Frechen, Friedrich Schoendube, Peter Hanrath and Maurits A. AllessieUlrich Schotten, Jannie Ausma, Christoph Stellbrink, Ingo Sabatschus, Miriam Vogel, Dirk

FibrillationCellular Mechanisms of Depressed Atrial Contractility in Patients With Chronic Atrial

Print ISSN: 0009-7322. Online ISSN: 1524-4539 Copyright © 2001 American Heart Association, Inc. All rights reserved.

is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231Circulation doi: 10.1161/01.CIR.103.5.691

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Cellular Mechanisms of Depressed Atrial Contractility inPatients With Chronic Atrial Fibrillation

Ulrich Schotten, MD; Jannie Ausma, PhD; Christoph Stellbrink, MD; Ingo Sabatschus; Miriam Vogel;Dirk Frechen; Friedrich Schoendube, MD; Peter Hanrath, MD; Maurits A. Allessie, MD, PhD

Background—After cardioversion of atrial fibrillation (AF), the contractile function of the atria is temporarily impaired.Although this has significant clinical implications, the underlying cellular mechanisms are poorly understood.

Methods and Results—Forty-nine consecutive patients submitted for mitral valve surgery were investigated. Twenty-threewere in persistent AF ($3 months); the others were in sinus rhythm. Before extracorporal circulation, the right atrialappendage was excised.b-Adrenoceptors were quantified by radioligand binding, and G proteins were quantified byWestern blot analysis. The isometric contractile response to Ca21, isoproterenol, Bay K8644, and the postrestpotentiation of contractile force were investigated in thin atrial trabeculae, which were also examined histologically. Thecontractile force of the atrial preparations obtained from AF patients was 75% less than that in preparations from patientsin sinus rhythm. Also, the positive inotropic effect of isoproterenol was impaired, and Bay K8644 failed to increase atrialcontractile force. In contrast, the response to extracellular Ca21 was maintained, and the postrest potentiation waspreserved.b-Adrenoceptor density and G-protein expression were unchanged. Histological examination revealed 14%more myolysis in the atria of AF patients.

Conclusions—After prolonged AF, atrial contractility was reduced by 75%. The impairment ofb-adrenergic modulationof contractile force cannot be explained by downregulation ofb-adrenoceptors or changes in G proteins. Dysfunctionof the sarcoplasmic reticulum does not occur after prolonged AF. Failure of Bay K8644 to restore contractility suggeststhat the L-type Ca21 channel is responsible for the contractile dysfunction. The restoration of contractile force by highextracellular Ca21 shows that the contractile apparatus itself is nearly completely preserved after prolonged AF.(Circulation. 2001;103:691-698.)

Key Words: arrhythmian contractility n receptors, adrenergic, betan remodelingn signal transduction.

Contractile dysfunction of the atria after cardioversion of atrialfibrillation (AF) has been recognized for.30 years.1 Echo-

cardiographic studies showed that the transmitral blood flow duringatrial contraction was markedly reduced after restoration of sinusrhythm (SR).2 The degree of atrial contractile dysfunction and thetime required for restoration of normal atrial transport functiondepend on the duration of AF.2,3 This temporary loss of atrialcontraction may lead to thromboembolic events even when atrialthrombi are not present at the time of cardioversion.4

The cellular mechanisms responsible for AF-induced contractiledysfunction are still poorly understood. First, it has been thoughtthat the electric energy applied during DC cardioversion caused“atrial stunning.”5 However, later studies have shown that contrac-tion of the atria was also impaired after pharmacological6 andspontaneous7 cardioversion. Sustained AF has been shown to causealterations in atrial cellular ultrastructure. Myolysis and fragmenta-tion of the sarcoplasmic reticulum might explain the contractiledysfunction in remodeled atria.8 Prolonged rapid atrial rhythmshave also been shown to cause a pronounced reduction in the L-type

Ca21 current.9–11This offers an alternative explanation for the lossof contractile force in the course of prolonged AF. Tachycardia-induced changes of intracellular Ca21 handling have been studied bySun et al12 in canine atrial cardiomyocytes. They found that bothcontractility and intracellular Ca21 transients were markedlyreduced.

In the present study, we investigated the cellular mechanisms ofpostfibrillatory atrial contractile dysfunction in humans. In atrialtrabeculae isolated from patients with and without AF, theb-adrenergic response, the positive inotropic effect of Ca21 and BayK8644, and the postrest potentiation were evaluated. In addition, themost important proteins involved in theb-adrenergic signal trans-duction pathway were biochemically determined.

