Isolation and characterization of IKr in cardiac myocytes by Cs+ permeation

Isolation of the rapidly activating delayed rectifier potassium current (I-Kr) from other cardiac currents has been a difficult task for quantitative study of this current. The present study was designed to separate I-Kr using Cs+ in cardiac myocytes. Cs+ have been known to block a variety of K+ channels, including many of those involved in the cardiac action potential such as inward rectifier potassium current I-K1 and the transient outward potassium current I-to. However, under isotonic Cs+ conditions (135 mM Cs+), a significant membrane current was recorded in isolated rabbit ventricular myocytes. This current displayed the voltage-dependent onset of and recovery from inactivation that are characteristic to I-Kr. Consistently, the current was selectively inhibited by the specific I-Kr blockers. The biophysical and pharmacological properties of the Cs+-carried human ether-a-go-go-related gene (hERG) current were very similar to those of the Cs+-carried I-Kr in ventricular myocytes. The primary sequence of the selectivity filter in hERG was in part responsible for the Cs+ permeability, which was lost when the sequence was changed from GFG to GYG, characteristic of other, Cs+-impermeable K+ channels. Thus the unique high Cs+ permeability in I-Kr channels provides an effective way to isolate I-Kr current. Although the biophysical and pharmacological properties of the Cs+-carried I-Kr are different from those of the K+-carried I-Kr, such an assay enables I-Kr current to be recorded at a level that is large enough and sufficiently robust to evaluate any I-Kr alterations in native tissues in response to physiological or pathological changes. It is particularly useful for exploring the role of reduction of I-Kr in arrhythmias associated with heart failure and long QT syndrome due to the reduced hERG channel membrane expression.