마취유도제인 Thiopental이 쥐 심실근 세포의 transient outward current (Ito) 및 Inwardly rectifying K+ current (Ik1)에 미치는 영향

Abstract

[한글]

심전도에서 long QT syndrome(LQTS, QTc interval 〉440 ms)을 보이는 환자는 torsade de pointes와 같이 생명을 위협할 수 있는 심한 심실성 부정맥이 발생할 수 있다. LQTS 환자에서는 심실의 재분극을 연장시키는 약제의 사용시 상승작용을 일으켜 이와 같은 부정맥을 촉진시키는 결과를 초래할 수 있다. 마취 유도시 사용되고 있는 정맥 마취제인 thiopental은 사람에서 QT interval을 연장시키는 효과가 있으며, thiopental의 이러한 효과는 또한 동물실험에서 활동전위기간을 연장시키는 결과와 일치한다. Thiopental은 Ik, Ik1 및 Ito를 억제시키나 현재 동일한 종의 동물에서 재분극에 관여하는 전류에 대한 종합적인 연구는 이루어져 있지 않다.

Langendorff 장치에 쥐의 심장을 현수후 효소 처리하여 단일세포로 분리하였다. 분리된 심근세포는 whole cell mode 전압고정법을 사용하였으며, Ito와 Ik1은 관류액에 0.5 mM CdCl2를 첨가하여 Ca+ 내향전류를 차단시킨 상태에서 각각의 voltage protocol을 적용하여 측정하였다. 쥐 우심실 유두근을 이용하여 심근수축을 측정하였으며, 통상적인 microelectrode technique을 이용하여 정상 활동전위를 측정하였다.

50 μM thiopental은 +60 mV에서 측정한 Ito의 peak current를 18 ± 1% (mean ± SEM) 감소시켰다. Thiopental 투여농도에 따른 Ito의 변화에서 IC50는 163 μM을 보였다. Ramp protocol을 적용한 실험에서 50 μM thiopental은 -130 mV에서 측정한 Ik1의 내향전류를 13 ± 2% 감소시켰다. -140 mV의 막전위에서 50 및 100 μM thiopental은 Ik1의 내향전류를 각각 14 ± 2% 및 22 ± 4% 감소시켰다. 50 μM thiopental은 ICa,L을 43 ± 5% 감소시켰다. 50 μM thiopental은 심근 수축에 별다른 영향을 미치지 않았다. 정상활동 전위에서 50 μM thiopental은 -5 mV의 탈분극을 보였고, 재분극 90%에서 활동전위 기간을 76% 연장시켰다.

쥐 심근세포에서 활동전위의 연장 효과는 Ito 및 Ik1의 억제에 의한 것으로 생각되며, 쥐 심근 세포에서 Ito의 current density가 매우 높은 점을 고려해 볼 때 Ito의 억제가 활동전위의 연장에 주된 원인으로 생각된다. Ca2+ 내향전류의 감소에도 불구하고 심근수축의 변화가 없었던 것은 thiopental의 Ito 및 Ik1 억제 효과가 Ca2+ 내향전류의 감소로 인한 활동전위의 단축 효과를 상쇄시켜 세포내로의 Ca2+ 유입을 증가시키기 때문인 것으로 생각된다.

[영문]Patients with the long QT syndrome, either congenital or acquired, have an increased risk of a serious ventricular arrythmia, called Torsade de Pointes. Thiopental (5 mg/kg) has been reported to prolong the QTc interval in patients undergoing surgery with normal repolarization. Recent studies have indicated that the clinical concentration of thiopental prolonged the action potential duration (APD), which was attributed to inhibition of the delayed rectifier (Ik) and/or the inwardly rectifying (Ik1) K+ currents at various animal myocardial preparations. The rat ventricular cells were used to study the contribution of transient outward current (Ito) and Ik1 because they possess a variety of K+ channel subtypes including Ito and Ik1 with little or no Ik, similar to those of human ventricular myocytes. The effect on Ca2+ current, ICa,L, which can alter the K+ conductance, was also observed.

With approval of the animal research committee in Yonsei University Medical College, isolated ventricular cells were obtained from enzymatically treated rat heart. The ICa,L was elicited from a holding potential of -40 mV to +60 mV under the normal Tyrode solution. The Ik1 and Ito was obtained from a holding potential of -40 mV before their membrane potential was changed from -130 to +50 mV. Ik1 was also measured by voltage steps from -140 to -40 mV in 20 mV increments. The holding potential was -50mV. Ito was recorded during depolarizing steps from -80 mV followed by inactivation of Na+ current by short pulses to -40 mV and then depolarized with 10 mV increments to test potentials up to +60 mV. Ito was measured as the peak current. The ICa,L was blocked by adding 0.5 mM CdCl2 during measurement of Ito. The contractile force was measured using papillary muscles and action potential was measured using conventional microelectrode technique.

At membrane potential of +60 mV, 50 μM thiopental caused modest depression of Ito to 82 ± 1% of control. From the dose-response curve from 1 to 1000 μM, the IC50 of thiopental was 163 μM. While 50 μM thiopental caused modest depression of Ik1 to 87 ± 2% of control at a test potential of -120 mV, ICa,L was significantly reduced to 57 ± 5% of control. 50 μM thiopental did not alter the contractile force. The APD90 was prolonged by 76% following application of 50 μM thiopental. The results are mean ± SEM.

Conclusion: Prolongation of APD induced by thiopental is likely to be attributed to the reduction of Ito and Ik1. Considering the high current density of Ito in rat ventricular myocytes, inhibition of Ito seems to be the main cause. It may suggest that inhibition of Ito and Ik1 counteracts the shortening of APD by moderate reduction of ICa,L, resulting in enhancement of Ca2+ current, which may result in maintenance of contractile force under thiopental.