MethodsPatientsRight atrial appendages were obtained from 49 patients undergoingmitral valve surgery (Table 1). Twenty-three patients were in chronicAF ($3 months); the others were in SR. Despite a tendency for a

Received July 31, 2000; revision received September 27, 2000; accepted September 27, 2000.From the Cardiovascular Research Institute Maastricht (U.S., J.A., M.A.A.), University of Maastricht, Maastricht, the Netherlands, and the Departments

of Cardiology (C.S., I.S., M.V., D.F., P.H.) and Thoracic and Cardiovascular Surgery (F.S.), University Hospital Aachen, Aachen, Germany.Correspondence to Dr Ulrich Schotten, Department of Physiology, Cardiovascular Research Institute Maastricht, University of Maastricht, PO Box 616,

6200 MD Maastricht, Netherlands. E-mail Schotten@fys.unimaas.nl© 2001 American Heart Association, Inc.

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lower cardiac index and a higher wedge pressure in AF patients,hemodynamics did not differ significantly between the 2 groups. AFpatients more often received Ca21 antagonists and digitalis forcontrol of their ventricular rate. The patients did not receive drugs atleast 12 hours before surgery. All patients gave written informedconsent, and the study was approved by an institutional reviewcommittee.

Contractility StudiesImmediately after surgical excision, right atrial appendages wereplaced into Tyrode’s solution (pH 7.4, gassed with 5% CO2/95% O2).Thin myocardial muscle bundles were prepared in parallel with themuscle fiber direction under stereomicroscopic control. The lengthof the bundles ranged between 3 and 6 mm, and the diameters were0.4560.03 mm (n555) in SR patients and 0.4360.03 mm (n551) inAF patients (P5NS). They were connected to isometric forcetransducers with silk threads and placed in an organ bath filled withprewarmed (37°C) bathing solution. After an equilibration period of30 minutes, the muscles were stretched to a resting tension of 1.0mN. External field stimulation was performed with rectangularpulses (5 ms, 5% to 10% above threshold) at a frequency of 1 Hz.Resting tension was increased in 0.2-mN steps until the musclelength providing maximal active force generation was reached (Lmax,which was 5.160.2 mm in SR patients and 5.260.2 mm in AFpatients;P5NS). The resting tension at Lmax was not different in the2 groups (1.8060.04 mN [n555] in SR patients and 1.7460.06 mN[n551] in AF patients,P5NS). The muscles were allowed toequilibrate for 30 minutes. All muscles showing a decline of force ofcontraction (FC) of.5% during this period were excluded from thestudy. In 20 preparations obtained from 16 patients in SR and in 19preparations from 13 AF patients, first a cumulative dose-responsecurve of isoproterenol was determined. After isoproterenol waswashed out and baseline FC was restored, the positive inotropiceffect of Ca21 was studied for comparison. Another subgroup ofpreparations (19 bundles prepared from 16 SR patients and 16bundles prepared from 12 AF patients) was first exposed to 10.8mmol/L extracellular Ca21 followed by washout. Thereafter, BayK8644 was cumulatively added to the organ bath.

To study the postrest potentiation of contractile force, 1-Hzexternal field stimulation was interrupted for 10 seconds, and FC ofthe first twitch after the pause was compared with the steady-stateforce amplitude (16 muscle preparations from 16 patients in bothpatient groups).

Histological ExaminationAt the end of the experiments, the bundles were examined histolog-ically. To compare the atrial morphology with less diseased hearts,

right atrial specimens were obtained from 19 patients undergoingcoronary artery bypass graft surgery (10 in SR, 9 in AF). Allpreparations were fixed in 3% glutaraldehyde in 90 mmol/L KH2PO4

for 48 hours. After postfixation with OsO4, the specimens weredehydrated by a graded ethanol series and embedded in epoxy resin.The sections (2mm) were stained with periodic acid Schiff (PAS)and toluidine blue. Morphometric quantification was carried out intransverse sections of each bundle by using a grid of 500 intersec-tions as described previously.8 At each intersection, the cellular andextracellular structures were classified as follows: (1) sarcomeres,(2) nuclei, (3) other intracellular structures of cardiomyocytes(mostly glycogen), (4) extracellular matrix, including fibroblasts andother nonmyocytes, and (5) vessels, including perivascular cells. Thecontent of each structure was expressed as the percentage of all 500measuring points. The transnuclear cell diameter of 50 cardiomyo-cytes (shortest axis) was measured with a digital imaging system(Sony CCD camera equipped with NIH image software).

