Skip Navigation

Cardiovascular Research 2002 54(2):397-404; doi:10.1016/S0008-6363(01)00555-7
© 2002 by European Society of Cardiology
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow E-letters: Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when E-letters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Dobrev, D.
Right arrow Articles by Ravens, U.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Dobrev, D.
Right arrow Articles by Ravens, U.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

Copyright © 2002, European Society of Cardiology

Human inward rectifier potassium channels in chronic and postoperative atrial fibrillation

Dobromir Dobreva,*, Erich Wettwera, Ariane Kortnerb, Michael Knautb, Stephan Schülerb and Ursula Ravensa

aDepartment of Pharmacology and Toxicology, Carl Gustav Carus Medical School, Dresden University of Technology, Dresden, Germany
bCardiovascular Centre, Carl Gustav Carus Medical School, Dresden University of Technology, Dresden, Germany

dobrev{at}rcs.urz.tu-dresden.de

* Corresponding author. Department of Pharmacology and Toxicology, TU Dresden, Fetscher Str. 74, D-01307 Dresden, Germany. Tel.: +49-351-458-6279; fax: +49-351-458-6315

Received 10 September 2001; accepted 14 November 2001


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: We showed recently that the 825T allele of the G-protein β3-subunit C825T polymorphism is associated with large inward rectifier K+ currents IK1 but low acetylcholine-activated K+ current IK,ACh amplitudes. During chronic atrial fibrillation (AF), IK1 and IK,ACh current densities were increased when compared to sinus rhythm (SR). It is unknown whether chronic AF and Gβ3 gene status are independent contributors to atrial K+ current activity. We measured IK1 and IK,ACh in tissue from AF patients with different Gβ3 genotypes and assessed the relation between the IK1 and IK,ACh amplitudes and the incidence of postoperative AF. Methods: We measured the amplitudes of IK1 and IK,ACh in atrial myocytes from 26 patients with sinus rhythm (SR) and from 16 patients with chronic AF (<6 months). The K+ currents were measured with standard patch-clamp techniques. The Gβ3 gene status of the patients was determined by PCR and restriction analysis. Results: At –100 mV, the amplitude of IK1 was larger in AF (10.9±1.0 pA/pF, n=49/16, cells/patients) than in SR (6.3±0.6 pA/pF, n=68/26, P<0.05), whereas the amplitude of IK,ACh was smaller in chronic AF (2.9±0.7 pA/pF, n=49/16) than in SR (6.3±0.7 pA/pF, n=68/26, P<0.05). These changes were independent of the patient Gβ3 gene status. Eight patients out of 26 in the SR group (31%) developed postoperative AF. When analysed based on incidence of postoperative AF, current amplitudes did not differ significantly. Conclusion: We provide evidence for up-regulation of IK1 but down-regulation of IK,ACh in chronic AF which are independent of Gβ3 gene status. Atrial myocytes from patients who are in SR but later develop postoperative AF have no manifestation of altered IK1 and IK,ACh at the time of cardiac surgery. Our results suggest that the AF-related changes of IK1 and IK,ACh may be a consequence of or a contributory factor to chronic AF.

KEYWORDS Atrial function; Gene expression; K-channel


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
Chronic atrial fibrillation (AF) is characterized by shortened effective refractory period (ERP), shortened action potential duration (APD) and by blunted cycle length-dependent rate adaptation ("electrical remodeling"; for review, see Ref. [1]). On a cellular basis, L-type Ca2+ currents are significantly decreased in AF-induced remodeling [2,3], whereas changes in K+ currents are less consistent [2,4–6]. The inward rectifier K+ current IK1 was increased in atria of patients with chronic AF [4], whereas its density remained unchanged in the canine rapid-atrial pacing model [7]. Vagal stimulation causes bradycardia and decreases contractility via activation of another inward rectifier current, the acetylcholine-activated K+ current (IK,ACh). While the amplitude of IK,ACh was reported to be increased in patients with chronic AF [2], mRNA and protein levels of these channels were decreased [8].

