Small-conductance Ca2+-activated K+ Channels in Ventricular Arrhythmia

• Main Authors: Chih-Chieh Yu, 游治節
• Other Authors: 賴凌平
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/rq633p

博士 === 國立臺灣大學 === 臨床醫學研究所 === 105 === Small conductance Ca2+-activated K+ (SK) current is a repolarization current gated by increased intracellular calcium concentration. It was firstly found abundant in the central nervous system and was responsible for afterhyperpolarization of the neurons. It was later found to be important in the repolarization of atrial myocytes and play important roles in automaticity and atrioventricular node conduction. Recently, some animal studies showed SK current was found to be upregulated in diseased heart ventricles, and was involved in arrhythmogenesis. To further test the role of SK current in human ventricles, we first used patch clamp technique to test the specificity of SK current blocker, apamin. We tested all major human cardiac ion currents, including L-type Ca2+, Na+ and the major K+ currents (IKs, IKr, IK1, Ito) with high dose apamin (500 nM) and found no significant effects. We then used apamin for the following human explanted failing heart optical mapping study. We took left ventricle free wall wedge, infused with calcium-sensitive and voltage-sensitive dyes. We used high resolution camera to capture the optical change of the dyes before and after apamin. We found a significant increase of action potential duration after apamin, especially at high pacing cycle length at all three layers of endocardium, midmyocardium and epicardium. In M cell islands, less increase of SK current was observed, and electrical alternans was observed after apamin. Interestingly, we also found a significant reduction of conduction velocity after apamin, implying that SK current was involved in conduction. This observation was also strengthened by our immunostaining study of the wedge that SK protein was abundant in the intercalated discs. However, because upregulation of SK current shortened action potential duration, it enhanced the discrepancy of action potential duration and calcium transient duration, which has been proposed as a pro-arrhythmic character. We further tested the association between SK current and human arrhythmia by conducting a genetic association study between KCNN2 and human sudden cardiac death. We collected the blood sample of the patients with history of aborted sudden cardiac death due to ventricular tachyarrhythmias. We tested the association of tachyarrhythmias and KCNN2 polymorphisms and found 2 strongly associated tag single nucleotide polymorphisms (SNPs). The odds ratio was around 2.6~2.9, proving the involvement of SK current in human ventricular tachyarrhythmias. SK current acts like a protective current when the myocardium is exposed in a condition that may increase intracellular calcium, such as heart failure, long QT syndrome and the recently rising syndrome, calmodulinopathy. We used patch clamp technique to test those human arrhythmogenic mutant calmodulin on SK current and found a failure of the mutant calmodulin to fully open SK2 channels in response to elevated intracellular calcium. Our studies in human tissues further strengthened the observation in animal studies that SK current was involved in ventricular arrhythmias when the heart was exposed in diseased status. In these studies, we also had some very novel findings that needed further studies to confirm and explore the mechanisms, such as involvement of SK current in conduction and the increased risk of sudden cardiac death when carrying KCNN2 polymorphisms. We’ve moved a step forward on the way of understanding SK current, but more effort is warranted until we achieve the goal of using SK current as the target to prevent sudden cardiac death.