Effect of the Inhibition of the I_Kur on the Action Potential Shape in Isolated Human Right Atrial Preparations and Computer Simulations

Otto Hála^a, Erich Wettwer^c, Dobromir Dobrev^c, Torsten Christ^c, Jürgen Heubach^c,
András Varró^a, Julius Gy Papp^ab, and Ursula Ravens^c

^aDepartment of Pharmacology and Pharmacotherapy, University of Szeged, Szeged, Hungary
^bResearch Unit for Cardiovascular Pharmacology, Hungarian Academy of Sciences, Szeged, Hungary
^cDepartment of Pharmacology and Toxicology, Medical Faculty, Dresden University of Technology,
Dresden, Germany

Correspondence: András Varró, Department of Pharmacology and Pharmacotherapy, University of Szeged, P.O. Box 427,
H-6720 Szeged, Hungary. E-mail: varro@phcol.szote.u-szeged.hu, phone +36 62 545 683, fax +36 62 545 680

1.  Introduction

Expression of the ultrarapid delayed rectifier K⁺ current (I_Kur) encoded by KCNA5 is confined to the atrial myocytes in the human heart. According to the present view, a targeted antiarrhythmic therapy could be achieved by inhibition of these channels in atrial fibrillation. The aim of the study was to validate this hypothesis by using action potential measurements in preparations from SR and AF patients.

2.  Material and Methods

Right atrial appendages were obtained from patients undergoing coronary bypass surgery. Atrial trabeculae were prepared at room temperature and were mounted onto the bottom of an organ bath perfused with Tyrode’s solution (composition in mM: NaCl: 125, Na₂HCO₃: 22, NaH₂PO₄: 0.4, KCl: 5, CaCl₂: 1, MgCl₂: 0.6, D-glucose: 10; pH: 7.4 when equilibrated with 5% CO₂ in O₂ at 35°C). The preparations were electrically driven with square-wave stimuli of 1 ms in duration, 1.25-times threshold in intensity and 1 Hz in frequency. Transmembrane potentials were recorded with microelectrode technique. In addition to the conventional electrophysiological parameters (resting membrane potential: RMP, action potential amplitude: AMP, maximum rate of depolarization: V_max; and action potential durations at 20, 80 and 90% repolarizations: APD_{20 - 90}) notch and plateau potentials were analyzed. Action potential forms were characterized by their triangularity (ln{ 8×APD₂₀/2APD₈₀}). Action potential simulation was based on known models [Courtemanche et al. 1998, Luo and Rudy, 1994], was written in PASCAL and was calculated on a 800 MHz PC in DOS mode. Numerical integration was carried out by means of a modified Euler method. The length of the time-steps were changed between 0.002 and 2 ms

Figure 1. Effect of 5 mM 4-aminopyridine (4-AP) on the action potentials in human right atrial preparations (A) obtained from patients with sinus rhythm (SR) and chronic atrial fibrillation (AF), and the I_Kur block –induced action potential changes in SR and AF as it was shown by the simulation (B). C, Control.

3.  Results

In SR preparations increasing 4-AP concentrations ranging from 0.3 to 100 mM elevated the height of the action potential plateau (control: -22.6±0.8 mV, 100 mM 4AP: -2.8±1.9 mV, n=5) with an EC₅₀ of 13 mM. Besides the plateau elevation, the notch potential was shifted to more positive values (control: -26.1±1.2 mV, 100 mM 4AP: -6.3 ±1.1 mV, EC₅₀: 16 mM) and the APD₉₀ value was shortened (control: 293.7±15.1 ms, 100 mM 4AP: 243.2±11.8 ms, EC₅₀: 7.3 mM). The lowest 4-AP concentrations inducing statistically significant plateau elevation were found between 3 and 10 mM. In separate experiments, 5 mM of 4-AP significantly increased the triangularity of the action potentials (from -3.2±0.22 to -2.4±0.31, n=10). In AF preparations, 5 mM of 4-AP increased the amplitude of the action potential plateau by 15% (from: 0.36±3.3 to 12.4±2.5 mV; n=6 [p<0.01]) In contrast to SR in AF APD₈₀ and APD₉₀ values were lengthened by 5 mM 4-AP (from 233.8±12.0 to 258.5±10.9 ms [p<0.01] and from 300.4±16.3 to 320.2±13.3 ms [p<0.01], respectively). Triangularity of AF action potentials was increased by 4-AP (from -0.71±28 to +0.18±0.05, Fig 1A). The obtained action potential changes by 4-AP could exclusively be simulated by reduction of I_Kur conductance only (Fig 1B). Interestingly, selective block of I_Kur was accompanied by an increased activity of L-type Ca²⁺ current (SR: 35%, AF: 3%) and the amplitude and time-integral of the [Ca²⁺]_i transient (SR: 12%, AF: 22%). In SR and AF, activity of I_Kr was markedly increased by inhibition of I_Kúr (80% and 43%, respectively).

4.  Conclusions

Selective inhibition of I_Kur exerts opposite effects on the APD in multicellular right atrial preparations taken from SR or AF patients, i.e. low 4-AP concentrations prolonged APD in AF but shortened it in SR. We speculate, that lengthening of APD in AF may have an antiarrhythmic effect, whereas the SR-related APD shortening induced by block of I_Kur and the accompanying shift of the action potential shape from a “healthy” (spike-and-dome) type into a more triangular form could enhance the propensity to arrhythmias. In the simulation model block of I_Kur was accompanied by substantial increase in intensity of the L-type Ca²⁺ current and the amplitude of the [Ca²⁺]_i transient. The I_Kur block –induced APD shortening significantly depends on the basal activity of I_Kr.

References

Courtemanche M, Ramirez JR, Nattel S. Ionic mechanisms underlying human atrial action potential properties: insight from a mathematical model. American Journal of Physiology, 275(44): 301-321, 1998.

Luo CH, Rudy Y. A dynamic model of the cardiac ventricular action potential. I. Simulations of ionic currents and concentration changes. Circulation Research, 74: 1071-1096, 1994.