Some patients in SR develop postoperative AF, and patients with the largest atrial L-type Ca2+ currents are at the highest risk [3]. This association suggests that a high cellular Ca2+ load may contribute to the initiation of AF. Experimentally, AF can be induced by vagal stimulation or by direct injection of ACh [9,10]. Vagal stimulation shortens action potential duration and increases the dispersion of refractoriness via cholinergic modulation of repolarization mediated by activation of IK,ACh [11]. Therefore, we investigated the electrophysiological properties of IK1 and IK,ACh in chronic AF and assessed the relation between their amplitudes and the incidence of postoperative AF. Since we recently demonstrated that a C825T polymorphism in the Gi-protein β3-subunit gene affects the IK1 and IK,ACh current density [12], all patients were genotyped to investigate whether chronic AF and Gβ3 gene status are independent contributors to atrial K+ current activity.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1 Patient characteristics
The present investigation conformed with the principles outlined in the Declaration of Helsinki. All patients gave written informed consent, which was approved by the local ethics committee of the Medical Faculty of the Dresden University of Technology (No. EK210/20.07.99). Patients with paroxysmal AF and those treated with antiarrhythmic drugs or having previous cardiac surgery were excluded from the study. A small piece (~1 cm3; 0.1–0.5 g) of the tip of the right atrial appendage was excised from 16 patients with chronic AF (AF >6 months) and from 26 patients without a history of AF as part of the routine atriotomy procedure.

2.2 Voltage-clamp recordings
Atrial myocytes were isolated as previously described in detail [12,13] and were kept in a storage solution containing (in mmol/l) KCl 20, KH2PO4 10, glucose 10, K-glutamate 70, β-hydroxybutyrate 10, taurine 10, EGTA 10, albumin 1; pH 7.4. Only well-striated, rod-shaped myocytes were used. The single electrode voltage-clamp technique was applied using a List EPC-7 amplifier to measure membrane currents [12,13]. Data were sampled and analyzed with pCLAMP 5.5 software (Axon Instruments, Foster City, USA). The currents were measured at 2 kHz and digitized at 1 kHz with a TL-125 A/D interface (Axon Instruments) for off-line analysis. The electrodes were fabricated from filamented borosilicate glass (Hilgenberg, Malsfeld, Germany) using a programmable horizontal puller (DMZ universal puller, Zeitz, München, Germany). When filled with pipette solution (in mmol/l: D,L K-aspartate 100, NaCl 10, KCl 40, Mg-ATP 5.0, EGTA 2.0, GTP–Tris 0.1, HEPES 10; pH 7.4) the microelectrodes had tip resistances of ~2–4 M{Omega}. Seal resistances were usually between 4 and 8 G{Omega}. Cell capacitance (CM) was measured using depolarizing ramps (1 V s–1) from –40 to –35 mV and was compensated up to 100 pF. The series resistance was compensated up to 70%. IK,ACh was activated by applying the muscarinic receptor agonist carbachol (2 µmol/l) to the bath solution (composition in mmol/l: NaCl 120, KCl 20, MgCl2 1, CaCl2 2.0, glucose 10, HEPES 10, pH 7.4 with NaOH) and analyzed at –100 mV as inward current. The clamp protocol consisted of a 50-ms step to –100 mV from a holding potential of –80 mV, followed by a depolarizing ramp to –10 mV (800 ms), a 100-ms step to –50 mV, and return to the holding potential (pulse frequency 0.5 Hz). All experiments were performed at room temperature.

In all experiments, the inward current at –100 mV and the difference current between the inward current in the absence and presence of the muscarinic receptor agonist carbachol (2 µM) were identified as IK1 and IK,ACh by their complete suppression with Ba2+ (1 mmol/l, see Fig. 1). The measured IK1 and IK,ACh were corrected for linear conductance calculated from the reversal potential of IK,ACh after stimulation with carbachol as previously described [12]. We found that the mean leak conductance of the cells corresponded very well with the residual Ba2+-insensitive current obtained after Ba2+ application.


Figure 1
View larger version (17K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 1 Original tracings of IK1 and IK,ACh. Left panel: control IK1 current and current in response to 2 µmol/l carbachol (difference current taken as IK,ACh) in the absence and presence of Ba2+ (1 mmol/l) in a typical myocyte from a patient with sinus rhythm (SR). Right panel: similar tracings but in a myocyte from a patient with atrial fibrillation (AF). Inset: clamp protocol (0.5 Hz). Only data during ramp pulse from –100 to –10 mV within 800 ms are shown.