b-Adrenoceptorsb-Adrenoceptor density was determined by radioligand binding asdescribed previously.13 Briefly, a crude membrane preparation of theatrial tissue was incubated with [125I]iodocyanopindolol (4 to 350pmol/L). After filtration through Whatman GF/C filters, the retainedradioactivity was counted in ag-counter. CGP 12177 (1mmol/L)was used to determine nonspecific binding. Protein content of themembrane preparations was measured according to the method ofBradford.14

Western Blot AnalysisTo quantify G proteins, electrophoresis of homogenate aliquotscontaining 80mg protein per lane was carried out with the use of10% polyacrylamide gels.13 After they were tank-blotted, the nitro-cellulose membranes were exposed to primary antibody (antiserumAS/7 [inhibitory G protein] or RM/1 [stimulatory G-protein], NENLife Sciences). After they were labeled with125I-protein A, thenitrocellulose membranes were cut, and single band signals werequantified in ag-counter.

Statistical AnalysisData are expressed as mean6SEM. For EC50 values, 95% CIs aregiven. Statistical significance was determined with the unpairedStudent t test or by 1-way ANOVA for comparison of multiplegroups. Significance of differences in medication or sex werecalculated byx2 test. A value P,0.05 was considered to bestatistically significant.

ResultsLight MicroscopyRepresentative examples of longitudinal and transverse sec-tions of atrial muscle preparations are shown in Figure 1. InSR and AF patients, marked myolytic alterations and pro-nounced interstitial fibrosis were present. However, the de-gree of myolysis was more pronounced in chronically fibril-lating atria. Also, the cell size was larger in AF patients, withthe average transverse cell diameter being 11.260.4 mm inSR patients and 17.360.6mm in AF patients (155%,P,0.01).

In all bundles, the sarcomere content was 14% lower in AFpatients compared with SR patients (Figure 1, Table 2). Theamount of noncontractile material (mostly glycogen) wasnearly 2 times higher in AF patients. The extracellular matrixconstituted'40% of the muscle bundles in both groups. Thisrelatively high amount of extracellular matrix is consistentwith changes described as the result of mitral valve orrheumatic heart disease.15,16 In the additional group of pa-tients with coronary heart disease, the amount extracellular

TABLE 1. Patients: Hemodynamic and Clinical Data

SR (n526) AF (n523)

Age, y 5764 5965

Sex, M/F 10/16 9/14

Cardiac index, L z min21 z m22 3.160.3 2.960.2

Mean pulmonary capillary wedgepressure, mm Hg

13.361.8 14.963.0

Mean right atrial pressure,mm Hg

5.460.8 7.360.9

Medication, n

Diuretics 10 11

b-Blockers 5 5

Digitalis 6 14*

ACE inhibitors 10 11

Ca21 antagonists 6 12*

Values are mean6SEM.*P,0.05.

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matrix was less (Table 2). However, the differences betweencoronary artery disease patients in SR compared with those inAF were similar to the observations in patients with mitralvalve disease.

ContractilitySingle contractions at baseline conditions (37°C, 1 Hz, and2.5 mmol/L Ca21) of all muscle preparations are superim-posed in Figure 2. In trabeculae isolated from patients in SR,the strength (FC) was 11.5 mN/mm2 (n555, range 4.1 to 21.9mN/mm2). In contrast, in muscle bundles of AF patients, FCwas only 3.1 mN/mm2 (n551, range 0.9 to 7.4 mN/mm2,P,0.01). Thus, in the atrial myocardium of AF patients,

baseline contractility was reduced by'75%. Increasing theextracellular Ca21 concentration increased FC in SR and AFpatients (Figure 3). However, the relative increase was morepronounced in the AF group. As a result, at Ca21 concentra-tions $7.4 mmol/L, FC was no longer significantly lowerthan in the SR group (but still slightly below the force in theSR group). Thus, high extracellular Ca21 concentrationsnearly overcame the contractile dysfunction in muscle prep-arations of AF patients. In 31 bundles of 19 SR patients and30 bundles of 15 AF patients, maximal FC (Fmax) was directlycompared with the differences in sarcomere content. In AFpatients, the reduction in sarcomere content (214%) wassimilar to the slight decrease in Fmax (215%), although the

Figure 1. Top, Light microscopy of trans-verse and longitudinal sections (2 mm thick)of atrial muscle preparations. Periodic acidSchiff (PAS) staining (red), representing gly-cogen accumulation, was more pro-nounced in AF patients. Sarcomeresstained with toluidine blue are partly dis-placed by glycogen in these patients. Mag-nification 3850 (transverse sections) and3350 (longitudinal sections). Bottom, Rela-tive tissue composition of atrial musclebundles. *P,0.05.

