A Major Role for hERG in Determining Frequency of Reentry in Neonatal Rat Ventricular Myocyte Monolayer

Abstract

Rationale:The rapid delayed rectifier potassium current, I Kr , which flows through the human ether-a-go-gorelated (hERG) channel, is a major determinant of the shape and duration of the human cardiac action potential (APD). However, it is unknown whether the time dependency of I Kr enables it to control APD, conduction velocity (CV), and wavelength (WL) at the exceedingly high activation frequencies that are relevant to cardiac reentry and fibrillation.Objective: To test the hypothesis that upregulation of hERG increases functional reentry frequency and contributes to its stability. Methods and Results:Using optical mapping, we investigated the effects of I Kr upregulation on reentry frequency, APD, CV, and WL in neonatal rat ventricular myocyte (NRVM) monolayers infected with GFP (control), hERG (I Kr ), or dominant negative mutant hERG G628S. Reentry frequency was higher in the I Kr -infected monolayers (21.12؎0.8 Hz; n‫34؍‬ versus 9.21؎0.58 Hz; n‫;61؍‬ P<0.001) but slightly reduced in G628S-infected monolayers. APD 80 in the I Kr -infected monolayers was shorter (>50%) than control during pacing at 1 to 5 Hz. CV was similar in both groups at low frequency pacing. In contrast, during high-frequency reentry, the CV measured at varying distances from the center of rotation was significantly faster in I Kr -infected monolayers than controls. Simulations using a modified NRVM model predicted that rotor acceleration was attributable, in part, to a transient hyperpolarization immediately following the AP. The transient hyperpolarization was confirmed experimentally. Key Words: hERG Ⅲ delayed rectifier potassium channel Ⅲ reentry Ⅲ ventricular fibrillation Ⅲ optical mapping V entricular fibrillation (VF) is a major cause of sudden cardiac death. Evidence suggests that, in mammalian species ranging in size from the mouse to human, VF may be maintained by highly periodic reentrant waves called rotors. 1,2 The spiral waves that emanate from high-frequency rotors propagate through the ventricles and undergo intermittent, spatially distributed wavebreaks, resulting in complex patterns of conduction also known as fibrillatory conduction. 1,3 Although the existence of rotors has been known for years, the ionic mechanisms responsible for their behavior remain incompletely understood. Recently, it was shown that the frequency and the stability of rotors are determined in great measure by the resting membrane potential (RMP) and the conduction velocity (CV), which are controlled by the dynamic interplay between the outward component of the inward rectifier potassium current (I K1 ) and the rapid inward sodium current (I Na ). 4 Other experiments have demonstrated that overexpression of the slow delayed rectifier current (I Ks ) does not affect reentry frequency but significantly increases the incidence of fibrillatory conduction in cardiomyocyte monolayers. 5 However, the consequences of upregulation or gain of function in the rapid delayed rectifier current (I Kr ) on reentry frequency and dynamics has never been directly i...

The Na+/K+ pump is an important modulator of refractoriness and rotor dynamics in human atrial tissue

“…As in previous studies (22,34,50), the choice of the two-dimensional tissue geometry avoids confounding interpretation of the ionic mechanisms with added complexities such as structural and electrophysiological heterogeneities. A planar wavefront was initiated by a stimulus applied at the lower tissue edge, followed by another stimulus applied on a square area in the tissue bottom-left corner at a CI within the vulnerable window.…”

Ionic mechanisms of arrhythmogenesis

“…the fast and the slow delayed rectifier K + currents, viz. I Kr and I Ks [44], we utilized monolayers of confluent, electrically coupled neonatal rat ventricular myocytes (NRVMs) [45,46]. The density of either I Ks or I Kr was increased by adenoviral overexpression of either KvLQT1-minK, or hERG, respectively in NRVM monolayers.…”

“…Experiments in isolated HEK cells and suggested that the increased wavebreaks was because of the phenomenon of post-repolarization refractoriness, occurring due to residual outward I Ks current, following the action potential [45]. In contrast to I Ks , a larger density of I Kr caused the spiral to accelerate significantly compared to control (∼21 Hz in the former, versus ∼9 Hz in the latter case) [46]. Both experiments and simulations showed that the faster spiral was due to APD shortening, as well as transient V rest hyperpolarization, which again would indirectly affect spiral frequency by modifying I Na availability [46].…”

A device for rapid and quantitative measurement of cardiac myocyte contractility

“…First, we utilized primary cultures of neonatal rat cardiomyocyte monolayers. 25 We also measured contractility in monolayers of human embryonic stem cell-derived cardiomyocytes (hESC-CMs) that have been used to generate specific cardiac myocyte lineages 26,27 and approved on the NIH stem cell registry. 28,29 The human cardiac cell line used was a control or healthy stem cell derived cardiomyocyte, UM22-2.…”