 
2.3 Molecular analysis of the G-protein β3-subunit C825T polymorphism
Genomic DNA (100 ng) extracted from peripheral blood was amplified in a 25-µl standard reaction mix containing GSP (primers: sense, 5'-TGA CCC ACT TGC CAC CCG TGC-3'; antisense, 5'-GCA GCA GCC AGG GCT GGC-3'; accession No: Y12057 [GenBank] ) and 1.1 mmol/l MgCl2. Cycling conditions were: denaturation at 94 °C for 5 min followed by 35 cycles of 94 °C for 1 min, 63.5 °C for 45 s, and 72 °C for 45 s (final extension for 7 min at 72 °C). The Gβ3 gene status at position 825 of the cDNA was determined by digesting PCR products (10 µl) with BseDI (Fermentas, Vilnius, Lithuania). After electrophoresis, digests were ethidium bromide stained and visualized under UV light. The undigested product (TT genotype) has a size of 268 bp; complete digestion (CC genotype) results in bands of 116 and 152 bp, respectively.

2.4 Statistical analysis
Since the number of myocytes investigated varied between the patients, the means of the groups were calculated from average values for each patient. The limited number of cells (two to seven cells per patient) analysed from a single patient were selected by the following criteria: rod shape, clear cross striations, and seal resistance >3 G{Omega}. Thus we did not pool cells from different patients as a group, because the variability of data from different myocytes for each patient would result in a deviation from the expected mean value if the number of cells are not balanced between patients. In addition, in order to extrapolate the K+ current data to the patient population (selected clinical variables and preoperative medication), all statistical analyses concerning the current data were performed with "n" indicating number of patients not cells, unless stated otherwise (mean±S.E.M.). Univariate ANOVA was applied to associate the preoperative variables with electrophysiological data. Independent variables were chronic AF, selected clinical variables and medication (Table 1). To test for interactions between chronic AF and other clinical parameters or the nature of preoperative medication, interaction terms were included in separate two-way ANOVAs. Differences between continuous data from patients were compared by unpaired Student's t-test. Frequency data were analyzed with likelihood {chi}2 statistics (SPSS for Windows, version 10.0). P<0.05 was considered statistically significant.


View this table:
[in this window]
[in a new window]

  		Table 1 Characteristics of patients in the SR and chronic AF group

 

   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
The patients* characteristics are shown in Table 1. Statistically significant differences between the patient groups were found with respect to gender, the underlying cardiac disease and size of the left atrium. With univariate analysis, however, chronic AF was the only preoperative clinical variable that significantly associated with density of IK1 or IK,ACh.

Medication was different between the patient groups (Table 1). Use of digitalis and Ca2+ channel blockers of non-dihydropyridine type was more prevalent in chronic AF (8% vs. 50%, P=0.001 and 0% vs. 31%, P=0.002), whereas nitrates were more common in SR (44% vs. 85%, P=0.010). However, no significant associations were found between K+ current density and preoperative therapy with either Ca2+ channel blockers of non-dihydropyridine type (P=0.485) or nitrates (P=0.965). Digitalis was the only preoperative medication significantly associated with density of K+ currents (P=0.016, see below).

3.1 Influence of human chronic AF on IK1 and IK,ACh
The resting membrane potential was not corrected for the calculated liquid junction potential (–12 mV) of the electrode filling solution [12]. Mean values were more negative in chronic AF than in SR (–25.4±1.4 mV, n=41, vs. –21.5±1.1 mV, n=58, respectively; P=0.034; statistical analysis on a per cell basis). Accounting for junction potential [12] and assuming [K+]i to be determined mainly by the pipette solution (140 mmol/l), the resting membrane potentials were –37 and –33 mV, respectively, the deviation from the potassium equilibrium potential of –49 mV as calculated with the Nernst equation being explained by a voltage drop resulting from the ratio of the relatively high input resistance of human atrial cells and the seal resistance [14].