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latter only nearly reached the level of statistical significance(P50.069). The correlation coefficient between sarcomerecontent and Fmax was 0.89 (P,0.001).

The b-adrenoceptor agonist isoproterenol exerted a pro-nounced positive inotropic effect in the SR and AF groups(Figure 4A). However, in AF patients, the concentration-response curve was shifted downward and to the right. FC atthe maximal isoproterenol concentration (1026 mol/L) waslower in the AF group (18.361.8 versus 30.061.3 mN/mm2,P,0.01). In the SR group, the half-maximal positive inotro-pic effect of isoproterenol was reached at 2.6 (1.8 to 4.4)nmol/L (EC50). A nearly 10-fold higher isoproterenol concen-tration was needed in the AF group to elicit the half-maximalresponse (EC50 22 [14 to 41] nmol/L,P,0.01). Thus, thepositive inotropic potency of isoproterenol was markedlyreduced in atrial muscle preparations from AF patients.Neither in the SR patients nor in the AF patients didb-blocker therapy alter the maximal response to isoproterenol(for SR patients, 32.264.3 mN/mm2 [n54, with b-blocker]versus 29.261.9 mN/mm2 [n512, nob-blocker],P5NS; forAF patients, 20.162.1 mN/mm2 [n54, with b-blocker] ver-sus 17.461.8 mN/mm2 [n59, no b-blocker], P5NS). Simi-larly, the potency of isoproterenol (EC50) was not changed.

b-Adrenergic Signaling[125I]Iodocyanopindolol binding experiments on atrial membranepreparations showed saturation characteristics in SR and in AF

patients (Figure 4B). Theb-adrenoceptor density was 5865fmol/mg in the SR group (n511) and 5465 fmol/mg in the AFgroup (n510) (P5NS). Thus, compared with the right atrialmyocardium of 6 healthy donor hearts (8167 fmol/mg), rightatrial b-adrenoceptor density was reduced to a similar extent inmitral valve disease patients in SR or AF.

Figure 4C shows representative immunoblots of the stim-ulatory and the inhibitory G protein. The antiserum AS/7bound specifically to a single band at 40 kDa, whichrepresents the Gia-2 subunit. RM/1 was bound to a 45-kDa anda 52-kDa protein representing 2 splicing variants of thestimulatory G-proteina subunit.13 Neither the level of thestimulatory G protein (12 7196883 cpm/mg in 14 SR pa-tients versus 14 67661243 cpm/mg in 12 AF patients,P5NS) nor the level of the inhibitory G protein (39516300cpm/mg in 14 SR patients versus 34766309 cpm/mg in 12AF patients,P5NS) was affected by AF.

Loss of Effect of Bay K8644The positive inotropic effect of Bay K8644 is shown in Figure 5.In the SR group, the Ca21 channel agonist exerted a pronouncedpositive inotropic effect. At 1025 mol/L Bay K8644, FC wascomparable to FC at 10.8 mmol/L extracellular Ca21. In contrast,the response to Bay K8644 was abolished in the AF group,although these muscle preparations did respond to an increase inextracellular Ca21 concentration. At the highest concentration ofBay K8644 (1025 mol/L), FC was 4.561.1 mN/mm2 comparedwith 28.062.6 mN/mm2 at 10.8 mmol/L Ca21 (P,0.01). Treat-ment with Ca21 antagonists did not change the response to BayK8644 in the AF patients. In the treated patients, FC at 1025

mol/L Bay K8644 was 4.761.3 mN/mm2 (n57) versus 4.261.3mN/mm2 (n55) in the patients who were not treated with Ca21

antagonists (P5NS). Digitalis did also not affect the response toBay K8644.

