Distinct Transient Outward Potassium Current (I_to) Phenotypes and Distribution of Fast-inactivating Potassium Channel Alpha Subunits in Ferret Left Ventricular Myocytes

The biophysical characteristics and α subunits underlying calcium-independent transient outward potassium current (I_to) 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 I_to phenotypes were measured (21–22°C) in the majority of LV epi and LV endo myocytes studied. The two I_to phenotypes displayed marked differences in peak current densities, activation thresholds, inactivation characteristics, and recovery kinetics. I_to,epi recovered rapidly [τ_{rec, −70 mV} = 51 ± 3 ms] with minimal cumulative inactivation, while I_to,endo recovered slowly [τ_{rec, −70 mV} = 3,002 ± 447 ms] with marked cumulative inactivation. Heteropoda toxin 2 (150 nM) blocked I_to,epi in a voltage-dependent manner, but had no effect on I_to,endo. Parallel FISH and IF measurements conducted on isolated LV epi and LV endo myocytes demonstrated that Kv1.4, Kv4.2, and Kv4.3 α 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 α subunits, respectively, are the molecular substrates underlying the I_to,epi and I_to,endo phenotypes. FISH and IF measurements were also conducted on ferret ventricular tissue sections. The three I_to α 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 I_to α subunits not only exists from LV epicardium to endocardium but also from apex to base.