The mean cell size of the myocytes in the AF group as measured as CM was larger than that in SR (CM=139±11 pF, n=49, for AF vs. 84±4 pF, n=68, for SR; P<0.001; statistical analysis on a per cell basis). Therefore, currents were corrected for cell size and are presented in terms of densities (in pA/pF). At –100 mV, mean values for IK1 were significantly larger in AF (–10.9±1.0 pA/pF, n=49 cells from 16 hearts (49/16)) than in SR (–6.3±0.6 pA/pF, n=68/26, Figs. 1 and 2AGo). In contrast, the amplitude of IK,ACh, defined as the difference between total current and IK1, was significantly smaller in AF (–2.9±0.7 pA/pF, n=49/16) than in SR (–6.3±0.7 pA/pF, n=68/26), whereas the total current amplitudes (IK1+IK,ACh) were similar in both patient groups (–14.1±1.1 pA/pF, n=49/16, vs. –12.6±0.8 pA/pF, n=68/26, for chronic AF and SR, respectively; Figs. 1 and 2AGo). These differences were also observed at other potentials (data not shown). In addition, when we compared the data on a per patient basis with those on a per cell basis for SR and AF separately, we did not find significant differences in the mean density of the respective currents (data not shown). In the presence of the non-selective muscarinic receptor antagonist atropine (1 µM), carbachol failed to activate IK,ACh in both SR and AF cells (data not shown).


Figure 2
View larger version (16K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 2 (Left) Mean values of IK1 and IK,ACh (in pA/pF at –100 mV) in myocytes from SR and AF patients, respectively. (Right) Mean values of IK1 and IK,ACh (in pA/pF at –100 mV) in myocytes from SR patients without postoperative AF (post-AF –) and those who experienced postoperative AF (post-AF +), respectively. IK,ACh is defined as the difference between the maximum current in response to 2 µmol/l carbachol and IK1. Numbers within the columns indicate the number of myocytes/patients investigated. *P<0.05 vs. IK1 and #P<0.05 vs. IK,ACh in SR, respectively.

 
Since we recently found that the Gi-protein β3-subunit C825T polymorphism is associated with increased current density of IK1 and IK,ACh [12], all patients were genotyped. In SR patients, the genotype at position 825 was CC in 12, and CT in 14, and TT in four patients. In contrast, in AF patients the genotype was CC in seven, and CT in nine, but no homozygous 825T allele carriers could be recruited. Therefore, all homozygous 825T allele carriers were excluded from the data analysis in the SR group (see above). As shown in Fig. 3, chronic AF and the G-protein β3-subunit C825T polymorphism appear to be independent contributors to the activity of IK1 and IK,ACh, because the AF-related differences in current density are still present in the respective genotype groups.


Figure 3
View larger version (18K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 3 Mean values of IK1 and IK,ACh (in pA/pF at –100 mV) in myocytes from SR and AF patients with different Gβ3 gene status. Same layout as in Fig. 2. #P<0.05 vs. IK1, *P<0.05 vs. IK,ACh in mocytes from patients with SR and CC and CT genotype, respectively.

 
3.2 Postoperative AF
After open-heart surgery, many patients with SR develop AF in the postoperative period (typically in the first few days after cardiac surgery). Therefore, we reanalysed the current data in the SR group on the basis of the occurrence of postoperative AF. Eight of 26 patients with SR (31%) experienced postoperative AF during the in-hospital recovery period. When separated by the occurrence of postoperative AF, there was no significant difference in CM (86±7 pF, n=26, vs. 83±8 pF, n=42, for with and without postoperative AF, respectively; statistical analysis on a per cell basis), resting membrane potential (–21.6±1.9 mV, n=26, vs. –21.5±1.1 mV, n=42, for with and without postoperative AF, respectively; statistical analysis on a per cell basis) or clinical characteristics, including medication (data not shown). The only significant difference between the groups was the age of the patient (72±5 vs. 66±5 years, for SR with and SR without postoperative AF, P<0.01). In these two groups of patients we detected no differences in the density of IK1 (–5.6±1.2 pA/pF, n=26/8 vs. –6.5±0.6 pA/pF, n=42/18 for SR with and without postoperative AF, respectively) or IK,ACh (–6.4±1.5 pA/pF, n=26/8 vs. –6.2±0.7 pA/pF, n=42/18, Fig. 2).