Preserved Postrest Potentiation of FCFigure 6 shows representative experiments on the postrestpotentiation of FC (rest interval 10 seconds). FC of the firstpostrest twitch was markedly increased in the SR and the AFpatients. Compared with the force amplitude at steady-stateconditions, the postrest FC was equally enhanced in bothpatient groups (4.161.0 mN/mm2 [SR, n516] versus 3.961.1mN/mm2 [AF, n516], P5NS).

TABLE 2. Relative Tissue Composition of Right Atrial Muscle Bundles

Mitral Valve Disease Coronary Artery Disease

SR AF SR AF

Muscle preparations/patients, n/n 31/19 30/15 12/10 11/9

Cardiomyocytes

Sarcomeres 50.462.0 43.562.0* 60.762.9† 52.761.4*†

Intracellular noncontractile material 6.860.3 11.760.3* 8.160.8 13.060.8*

Nuclei 1.260.1 1.160.1 1.460.2 1.560.2

Extracellular space

Extracellular matrix including fibroblasts 38.762.0 41.161.9 26.762.6† 29.761.4†

Vessels including perivascular cells 2.960.2 2.660.2 3.160.4 3.160.2

Values are mean6SEM (percentage of total tissue volume).*P,0.05 for AF vs SR; †P,0.05 for coronary artery disease vs mitral valve disease.

Figure 2. Superimposed force recordings of atrial muscle prep-arations from 25 SR patients (55 preparations) and from 22 AFpatients (51 preparations) recorded under isometric conditions(37°C, 1 Hz).

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DiscussionThe present study provides the first insight into the cellularmechanisms of the postfibrillatory atrial contractile dysfunc-tion in humans. It demonstrates that the contractility of atrial

muscle bundles isolated from AF patients is reduced by'75%. The data confirm previous observations in a dogmodel of tachycardia-induced atrial contractile dysfunction inwhich the systolic shortening of atrial cardiomyocytes wasfound to be reduced to a similar extent.12 The present studydemonstrates that AF-induced myolysis is only of limitedimportance as a cause of atrial contractile dysfunction.Instead, downregulation and/or altered function of the L-typeCa21 channels seems to underlie AF-induced atrialdysfunction.

Figure 3. Top, Effect of extracellular Ca21 concentration on FC(mN/mm2) of muscle preparations from patients in SR (20 bun-dles prepared from 16 patients) or in AF (19 bundles preparedfrom 13 patients) is shown. High extracellular Ca21 concentra-tions nearly overcame depression of FC due to AF. *P,0.05 forAF vs SR. Middle, In 31 muscle preparations from 19 SRpatients and in 30 muscle preparations from 15 AF patients,maximal FC during exposure to 10.8 mmol/L extracellular Ca21

concentration (Fmax) and structural changes were studied. Onaverage, sarcomere content was reduced in AF patients by14%. Slight reduction in Fmax was of similar extent (215%),nearly reaching statistical significance (P50.069). Bottom, Rela-tive sarcomere content was positively correlated with Fmax.

Figure 4. A, Effect of b-adrenoceptor agonist isoproterenol onFC (mN/mm2) of isolated muscle preparations (same prepara-tions as in Figure 3, top). Compared with response to 10.8mmol/L Ca21, positive inotropic effect of isoproterenol wasreduced in AF patients. B, Representative binding experimentson right atrial membrane preparations with use of [125I]iodocyan-opindolol (125I-ICYP) as radioligand. Neither affinity of radioligandto b-adrenoceptors nor density of receptors was different inthese patients. C, Representative immunoblot of stimulatory andinhibitory G-protein quantification. Each lane of autoradiographyrepresents 1 patient with either SR or AF. STD indicates stan-dard sample, which was subjected to each gel to promote com-parability of determination from different blots.

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Myolysis and AF-Induced Loss ofAtrial ContractilityIn patients with chronic AF, recovery of atrial contractilefunction after cardioversion may require several weeks ormonths.3 Restoration of the cellular (ultra)structure mightexplain such a long recovery of contractile function.8 Indeed,the present study shows that myolysis and replacement ofsarcomeres by glycogen was more pronounced in the trabec-ulae of AF patients. The same alterations have previouslybeen reported in the goat model of AF,8 and it has beenconcluded that the loss of sarcomeres might contribute to thedelayed recovery of atrial function after conversion to SR.However, in the latter study, a direct relation betweenstructural changes and the contractile function of the atrialmyocardium was not investigated. In the present study, theloss of sarcomeres was compared with the maximal achiev-able FC in the same muscle preparations to evaluate thecontribution of structural remodeling to the AF-induced atrialcontractile dysfunction. At high extracellular Ca21 concentra-tions, atrial FC was only slightly reduced in the AF group

