| Objectives: Treatment of atrial fibrillation/flutter with available potassium channelblockers (class III antiarrhythmic agents which mainly block the rapid delayed rectifiercurrent, Ikr) associated with not only the prolongation of the atrial effective refractoryperiod but also with the elongation of the QT interval and the subsequence ofventricular proarrhythmia (early after-depolarization leading to torsades de pointesarrhythmia). This maybe due to the indifferent distribution of IKr between atrial andventricular myocardiocyte and limits the efficiency and security of theiranti-fibrillation/flutter activity. Since the ultra rapid rectifier current (Ikur) has beenfound only in the atrium but not in the ventricle, and the transient outward current (Itol)density distributed several times more in the atrium than in the ventricle, they may bethe key currents contribute to the action potential repolarization in the atrium. So it isreasonable to hypothesize that selective blockade of the two currents may behave asselective anti-atrial fibrillation/flutter activity without the risk of ventricularproarrhymia. It is well known that 4-aminopyridine(4-AP) behave as selective blockerof Ikur at u mol/L level and selective blocker of Itol at mmol/L level, so we areintending to establish an atrial fibrillation model on rabbit by rapid atrial pacing and use4-AP to evaluate the effect of selective blockade of Ikur or Itol on the cellularelectrophysiology properties and their potential anti-fibrillation/flutter effects.Methods: 1) Cellular electrophysiology study: By establishing a whole-cell-clampmodel on the enzymaticaly isolated single rabbit atrial and ventricular myocytes, firstly,we use V-clamp mode to observe the distribution characters of the Itol, Ikur, Ikr+sbetween atrium and ventricle, and the selective blockade of the 0.1 mmol/L and2mmol/L 4-AP on them. Secondly, we use I-clamp mode to observe the effect on theaction potential morphology of the two types myocardiocyte imposed by perfusion of0.1mmol/L and 2mmol/L 4-AP. 2) In vivo study of the two levels 4-AP'santi-fibrillation/flutter effects. We firstly establish a rabbit atria fibrillation model byrapid right atrial pacing(RAP) at frequency of 10Hz and 4 times threshold and 2mspulse wide for 2 hours (including 7 rabbits without RAP for physiological control, and16 rabbits with RAP for 2 hours to establish acute atrial fibrillation model). Then toobviate the 4-AP's potential side effects of the neuro-toxity, we cut off the heart andestablish an isolated Langendorff-perfused rabbit heart model, and put 4 bipolarelectrodes on high right atrium(HRA), right appendage(RAA), ostium of left superiorpulmonary vein(LPV) and left appendage(LAA), and another 2 electrode catheters atendoventricular and into midventricular layer. By these electrodes we can recordmultisites endomyocardiaograme and carryout the classic electrophysiology study(EP)by S1S2 stimulus or burst pacing to measure local effect refractory period or inducearrhythmias. For the normal physiology control group, we directly observe the sinusrate(SR), intra atrial conduction time between HRA and LAA(CT), Wenckbach cyclelength of A-V conduction(WCL), inducibility of atrial fibrillation/flutter(Af/AF), andinducibility of ventricular arrhythmias, at baseline and 0.1 mM and 2.0mM 4-AP toevaluate their effects on normal heart by self control. For the 16 RAP induced Af rabbits, after establishment of the isolated Langendorff-perfused rabbit heart model, additional 30min's RAP was reimposed for every 30 mimutes during the entire study to enhance the remodeled atrial by RAP. The 16 RAP induced Af rabbits were randomly divided into Af control group(n==7) and Af treat group(n=9). In the Af control group, the same EP study as the normal control group was underwent at 30min(after 1st 30mins' RAP, as baseline), 1.5hour (after 2nd 30mins' RAP)and 3hour(after 3rd 30mins' RAP) from the time of the isolation of the heart to evaluate the maintenance of the 2hours' RAP induced atrial remodeling by repeated RAP during the entire study course of isolated heart model, which was same as Af treat group, making quality control of this model used in the Af treat group(with 4-AP). For the Af treat group we underwent the same EP study as mentioned above at baseline of RAP induced Af model and after perfusion of normal Tyroid's solution containing O.lmM and 2.OmM 4-AP to evaluate the anti-fibrillation/flutter effects of the two concentration 4-AP representing the selective blockade of Ikurandltoi respectively, by a self-controlled study.Results: Part I: (Dlkur expressed only in rabbit atrium but not in ventricle, and the current density of I,ol is significant higher in atrium than ventricle. (2) O.lmM 4-AP selectively block Ikur (inhibiting Ikur density by 95.54%+10.30%, pO.OOl; while inhibiting Itoi by 17.30%±5.88%, p>0.05). ? 2.0mM 4-AP selectively inhibit Ito, by >95% for both atrium and ventricle (pO.OOl) without influence on Ik. Part II: ? Blockade of Ikur by O.lmM 4-AP significantly prolong APD20, APD50 and APD90 of atrial myocyte without AP waveform change of ventricle. (2) Blockade of Ikur and Itoi by 2.OmM 4-AP significantly shows higher AP amplitude(APA) and additional prolongation of APD20 of atrial myocyte without additional prolongation of APDsoand APD90 compared with 0.1 raM 4-AP. (3) 2.0mM 4-AP manifest minor effect of APA increment and APD20 prolongation on ventricle myocyte but significant prolongation of APD50 and APD90. Part III: ? Compared with normal control group RAP for 2 hours significantly decrease AERP and increase the inducibility of Af and AF. (2)Af control group shows our protocol of repeat RAP can maintain stable remodeling throughout the entire study course, with sable shortened AERP and stable inducibility Af/AF inducibility. (3) Af treat group(by 4-AP) manifest significant prolongation of shortened AERP by RAP, and significantly decrease Af/AF inducibility without ventricular proarrhythmic effects at both 0.1 mM and 2.OmM 4-AP perfusion compared t o b aseline. F or 2 .OmM 4 -AP m anifest a dditional A ERP p rolongation b ut without any additional gain in reduction Af/AF inducibility compared to O.lmM 4-AP. Moreover, 2.OmM 4-AP showed significant VERP prolongation and significant inhibition of sinus nodal and atrial ventricular nodal function. ? For the normal control group O.lmM 4-APshows minor AERP prolongation without ventricular proarrhythmia or brandycardia proarrhythmia effects. However 2.0 mM 4-AP also shows significant brandycardia and AV conduction dysfunction. Conclusions: Ikur and Itoi, especially the former, may be the predominant K+ currents... |
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