3.3 Impact of the patients* medication on AF-related changes in K+ current density
Digitalis was the only medication significantly associated with current density (P=0.016, univariate analysis). Myocytes from AF patients on digitalis therapy showed a significantly lower density of IK1 (P=0.037, Fig. 4). Test of interaction effects between chronic AF and medication with digitalis indicated that these are independently associated with IK1, because no significant interaction was found (P=0.420).


Figure 4
View larger version (10K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 4 Mean values of IK1 (in pA/pF at –100 mV) in myocytes from patients with SR and chronic AF in the presence (+) and absence (–) of therapy with digitalis (DIG). Numbers within the columns indicate the number of myocytes/patients investigated. *P<0.05 vs. IK1 in the absence of digitalis by patients with chronic AF.

 
3.4 Impact of the underlying heart disease on AF-related changes in K+ current density
Valvular heart disease was more common in the chronic AF group (Table 1), which may explain the significantly larger left atrial diameter in the AF group. Thus, current changes during AF may be confounded by atrial dilation. Therefore, the data for current density of IK1 and IK,ACh in patients with chronic AF were separated according to the patient's underlying heart disease (coronary artery disease or valvular heart disease) and only patients with coronary artery disease were compared. Exclusion of all patients with valvular heart disease had no effect on detected differences in density of IK1 (–9.5±1.8 pA/pF, n=22/7 vs. –6.3±0.6 pA/pF, n=68/26, for AF and SR, respectively; P=0.031) and IK,ACh (–3.0±1.5 pA/pF, n=22/7 vs. –6.3±0.7 pA/pF, n=68/26; P=0.045).

3.5 Relationship between myocyte capacitance and density of IK1 and IK,ACh
Since the mean cell size as measured as CM was larger in AF than in SR, we evaluated the impact of cell size on the measured current densities. The densities of IK1 and IK,ACh at –100 mV were plotted for small (<70 pF), medium-size (70 to 100 pF), and large (<100 pF) atrial myocytes (Table 2). The increased density of IK1 in myocytes from AF patients was independent of cell size (although not significant with cells <70 pF). Interestingly, the AF-related reduction in IK,ACh was observed only in myocytes with CM greater than 100 pF (Table 2).


View this table:
[in this window]
[in a new window]

  		Table 2 Distribution of current density (in pA/pF at –100 mV) of IK1 and IK,ACh as a function of cell capacitance for myocytes from SR and AF patients

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
Our study demonstrates that chronic human AF is associated with an increased density of IK1 but reduced activity of IK,ACh. Patients with SR who developed postoperative AF revealed no changes in K+ current density at the time of cardiac surgery. The AF-related changes in K+ current density are independent of G-protein β3-subunit C825T polymorphism and may be a consequence of, or a contributory factor to, chronic AF, providing a potential electrophysiological basis for the self-perpetuating nature of AF.

4.1 Activity of IK1 and IK,ACh in chronic AF
The AF-related increase of IK1 confirms previous results reported in the right [2] and left atrium [4]. Interestingly, the IK1 increase in the left atrium was observed only in cells smaller than 75 pF [4], while in the right atrium the increase appears to be present in medium and large cells (present study and Ref. [2]). The reason for these differences is unclear. Since IK1 (and to some extent also IK,ACh) determines the resting membrane potential, the larger current density of IK1 in AF compared with SR myocytes is expected to hyperpolarize the cell membrane. Indeed, we found more negative resting potentials in AF cells. At present, the cause for the increased IK1 is unknown. Two possibilities must be taken into consideration: (i) increased channel density due to enhanced expression or (ii) enhanced single channel activity. Current experiments in our laboratory are directed at elucidating these issues.

Here, we investigated IK,ACh, because clinical evidence points to an important role of the parasympathetic nervous system in human AF [15,16]. IK,ACh was reduced by 54% in myocytes from AF patients, suggesting that vagal tone is most likely involved in electrical remodeling. Our data correspond well with a very recent report on IK,ACh reduction by 34% of mRNA and by 62% of protein [8]. The discrepancy between the low (present study) and increased amplitude of IK,ACh described by Bosch et al. [2] can be explained by the different populations of myocytes investigated: we found significantly smaller amplitudes of IK,ACh only in cells with membrane capacitance >100 pF, while Bosch et al. investigated only cells of <80 pF, in which he found no change. In addition, these authors defined IK,ACh as the total current in the presence of acetylcholine, i.e. IK1 plus IK,ACh. If IK,ACh is calculated as the difference between total current and IK1—as in the present study—there is no change in amplitude of IK,ACh in patients with chronic AF [2].