(215%). This moderate reduction in maximal FC corre-sponded to a similar reduction in sarcomere content (214%).There was a strong correlation between the maximal FC andthe sarcomere content of the individual muscle preparations.Therefore, AF-induced myolysis can only partly explain thepronounced reduction of contractility in remodeled atria. Themain mechanism of postcardioversion atrial contractile dys-function is a disturbed activation rather than a loss of atrialmyofilaments.

b-Adrenergic SignalingAn important mechanism controlling myocardial force inatrial and ventricular myocardium is theb-adrenergic signaltransduction pathway. Our data demonstrate that in atrialmyocardium of AF patients, the positive inotropic effect ofisoproterenol was markedly reduced. Although the impairedb-adrenergic response cannot explain the reduced atrialcontractility in vitro, it might contribute to the atrial contrac-tile dysfunction in vivo by blunting the positive inotropicresponse to the physiological variations in sympathetic tone.

In human failing ventricular myocardium and in a caninemodel of pacing-induced heart failure, desensitization towardcatecholamines has been shown to be due to a reduceddensity of theb-adrenoceptors and an upregulation of theinhibitory G protein.17–19 In contrast, in the present study,atrial b-adrenoceptor density and G-protein levels were notaltered in AF patients. This indicates that the atrial contractiledysfunction after the cessation of AF is due to mechanismsdifferent from the ventricular cardiomyopathy in heart failureor experimental tachycardia-induced cardiomyopathy on theventricular level.

Role of the L-Type Ca21 ChannelBecause the positive inotropic effect of catecholamines ismainly due to activation of the L-type Ca21 channel, we testedthe hypothesis that alterations of the Ca21 channels wereresponsible for the diminished isoproterenol effect. A reduc-tion in Ca21 current has been shown to be the most importantionic mechanism of AF-induced electrical remodeling.9 Sev-eral studies have demonstrated a downregulation of L-typeCa21 channel subunits in AF.20–22 In our present study, theL-type Ca21 channel agonist Bay K8644 failed to increaseatrial FC in AF patients. Potential explanations includechanges in the number and/or function of the L-type Ca21

channels. Downregulation of channel subunits is expected toreduce the positive inotropic effect of the Ca21 channelagonist and might have caused the lack of response to BayK8644. Altered function of the channel due to changes of thephosphorylation state is a second possibility to explain theloss of effect of Bay K8644. In failing ventricular myocar-dium, the spatial relationship between the L-type Ca21 chan-nel and the Ca21-release channel of the sarcoplasmic reticu-lum is disturbed.23 This would provide a third explanation forthe absence of the Bay K8644 effect. The latter possibilitywould also explain why the positive inotropic effect of BayK8644 is abolished in atrial myocardium of AF patients,whereas Bay K8644 could partly overcome the decrease ofthe Ca21 inward current in the atrial cells of dogs undergoingrapid atrial pacing.9

Figure 5. Effect of Bay K8644 on FC (mN/mm2) of isolated mus-cle preparations from patients in SR (19 muscle preparationsprepared from 16 patients) or in AF (16 muscle preparationsprepared from 12 patients). In SR patients, Bay K8644 elicited apronounced positive inotropic effect, which was comparable tothe effect of 10.8 mmol/L Ca21. In AF patients, Bay K8644 failedto increase FC, although the same muscle bundles did respondto 10.8 mmol/L Ca21.

Figure 6. Representative recordings of postrest potentiation ofFC (rest intervals of 10 seconds). Postrest potentiation was fullypreserved in atrial muscle preparation of AF patient.

