The KCNE1 gene product, a small protein called IsK or minK, behaves as a K+-channel beta-subunit and coassembles with the proteins encoded by the KvLQT1 and HERG genes, to produce the cardiac IKs and IKr currents . Using gene-targeting strategies, two groups independently generated minK-deficient mice [27,28]. Drici et al.  reported in KCNE1-deficient mice, longer QT-intervals at slow heart rates and paradoxical shorter QT-intervals at higher heart rates, during anaesthesia-induced lengthening of the interval between consecutive QRS-complexes (RR, Table 2). An increase in RR cycle length from 100 to 400 ms induces a threefold increase in QT duration in wild-type compared to fivefold in KCNE1-deficient mice. This change in QT–RR relation has been suggested to be essential for LQTS . A similar approach to study the effect of invalidation of the KCNE1 gene product was used by Kupershmidt et al.  They generated minK-deficient mice in which the bacterial lacZ gene substituted the minK coding region such that beta-galactosidase expression was controlled by endogenous minK regulatory elements . In contrast to Drici et al. , they found no differences between wild-type and minK knock-out mice in any ECG interval recorded in anesthetised mice, nor was there any difference in QT-interval when heart rate was increased by an isoproterenol challenge. They concluded that in adult mice, IKs and IKr do not play an important role in determining cardiac repolarisation.
In contrast to the functional and molecular diversity of IKr and IKs, the properties of the transient outward current (Ito) in adult mammalian cardiac cells isolated from different species are quite similar. Barry et al.  introduced a point mutation in the pore region of the Kv 4.2 gene to produce a subunit (Kv 4.2-W362F) that exerts a dominant-negative effect. Electrophysiological studies revealed that in ventricular myocytes isolated from Kv 4.2-W362F mice, the fast component of Ito (Ito,fast or Ito,f) was absent, and thus that the Kv 4 subfamily underlies Ito,f in the mouse heart. Surface ECG recordings from Kv 4.2-W362F-expressing transgenic mice revealed a marked prolongation of the QT-interval and action potential duration. In addition, it was found that a novel current was upregulated in Kv 4.2-W362F-expressing transgenic mice, not expressed in wild-type littermates.
London et al.  reported the generation of long QT mice expressing a truncated Kv 1.1 protein (Kv 1.1-N206Tag) that also exerts a dominant-negative effect. In Kv 1.1-N206Tag transgenic mice, the slowly inactivating, 4-aminopyridine sensitive outward K+ current (Islow) is attenuated . This is consistent with the hypothesis that a member of the Kv1 subfamily, likely Kv 1.5, underlies this K+ current. In Kv 1.1-N206Tag mice, prolonged ventricular action potentials and QT-intervals were recorded. In contrast to Kv 4.2-W362F mice, in Kv 1.1-N206-Tag mice spontaneous ventricular tachyarrhythmias were documented .
The interpretation of the results obtained from studies in transgenic mice is hampered by a lack of knowledge, with respect to the relationship between action potential morphology and the underlying currents in wild-type mice. The relative contribution of different potassium channels to action potential duration and repolarisation in mice requires further investigation. Whether information gained from these mouse experiments can be translated directly to improved therapeutic interventions in man remains to be established.
5 Atrial fibrillation (AF)
In human, AF is a common disorder affecting far more patients than ventricular tachyarrhythmias. At the age of 50 to 59 years, 0.5% of the population suffers from AF increasing to almost 9% of all people at the age of 80 to 89 years . Although AF appears to be a benign disorder, AF is an independent risk factor for mortality (1.3–1.9-fold increased risk). As 15% of all strokes occur in people with AF, it is a major risk factor for cerebrovascular accidents .
During sinus rhythm, impulse conduction via cardiomyocytes occurs unidirectional, exiting myocytes in a fixed order. This guarantees the coordinated contraction of the atria and ventricles. The direction of the wave fronts can change due to anatomical and/or functional obstacles. The wavefronts can then encounter previously excited tissue, hereby reexciting it, and cause a phenomenon called reentry. Obstacles leading to reentry can be a disturbed morphology of the heart or conduction system (e.g. infarction scars). In addition, shortening of the refractory period and slowing of conduction either due to a reduced depolarisation speed or high intercellular resistance, can contribute to arrhythmia initiation.