What may be the reasons for the inconsistent results obtained in the present study and Refs. [2,4]? In general, studies in human tissue are limited by various disease states, duration of the disease and drug therapy. We have shown that myocytes from atria of patients receiving digitalis had a significantly lower IK1 density than those from patients without this medication. Atria from patients receiving digitalis also have lower resting membrane potentials (i.e., less negative) than those receiving no digitalis [17]. Thus, our results of a higher IK1 density and resting membrane potential in myocytes from AF patients and the results of Refs. [2,4] may be underestimated. There are also differences when AF is considered as chronic. Both studies defined AF as chronic when persisting for at least 1 month. In our AF group, only patients with AF persisting for more than 6 months were included. Another important difference between the present study and Refs. [2,4] may be the presence of right atrial dilation. In the study of Van Wagoner et al. [4], the diameter of the right atria was not different between the SR and AF groups. Such data are not reported in Ref. [2] and were unavailable in most of our patients. However, since chronic AF leads to enlargement of both atria [18], we cannot exclude the effect of AF-induced dilation of the right atrium in our AF patients. In any case, IK1 was found to remain unaffected in dilated atria without AF [19]. The underlying heart disease may also affect channel activity. Some patients in the AF group had valvular heart disease, but the observed current differences remained the same when these were excluded from the analysis. Reduced functional activity of atrial IK1 and IK,ACh has been documented in patients with heart failure [20]. Since there was no evidence for heart failure in our and Bosch's patients [2], and no data were reported by Van Wagoner et al. [4], it may be speculated that the observed inter-study differences could be due to differences in left ventricular function. Finally, the presence of homozygous 825T allele carriers of the Gi-protein β3-subunit gene, which exhibit larger IK1 and smaller amplitudes of IK,ACh than carriers of the C825 allele, might also affect the data of the two studies [12]. To prevent this genetic influence from confounding the results, our patients were genotyped to examine homozygous 825T allele carriers separately. Interestingly, there was not a single patient of TT genotype with chronic AF, so that patients with TT genotype had to be excluded from the SR group. The AF-related changes are independent of Gβ3 gene status, indicating that Gβ3 genotype and chronic AF are probably independent contributors to atrial inward rectifier K+ channel activity. The lack of homozygous 825T allele carriers in the AF group impeded clarification of any possible interaction effect between the 825T allele and chronic AF on the activity of atrial K+ currents.

4.2 Activity of IK1 and IK,ACh in postoperative AF
One frequent complication following cardiac surgery is development of postoperative AF [21]. In myocytes from these patients, the L-type Ca2+ current is larger than in patients without postoperative AF [3]. Thus, Ca2+ overload of the myocytes may be an important contributor to the onset of postoperative AF. The relationship between IK,ACh as an effector of vagal activity and postoperative AF is less clear. In the present study, there were no differences in the density of IK,ACh between patients with and without postoperative AF. Thus, changes in vagal tone may not play a major role in the initiation of postoperative AF.

4.3 Study limitations
The AF-related decrease of IK,ACh may also be a consequence of altered density of muscarinic receptors and/or inhibitory (Gi) G-protein levels. To the best of our knowledge, the density of muscarinic receptors has not been investigated in atria from AF patients and Gi-protein expression is unaffected by AF [22]. Thus, the observed changes in response to carbachol are probably due to down-regulation of IK,ACh density.

The incidence of the TT genotype in the Caucasian population is 5–10% [23]. Therefore, the absence of the TT genotype among the AF group may indicate that we have studied too few patients and could possibly encounter homozygous 825T allele carriers when more AF patients are recruited in future studies.