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The blunted response to Bay K8644 not only emphasizesthat a change in the function and/or number of the L-typeCa21 channels is the key to understanding post-AF atrialcontractile dysfunction, it also explains why the positiveinotropic effect of isoproterenol was reduced in AF patients,althoughb-adrenoceptor density and G-protein levels wereunaltered. Isoproterenol increases FC by increasing the Ca21

inward current and by enhancing the Ca21 reuptake by theCa21-ATPase of the sarcoplasmic reticulum (SERCA). As aresult of both effects, the Ca21 load of the sarcoplasmicreticulum is increased. In atrial myocardium of AF patients,the inotropic effect of isoproterenol was reduced, which canbe explained by the above-mentioned changes of the L-typeCa21 channel. Interestingly, isoproterenol still elicits somepositive inotropic effect in the atrial myocardium of AFpatients. This remaining stimulatory effect is most likely dueto stimulation of the Ca21 reuptake into the sarcoplasmicreticulum by SERCA, which would be consistent with ourhypothesis that the function of the sarcoplasmic reticulum ispreserved in the atrial myocardium of AF patients (seebelow). In contrast, Bay K8644, acting purely on the L-typeCa21 channel, lacks this alternative positive inotropic mech-anism and becomes ineffective in the atrial myocardium ofAF patients.

No Dysfunction of the Sarcoplasmic ReticulumAs reported in human failing ventricular myocardium and inexperimental tachycardia-induced heart failure,24 dysfunctionof the sarcoplasmic reticulum might also contribute to thereduced atrial contractility after the cessation of AF. Postrestpotentiation of contractile force critically depends on theability of the sarcoplasmic reticulum to store and release Ca21

and reflects the functional capacity of the sarcoplasmicreticulum.25 In atrial myocardium of AF patients, the postrestpotentiation was fully preserved, indicating that in fibrillatingatria, FC is not limited by the capacity of the sarcoplasmicreticulum to release and take up Ca21 again. This observationis in accordance with the finding of Sun et al,12 who reportedthat the postrest potentiation was still present in atrialcardiomyocytes from dogs with tachycardia-induced atrialcontractile dysfunction.

Limitations of the StudyAlthough the present study has demonstrated that in atrialmuscle bundles from AF patients, contractility is reduced by75%, a causative relationship between AF and the atrialcontractile dysfunction has not been studied. We cannotexclude the possibility that a common underlying factorcauses both the atrial contractile dysfunction and AF. How-ever, the L-type Ca21 current is the main determinant ofcontractile force and becomes downregulated by AF. There-fore, it is reasonable to assume that electrical remodeling goeshand in hand with the atrial contractile dysfunction. Thishypothesis is supported by the study of Sun et al,12 whoshowed that in dogs undergoing rapid atrial pacing, thedecrease in atrial contractility follows a time course similar tothe shortening of the action potential duration and thedecrease of the Ca21 inward current.

In all patients, the atria showed histological changes thatwere probably due to the underlying structural heart disease.Although the structural differences in atria from patients inAF and SR were similar to the changes found in the goatmodel of AF (resembling the situation in lone AF), extrapo-lation to AF in less diseased hearts should be performed withgreat caution. Further studies on atrial preparations frompatients with lone AF would certainly be worthwhile.

A further limitation concerns the heterogeneity of ourstudy population. Digitalis and Ca21 antagonists were morefrequently taken by the AF patients, and some patients fromour study population were onb-blockers. The subgroupanalysis revealed that drug treatment did not change contrac-tile force and the response to positive inotropes in the atrialmuscle preparations. Thus, although we cannot exclude thepossibility that differences in medication might have influ-enced the contractile behavior of the muscle preparations, themarked differences in atrial contractility between SR and AFpatients could not be explained by differences in drugtherapy.

AcknowledgmentsDr Schotten is the recipient of the Hendrick Casimir-Karl Zieglerscholarship 1998 of the Academy of Sciences of North-RhineWestfalia, Germany.

References1. Logan W, Rowlands D, Howitt G, et al. Left atrial activity following

cardioversion.Lancet. 1965;2:471–473.2. Shapiro EP, Effron MB, Lima S, et al. Transient atrial dysfunction after

conversion of chronic atrial fibrillation to sinus rhythm.Am J Cardiol.1988;62:1202–1207.

3. Manning WJ, Silverman DI, Katz SE, et al. Impaired left atrialmechanical function after cardioversion: relation to the duration of atrialfibrillation. J Am Coll Cardiol. 1994;23:1535–1540.

4. Black IW, Fatkin D, Sagar KB, et al. Exclusion of atrial thrombus bytransesophageal echocardiography does not preclude embolism after car-dioversion of atrial fibrillation: a multicenter study.Circulation. 1994;89:2509–2513.

5. Fatkin D, Kuchar DL, Thorburn CW, et al. Transesophageal echocardi-ography before and during direct current cardioversion of atrial fibril-lation: evidence for “atrial stunning” as a mechanism of thromboemboliccomplications.J Am Coll Cardiol. 1994;23:307–316.

