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Department of Biomedical Engineering, and ¶ Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710;
Department of Biochemistry and Cell Biology, State University of New York, Stony Brook, Stony Brook, New York 11794; and || Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, Missouri 63110
The biophysical characteristics and
subunits underlying calcium-independent transient outward potassium current (Ito) phenotypes expressed in ferret left ventricular epicardial (LV epi) and endocardial (LV endo) myocytes were analyzed using patch clamp, fluorescent in situ hybridization (FISH), and immunofluorescent (IF) techniques. Two distinct Ito phenotypes were measured (21–22°C) in the majority of LV epi and LV endo myocytes studied. The two Ito phenotypes displayed marked differences in peak current densities, activation thresholds, inactivation characteristics, and recovery kinetics. Ito,epi recovered rapidly [
rec, –70 mV = 51 ± 3 ms] with minimal cumulative inactivation, while Ito,endo recovered slowly [
rec, –70 mV = 3,002 ± 447 ms] with marked cumulative inactivation. Heteropoda toxin 2 (150 nM) blocked Ito,epi in a voltage-dependent manner, but had no effect on Ito,endo. Parallel FISH and IF measurements conducted on isolated LV epi and LV endo myocytes demonstrated that Kv1.4, Kv4.2, and Kv4.3
subunit expression in LV myocyte types was quite heterogenous: (a) Kv4.2 and Kv4.3 were more predominantly expressed in LV epi than LV endo myocytes, and (b) Kv1.4 was expressed in the majority of LV endo myocytes but was essentially absent in LV epi myocytes. In combination with previous measurements on recovery kinetics (Kv1.4, slow; Kv4.2/4.3, relatively rapid) and Heteropoda toxin block (Kv1.4, insensitive; Kv4.2, sensitive), our results strongly support the hypothesis that, in ferret heart, Kv4.2/Kv4.3 and Kv1.4
subunits, respectively, are the molecular substrates underlying the Ito,epi and Ito,endo phenotypes. FISH and IF measurements were also conducted on ferret ventricular tissue sections. The three Ito
subunits again showed distinct patterns of distribution: (a) Kv1.4 was localized primarily to the apical portion of the LV septum, LV endocardium, and approximate inner 75% of the LV free wall; (b) Kv4.2 was localized primarily to the right ventricular free wall, epicardial layers of the LV, and base of the heart; and (c) Kv4.3 was localized primarily to epicardial layers of the LV apex and diffusely distributed in the LV free wall and septum. Therefore, in intact ventricular tissue, a heterogeneous distribution of candidate Ito
subunits not only exists from LV epicardium to endocardium but also from apex to base.
Key Words: cardiac repolarization patch clamp Kv1.4 Kv4.2 Kv4.3
Abbreviations: AP, action potential; FISH, fluorescent in situ hybridization; HP, holding potential; HPTX, Heteropoda toxin; IF, immunofluorescent; LV endo, left ventricular endocardial; LV epi, left ventricular epicardial; RV, right ventricular; TnIC, troponin I cardiac
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