Valvular heart disease was frequently present in the AF group, but was absent in the SR group. Subgroup analysis, however, showed that the underlying heart disease had no effect on electrophysiological data. Digitalis and Ca2+ channel blockers of non-dihydropyridine type were more frequently prescribed in AF, whereas β-blockers, nitrates, and ACE inhibitors were more common in the SR group. Thus, we cannot exclude that these drugs altered the current density. However, among these drugs, only digitalis was significantly associated with K+ current density (IK1). Physiologically, the Na+–K+ pump creates a current in the outward direction [24]. Thus, inhibition of this pump current by digitalis would reduce its contribution to the background conductance, i.e. a larger IK1 density in patients treated with digitalis should be expected. Since the digitalis-related changes were in the opposite direction, i.e. the mean IK1 current density was smaller in patients treated with digitalis, the difference between SR and AF is expected to be even larger without this confounding factor.

4.4 Conclusions
IK1 and IK,ACh are the main K+ currents contributing to the late phase of repolarization of the atrial action potential [11]. Thus, these currents may play a key role in the frequency-dependent modulation of APD and ERP. Parasympathetic stimulation shortens APD and ERP and increases ERP dispersion, which may contribute to the perpetuation of AF [25]. Therefore, reduction of IK,ACh might serve as a compensatory mechanism for the AF-related decrease in APD and ERP. Catheter ablation of cardiac parasympathetic nerves in dogs prevents vagal atrial fibrillation [26], suggesting that, at least in some patients with AF, inhibition of the parasympathetic nervous system may have an antiarrhythmic effect.

Time for primary review 32 days.


   Acknowledgements
 
The authors thank Manja Schöne, Konstanze Fischer, and Renate Weitschat for excellent technical assistance. This study was supported by a faculty grant (MeDDrive 2001 to D.D.) and by the Deutsche Forschungsgemeinschaft (DFG Ra 222/9-1).


