Regulation of Potassium Channel in Ventricular Myocytes of Rat Following Volume Overload

Abstract

Heart failure (HF) is a leading cause of death in the US. Among the deaths of patients with HF, up to 50% are sudden and unexpected. The sudden deaths have been related to the lethal ventricular arrhythmias such as ventricular tachycardia or ventricular fibrillation. However, the mechanisms underlying arrhythmias in HF patients are unknown. Previous studies supported that the abnormal repolarizations may play an important role in arrhythmogenesis in patients with HF. In human and animal failing hearts, the most consistent electrophysiological change is prolonged action potential (AP), whereas downregulation of transient outward potassium current (Ito) is also a consistent change in ion currents. So far, the cellular and molecular mechanisms involving AP prolongation and Ito downregulation in HF patients is still incompletely understood. Recently growing evidence for the alteration of the ubiquitin-proteasome system (UPS) have been reported in various heart diseases. In addition, some ion channels have been reported to be degraded by the UPS. In the present study, using a rat model of volume overload induced by aortocaval fistula, we examined the electrophysiological characteristics in ventricular myocytes and possible molecular mechanisms underlying the electrophysiological alterations following ventricular remodeling. We found that AP duration was prolonged in fistula myocytes compared with control myocytes at 10- and 13-week post-fistula. Consistently, Ito densities were significantly decreased in fistula myocytes. In addition, depressed Kv4 α subunits were detected in protein level, but not in mRNA level, suggesting that posttranscriptional modification occurred on the Kv4 α subunits in fistula myocytes. Furthermore, elevated ubiquitinated Kv4 α subunits were detected in fistula myocytes compared with control. To determine whether the Kv4 α subunits are degraded by the proteasome, we employed MG-132, a proteasomal inhibitor, and chloroquine, a lysosomal inhibitor, respectively. Incubation with MG-132, but not chloroquine, for 24 hours led to increased Ito densities and protein expression of Kv4 α subunits in fistula myocytes, whereas neither MG-132 nor chloroquine altered Ito density and Kv4 α subunits expression in control myocytes. Accordingly, we conclude that the Kv4 α subunits are degraded by the proteasome and that the accelerated degradation of Kv4 α subunits results in the downregulation of transient outward potassium current therefore lengthening APD in rat ventricular myocytes following volume overload.