During AF, multiple reentering wavelets are propagated through the atria [34,35]. According to Allessie et al. , three to five independent wavelets are required to sustain AF. The mouse heart is very small, weighing only 0.15–0.18 g . The atria are thin-walled, consisting of only three to five cell layers. Therefore, the occurrence of multiple wavelets in the limited atrial tissue of the mouse would not be expected. Accordingly, in wild-type mouse strains it is not possible to induce atrial arrhythmias by pacing [6,8,37]. In contrast, to date three different transgenic mouse models are known to develop AF.
The first report on AF in mice originates from 1988 . Transgenic mouse lines were generated harbouring an oncogene (SV40 large T antigen) under the control of the atrial-specific atrial natriuretic factor promotor. This approach resulted in the hyperplastic growth of the right atrium, leading to a 10–20-fold increase in size compared to control. ECG recordings performed in these mice revealed, corresponding to the progression of atrial hyperplasia, the development of supraventricular arrhythmias including AF .
AF was observed after conditional ablation of cardiomyocytes, by transgenic expression of the diphtheria toxin A gene, controlled by a tetracycline-responsive promotor . Induction of the expression of the diphtheria toxin led to cell death, the severity of fibrosis and chamber dilatation was shown to be dependent on toxin expression levels. In addition to AF in vivo, ventricular fibrillation could be induced ex vivo and reentry was visualised with an optical action potential mapping system . Interestingly, even hearts without gross anatomical abnormalities were prone to arrhythmias, which was probably due to the impaired expression and altered localisation of the gap junctional protein connexin43, as observed in the diseased mouse hearts.
The gap junction channels, consisting of two hexamers of connexin proteins, are responsible for the electrical coupling of cardiomyocytes. They allow the direct intercellular exchange of ions and also of second messengers and small metabolites . The specific ablation of connexin40, a connexin preferentially expressed in atrial myocytes and the ventricular conduction system, resulted in a mouse strain with a predisposition for supraventricular arrhythmias . ECGs obtained from Cx40-deficient (Cx40−/−) mice reveal prolongation of all measured ECG parameters, the most prominent alteration being a 56% prolongation of the P-wave compared to wild-type mice [7,8]. In addition, the sinus node recovery time was prolonged and the Wenckebach periodicity occurred at significantly longer atrial pacing cycle lengths in Cx40−/− compared to wild-type mice. This shows that Cx40 is important for the fast conduction in the atrial tissue and ventricular conduction system. Cx40−/− mice showed spontaneous and inducible atrial arrhythmias including atrial ectopia, atrial flutter and AF (Fig. 2) [7,8]. As the expression pattern of Cx40 is comparable in mice and men, the findings in Cx40-deficient mice might implicate a role of Cx40 abnormalities in the genetic predisposition for AF in man.
Mouse model of atrial fibrillation. Three limb lead surface ECG showing continuous tracings of atrial fibrillation in a Cx40-deficient mouse heart. The arrhythmia was induced by burst stimulation via a transoesophageal lead and spontaneously converted to sinus rhythm (marked by arrow) after 3.5 s. P, P wave; SR, sinus rhythm.
Since the number and availability of transgenic mouse models with cardiac phenotype is increasing, the systematic evaluation of electrophysiological abnormalities in these small animals becomes more and more important. The examples shown above demonstrate that transgenic models are valuable for the exploration of genes responsible for human cardiac disease, and for the study of therapeutic drugs and specific intervention strategies to conquer these conditions.
The interest in mouse electrophysiology is expanding fast, despite the marked differences between mouse and man. The mouse genome can be altered with relative ease, which allows the molecular dissection of the various components that contribute to de- and repolarisation and the initiation and propagation of cardiac arrhythmias. The mouse IR model holds great potential to study preventive measures against myocyte damage and which parameters can be used in man to judge the effects. Therefore, it seems very likely that the paper by Goldbarg et al.  will be cited more often in the next decennium.
Dumont E, Hofstra L, Frederik J et al. Rapid onset of cardiomyocyte apoptosis induced by ischemia and reperfusion in murine hearts. In: Sidlowski JA, Cohen JJ, editors, Apoptosis and programmed cell death. Keystone meeting 1999;336 (Abstract).