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

  1. Nattel S, Li D. Ionic remodeling in the heart. Pathophysiological significance and new therapeutic opportunities for atrial fibrillation. Circ Res (2000) 87:440–447.[Abstract/Free Full Text]
  2. Bosch R.F, Zeng X.R, Grammer J.B, et al. Ionic mechanisms of electrical remodeling in human atrial fibrillation. Cardiovasc Res (1999) 44:121–131.[Abstract/Free Full Text]
  3. Van Wagoner D.R, Pond A.L, Lamorgese M, et al. Atrial L-type Ca2+ currents and human atrial fibrillation. Circ Res (1999) 85:428–436.[Abstract/Free Full Text]
  4. Van Wagoner D.R, Pond A.L, McCarthy P.M, Trimmer J.S, Nerbonne J.M. Outward K+ current densities and Kv1.5 expression are reduced in chronic human atrial fibrillation. Circ Res (1997) 80:772–781.[Abstract/Free Full Text]
  5. Grammer J.B, Bosch R.F, Kühlkamp V, Seipel L. Molecular remodeling of Kv4.3 potassium channels in human atrial fibrillation. J Cardiovasc Electrophysiol (2000) 11:626–633.[Web of Science][Medline]
  6. Lai L.P, Su M.J, Lin J.L, et al. Changes in the mRNA levels of delayed rectifier potassium channels in human atrial fibrillation. Cardiology (1999) 92:248–255.[CrossRef][Web of Science][Medline]
  7. Yue L, Feng J, Melnyk P, et al. Molecular mechanisms underlying ionic remodeling in a dog model of atrial fibrillation. Circ Res (1999) 84:776–784.[Abstract/Free Full Text]
  8. Brundel B.J.J.M, Van Gelder I.C, Henning R.H, et al. Ion channel remodeling is related to intraoperative atrial effective refractory periods in patients with paroxysmal and persistent atrial fibrillation. Circulation (2001) 103:684–690.[Abstract/Free Full Text]
  9. Liu L, Nattel S. Differing sympathetic and vagal effects on atrial fibrillation in dogs: role of refractoriness heterogeneity. Am J Physiol (1997) 273:H805–H816.[Web of Science][Medline]
  10. Moe G.K, Abildskov J.A. Atrial fibrillation as a self-sustaining arrhythmia independent of focal discharge. Am Heart J (1959) 58:59–70.[CrossRef][Web of Science][Medline]
  11. Yamada M, Inanobe A, Kurachi Y. G protein regulation of potassium ion channels. Pharmacol Rev (1998) 50:723–757.[Abstract/Free Full Text]
  12. Dobrev D, Wettwer E, Himmel H.M, et al. G-protein β3-subunit 825T-allele is associated with enhanced human atrial inward rectifier potassium currents. Circulation (2000) 102:692–697.[Abstract/Free Full Text]
  13. Himmel H.M, Meyer zu Heringdorf D, Graf E, et al. Evidence for edg-3 receptor-mediated activation of IK,ACh by sphingosine-1-phosphate in human atrial cardiomyocytes. Mol Pharmacol (2000) 58:449–454.[Abstract/Free Full Text]
  14. Amos G.J, Wettwer E, Metzger F, et al. Differences between outward currents of human atrial and subepicardial ventricular myocytes. J Physiol (London) (1996) 491:31–50.[Abstract/Free Full Text]
  15. Coumel P. Atrial flutter. Touboul P.C, Waldo A.L, eds. (1996) New York: Futura. 248–261.
  16. Chen Y.J, Chen S.A, Tai C.T, et al. Role of atrial electrophysiology and autonomic nervous system in patients with supraventricular tachycardia and paroxysmal atrial fibrillation. J Am Coll Cardiol (1998) 32:732–738.[Abstract/Free Full Text]
  17. Mary-Rabine L, Albert A, Pham T.D, et al. The relationship of human atrial cellular electrophysiology to clinical function and ultrastructure. Circ Res (1983) 52:188–199.[Abstract/Free Full Text]
  18. Sanfilippo A.J, Abascal V.M, Sheehan M, et al. Atrial enlargement as a consequence of atrial fibrillation. A prospective echocardiographic study. Circulation (1990) 82:792–797.[Abstract/Free Full Text]
  19. Le Grand B, Hatem S, Deroubaix E, et al. Depressed transient outward and calcium currents in dilated human atria. Cardiovasc Res (1994) 28:548–556.[Web of Science][Medline]
  20. Koumi S.-I, Arentzen C.E, Backer C.L, Wasserstrom J.A. Alterations in muscarinic K+ channel response to acetylcholine and to G protein-mediated activation in atrial myocytes isolated from failing human hearts. Circulation (1994) 90:2213–2224.[Abstract/Free Full Text]
  21. Hogue C.W, Hyder M.L. Atrial fibrillation after cardiac operation: risks, mechanisms, and treatment. Ann Thorac Surg (2000) 69:300–306.[Abstract/Free Full Text]
  22. Schotten U, Ausma J, Stellbrink C, et al. Cellular mechanisms of depressed atrial contractility in patients with chronic atrial fibrillation. Circulation (2001) 103:691–698.[Abstract/Free Full Text]
  23. Siffert W, Forster P, Jockel K.H, et al. Worldwide ethnic distribution of the G protein β3 subunit 825T allele and its association with obesity in Caucasian, Chinese, and Black African individuals. J Am Soc Nephrol (1999) 10:1921–1930.[Abstract/Free Full Text]
  24. Glitsch H.G. Electrophysiology of the sodium–potassium-ATPase in cardiac cells. Physiol Rev (2001) 81:1791–1826.[Abstract/Free Full Text]
  25. Smeets J.L.R.M, Allessie M.A, Lammers W.J.E.P, et al. The wavelength of the cardiac impulse and reentrant arrhythmias in isolated rabbit atrium: the role of heart rate, autonomic transmitters, temperature, and potassium. Circ Res (1986) 58:96–108.[Abstract/Free Full Text]
  26. Schauerte P, Scherlag B.J, Pitha J, et al. Catheter ablation of cardiac autonomic nerves for prevention of vagal atrial fibrillation. Circulation (2000) 102:2774–2780.[Abstract/Free Full Text]

Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?


This article has been cited by other articles:


Home page
 J Am Coll CardiolHome page
C.-T. Tsai, L.-P. Lai, J.-J. Hwang, J.-L. Lin, and F.-T. Chiang
Molecular Genetics of Atrial Fibrillation
J. Am. Coll. Cardiol., July 22, 2008; 52(4): 241 - 250.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow E-letters: Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when E-letters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Dobrev, D.
Right arrow Articles by Ravens, U.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Dobrev, D.
Right arrow Articles by Ravens, U.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?