6. Dethy M, Chassat C, Roy D, et al. Doppler echocardiographic predictorsof recurrence of atrial fibrillation after cardioversion.Am J Cardiol.1988;62:723–726.

7. Grimm RA, Leung DY, Black IW, et al. Left atrial appendage “stunning”after spontaneous conversion of atrial fibrillation demonstrated by trans-esophageal Doppler echocardiography.Am Heart J. 1995;130:174–176.

8. Ausma J, Wijffels M, Thoné F, et al. Structural changes of atrial myo-cardium due to sustained atrial fibrillation in the goat.Circulation. 1997;96:3157–3163.

9. Yue L, Feng J, Gaspo R, et al. Ionic remodeling underlying actionpotential changes in a canine model of atrial fibrillation.Circ Res.1997;81:512–525.

10. Bosch RF, Zeng XR, Grammer JB, et al. Ionic mechanisms of electricalremodeling in human atrial fibrillation.Cardiovasc Res. 1999;44:121–131.

11. Van Wagoner DR, Pond AL, Lamorgese M, et al. Atrial L-type Ca21-currents and human atrial fibrillation.Circ Res. 1999;85:428–436.

12. Sun H, Gaspo R, Leblanc N, et al. Cellular mechanisms of atrial con-tractile dysfunction caused by sustained atrial tachycardia.Circulation.1998;98:719–727.

13. Schotten U, Filzmaier K, Borghardt B, et al. Changes of theb-adrenergicsignal transduction in compensated human cardiac hypertrophy dependon the underlying disease.Am J Physiol. 2000;278:H2076–H2083.

Schotten et al Atrial Contractile Dysfunction in AF 697

by guest on February 20, 2013http://circ.ahajournals.org/Downloaded from

14. Bradford MM. A rapid and sensitive method for the quantitation ofmicrogram quantities of protein utilizing the principle of protein-dyebinding.Anal Biochem. 1976;72:248–254.

15. Boyden PA, Tilley LP, Pham TD, et al. Effects of left atrial enlargementon atrial transmembrane potentials and structure in dogs with mitral valvefibrosis.Am J Cardiol. 1982;49:1896–1908.

16. Pham TD, Fenoglio JJ Jr. Right atrial ultrastructure in chronic rheumaticheart disease.Int J Cardiol. 1982;1:289–304.

17. Bristow MR, Ginsburg R, Minobe W, et al. Decreased catecholaminesensitivity and beta-adrenergic-receptor density in failing human hearts.N Engl J Med. 1982;307:205–211.

18. Feldman AM, Cates AE, Veazey WB, et al. Increase of the 40,000-mol wtpertussis toxin substrate (G-protein) in the failing human heart.J ClinInvest. 1988;82:189–197.

19. Marzo KP, Frey MJ, Wilson JR, et al. Beta-adrenergic receptor-G-protein-adenylate cyclase complex in experimental canine congestiveheart failure produced by rapid ventricular pacing.Circ Res. 1991;69:1546–1556.

20. Brundel BJJM, Van Gelder IC, Henning RH, et al. Gene expression ofproteins influencing the calcium homeostasis in patients with persistentand paroxysmal atrial fibrillation.Cardiovasc Res. 1999;42:443–454.

21. Van Gelder IC, Brundel BJ, Henning RH, et al. Alterations in geneexpression of proteins involved in the calcium handling in patients withatrial fibrillation. J Cardiovasc Electrophysiol. 1999;10:552–560.

22. Gaspo R, Sun H, Fareh S, et al. Dihydropyridine and beta-adrenergicreceptor binding in dogs with tachycardia-induced atrial fibrillation.Car-diovasc Res. 1999;42:434–442.

23. Gomez AM, Valdivia HH, Cheng H, et al. Defective excitation-contraction coupling in experimental cardiac hypertrophy and heartfailure. Science. 1997;276:800–806.

24. Mittmann C, Eschenhagen T, Scholz H. Cellular and molecular aspects ofcontractile dysfunction in heart failure.Cardiovasc Res. 1998;39:267–275.

25. Bers DM. Ca-influx and sarcoplasmic reticulum Ca-release in cardiacmuscle activation during postrest recovery.Am J Physiol. 1985;248:H366–H381.

698 Circulation February 6, 2001

by guest on February 20, 2013http://circ.ahajournals.org/Downloaded from