UC Davis

Background

Rapid delayed rectifier K⁺ current (I_Kr) and late Na⁺ current (I_NaL) significantly shape the cardiac action potential (AP). Changes in their magnitudes can cause either long or short QT syndromes associated with malignant ventricular arrhythmias and sudden cardiac death.

Methods

Physiological self AP-clamp was used to measure I_NaL and I_Kr during the AP in rabbit and porcine ventricular cardiomyocytes to test our hypothesis that the balance between I_Kr and I_NaL affects repolarization stability in health and disease conditions.

Results

We found comparable amount of net charge carried by I_Kr and I_NaL during the physiological AP, suggesting that outward K⁺ current via I_Kr and inward Na⁺ current via I_NaL are in balance during physiological repolarization. Remarkably, I_Kr and I_NaL integrals in each control myocyte were highly correlated in both healthy rabbit and pig myocytes, despite high overall cell-to-cell variability. This close correlation was lost in heart failure myocytes from both species. Pretreatment with E-4031 to block I_Kr (mimicking long QT syndrome 2) or with sea anemone toxin II to impair Na⁺ channel inactivation (mimicking long QT syndrome 3) prolonged AP duration (APD); however, using GS-967 to inhibit I_NaL sufficiently restored APD to control in both cases. Importantly, I_NaL inhibition significantly reduced the beat-to-beat and short-term variabilities of APD. Moreover, I_NaL inhibition also restored APD and repolarization stability in heart failure. Conversely, pretreatment with GS-967 shortened APD (mimicking short QT syndrome), and E-4031 reverted APD shortening. Furthermore, the amplitude of AP alternans occurring at high pacing frequency was decreased by I_NaL inhibition, increased by I_Kr inhibition, and restored by combined I_NaL and I_Kr inhibitions.

Conclusions

Our data demonstrate that I_Kr and I_NaL are counterbalancing currents during the physiological ventricular AP and their integrals covary in individual myocytes. Targeting these ionic currents to normalize their balance may have significant therapeutic potential in heart diseases with repolarization abnormalities. Visual Overview: A visual overview is available for this article.