Four Kinetically Distinct Depolarization-activated K+ Currents in Adult Mouse Ventricular Myocytes

Abstract

In the experiments here, the time- and voltage-dependent properties of the Ca2+-independent, depolarization-activated K+ currents in adult mouse ventricular myocytes were characterized in detail. In the majority (65 of 72, ≈ 90%) of cells dispersed from the ventricles, analysis of the decay phases of the outward currents revealed three distinct K+ current components: a rapidly inactivating, transient outward K+ current, Ito,f (mean ± SEM τdecay = 85 ± 2 ms); a slowly (mean ± SEM τdecay = 1,162 ± 29 ms) inactivating K+ current, IK,slow; and a non inactivating, steady state current, Iss. In a small subset (7 of 72, ≈ 10%) of cells, Ito,f was absent and a slowly inactivating (mean ± SEM τdecay = 196 ± 7 ms) transient outward current, referred to as Ito,s, was identified; the densities and properties of IK,slow and Iss in Ito,s-expressing cells are indistinguishable from the corresponding currents in cells with Ito,f. Microdissection techniques were used to remove tissue pieces from the left ventricular apex and from the ventricular septum to allow the hypothesis that there are regional differences in Ito,f and Ito,s expression to be tested directly. Electrophysiological recordings revealed that all cells isolated from the apex express Ito,f (n = 35); Ito,s is not detected in these cells (n = 35). In the septum, by contrast, all of the cells express Ito,s (n = 28) and in the majority (22 of 28, 80%) of cells, Ito,f is also present. The density of Ito,f (mean ± SEM at +40 mV = 6.8 ± 0.5 pA/pF, n = 22) in septum cells, however, is significantly (P < 0.001) lower than Ito,f density in cells from the apex (mean ± SEM at +40 mV = 34.6 ± 2.6 pA/pF, n = 35). In addition to differences in inactivation kinetics, Ito,f, Ito,s, and IK,slow display distinct rates of recovery (from inactivation), as well as differential sensitivities to 4-aminopyridine (4-AP), tetraethylammonium (TEA), and Heteropoda toxin-3. IK,slow, for example, is blocked selectively by low (10–50 μM) concentrations of 4-AP and by (≥25 mM) TEA. Although both Ito,f and Ito,s are blocked by high (>100 μM) 4-AP concentrations and are relatively insensitive to TEA, Ito,f is selectively blocked by nanomolar concentrations of Heteropoda toxin-3, and Ito,s (as well as IK,slow and Iss) is unaffected. Iss is partially blocked by high concentrations of 4-AP or TEA. The functional implications of the distinct properties and expression patterns of Ito,f and Ito,s, as well as the likely molecular correlates of these (and the IK,slow and Iss) currents, are discussed.

Loss of Cardiomyocyte Integrin-Linked Kinase Produces an Arrhythmogenic Cardiomyopathy in Mice

“…The sustained component I ss was not altered. qPCR confirmed significant changes in subunit expression corresponding to the ionic-current changes at 10 weeks: Kv4.2 and KChIP2, putative α and β subunits corresponding to I to,f , 13,14 were significantly reduced, by 55% and 66%, respectively ( Figure IVA in the Data Supplement). Kv4.3 expression was not altered.…”

“…I to,s , a component with kinetics intermediate between I to,f and I K,slow , is difficult to detect in native myocytes but is likely encoded by Kv1.4. 13 We found that Kv1.4 was expressed at low levels that were significantly upregulated by ILK knockout. Two other subunits, Kv1.2 and Kv2.1, were unaltered.…”

“…Voltage-gated K + currents were reduced in ILK-knockout, with changes already present at 5 weeks ( Figure 6A) and continuing to 10 weeks ( Figure 6B). Kinetic analyses revealed that in addition to total (peak) current, 2 of the 3 kinetically dissociable components reported in wild-type mice 13 are downregulated by ILK knockout: I to,f and I K,slow . The sustained component I ss was not altered.…”

“…Kv4.3 expression was not altered. Kv1.5, the subunit thought to underlie I K,slow , 13 was also downregulated, by 30%. I to,s , a component with kinetics intermediate between I to,f and I K,slow , is difficult to detect in native myocytes but is likely encoded by Kv1.4.…”

“…The amplitude of I to,f , I K,slow , and I ss were determined from exponential fits to the decay phases of the total outward K + current as previously described. 13,14 Currents are expressed as densities (pA/pF).…”