The plateau outward current in canine ventricle, sensitive to 4‐aminopyridine, is a constitutive contributor to ventricular repolarization

Abstract

Background and purpose: I Kur (Ultra-rapid delayed rectifier current) has mM sensitivity to 4-aminopyridine (4-AP) and is an important modulator of the plateau amplitude and action potential duration in canine atria. Kv1.5 encodes I Kur and is present in both atria and ventricles in canines and humans. We hypothesized that a similar plateau outward current with mM sensitivity to 4-AP is present in canine ventricle. Experimental approach: We used established voltage clamp protocols and used 4-AP (50 and 100 mM) to measure a plateau outward current in normal canine myocytes isolated from the left ventricular mid-myocardium. Key results: Action potential recordings in the presence of 4-AP showed significant prolongation of action potential duration at 50 and 90% repolarization at 0.5 and 1 Hz (Po0.05), while no prolongation occurred at 2 Hz. Voltage clamp experiments revealed a rapidly activating current, similar to current characteristics of canine atrial I Kur , in B70% of left ventricular myocytes. The IC 50 of 4-AP for this current was 24.2 mM. The concentration of 4-AP used in our experiments resulted in selective blockade of an outward current that was not I to or I Kr . b-Adrenergic stimulation with isoprenaline significantly increased the 4-AP sensitive outward current density (Po0.05), suggesting a role for this current during increased sympathetic stimulation. In silico incorporation into a canine ventricular cell model revealed selective AP prolongation after current blockade. Conclusions and implications: Our results support the existence of a canine ventricular plateau outward current sensitive to micromolar 4-AP and its constitutive role in ventricular repolarization.

Repolarization abnormalities and afterdepolarizations in a canine model of sudden cardiac death

“…5A). We recently identified a 4-aminopyridine-sensitive "I Kur -like" plateau current in a majority (ϳ70%) of left ventricular midmyocardial myocytes (44). The VF ϩ myocytes had a reduced density of "I Kur -like" plateau current compared with controls and the VF Ϫ groups (P Ͻ 0.05; Fig.…”

“…Notably, similar to our finding in the midmyocardial region of a 4-aminopyridine-sensitive plateau current, Dun et al reported a C9356-sensitive (C9356 is a selective blocker of Kv1.5 channels) current in myocytes from both the EBZ and normal zone of canine hearts. We (44) recently reported the presence of a similar current that activates at plateau voltages and modulates canine midmyocardial APD. Because of its similarities with canine atrial I Kur (both in properties and inhibition with micromolar 4-aminopyridine), we suggest that this is a 4-aminopyridine sensitive "I Kur -like" plateau current.…”

“…IKr was measured as the D-sotalol-sensitive (100 M) current, while D-sotalolinsensitive tail currents were used to measure IKs (18,27). During IKr and IKs recordings, the bath solution also contained 4-aminopyridine (100 M) to prevent any potential contamination by "IKur-like" plateau current (44).…”

“…A combination of a Ϫ40-mV holding potential and an 80-ms prepulse to ϩ30 mV was used to inactivate Ito. A sustained 4-aminopyridine-sensitive plateau current was measured as the steady-state difference current, recorded after a minimum of 4 min of superfusion with 4-aminopyridine (44).…”

Multiple effects of 4-aminopyridine on feline and rabbit sinoatrial node myocytes and multicellular preparations

“…Until recently, 4-AP-sensitive current has been used as a measure of I Kur at concentrations of 50 and 100 µM (e.g., [27]) and at concentrations 1-5 mM as a measure of I to (e.g., [4,27,28]. It is concluded that in spite of the usefulness of 4-AP as I to blocker and being a I Kur blocker at concentrations lower to 100 µM in cardiac preparations, care should be taken with conclusions, since the drug produces multiple effects on cardiac preparations.…”