Effects of I Kr and I Ks Heterogeneity on Action Potential Duration and Its Rate Dependence

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Abstract
Background—A growing body of evidence suggests that heterogeneity of ion channel expression and electrophysiological characteristics is an important property of the ventricular myocardium. The 2 components of the delayed rectifier potassium current, IKr (rapid) and IKs (slow), play a dominant role in the repolarization of the action potential and are important determinants of its duration. Methods and Results—In this report, the effects of heterogeneities of IKr and IKs on action potential duration (APD) and its rate dependence (adaptation) are studied with the use of the LRd model of a mammalian ventricular cell. Results demonstrate the importance of IKs density variations in heterogeneity of repolarization. Cells with reduced IKs (eg, mid-myocardial M cells) display long APD and steep dependence of APD on rate. Mechanistically, accumulation of IKs activation and increased sodium calcium exchange current, INaCa, secondary to Na+ accumulation at a fast rate underlie the steep APD-rate relation of these cells. When cells are electrotonically coupled in a multicellular fiber through resistive gap junction, APD differences are reduced. The results demonstrate strong dependence of APD heterogeneity on the degree of intercellular coupling even in the normal physiological range. Highly reduced coupling maximizes APD heterogeneity. Conclusions—Heterogeneity of IKs:IKr density strongly influences APD and its rate dependence. However, in the intact myocardium, the degree of gap-junction coupling may be an important factor that determines the manifestation of APD heterogeneity and dispersion of repolarization. The clinical significance of this study is in the context of repolarization abnormalities and associated arrhythmias (eg, long QT syndrome and torsade de pointes).

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