Effect Of Cardiac Nerve Remodeling On Transient Outward K~+ Current Of Cardiomyocyte And The Underlying Molecular Mechanisms

Arrhythmias were influenced directly or indirectly by autonomic nerve in the heart. Dysfunction of autonomic nerve have been considered an important factor in the triggering and maintainance of arrhythmias in cardiacmocytes. Cardiac autonomic nerve remodeling (CANR) refers to the change of autonomic innervations of the heart due to any diseases which increase or decrease regional cardiac autonomic innervation, such as nerve regeneration or sprouting (in myocardial infarction) and reduction of nerve density or heterogeneous cardiac denervation (in congestive heart failure or other forms of cardiac injury). Arrhythmias is mainly caused by two critical risk factors: substrate and trigger. At present, the mechanism underlying CANR related arrhythmias is poorly understood.Transient outward current (Ito) increases greatly during phase 1 of the cardiac action potential and is responsible for the early repolarization of action potentials which can directly influence action potential duration (APD) of cardiacmocytes. In human and other mammals, the magnitude of Ito is the greatest in ventriclar epicardium and the lowest in endocardium. The variation in Ito magnitude causes a transmural voltage gradient during ventricular activation. When this transmural voltage gradient of Ito was disrupted by cardiac diseases, it can disorder electrical activity of the heart and cause cardiac arrhythmias. The protein channels that mediate the fast component of Ito are formed by co-assembly of four identical Kv4.2 (rodent) or Kv4.3 (human, dog) a subunits. A smaller and slower component of Ito is mediated by Kv1.4 channels. One auxiliary subunit, K~+ channel-interacting protein 2 (KChIP2) that interact with Kv4 channels, were discovered recently. KChIP2 is preferentially expressed in the heart and enhance the trafficking of Kv4 proteins from the endoplasmic reticulum to the plasma membrane. We hypothesize that down-regulation of Ito is one of the leading mechanisms accounts for the induction of arrhythmias by in heart with CANR. The purpose of the present study is to test this hypothesis.The present study focused on the molecular mechanisms of CANR related arrhythmias, especially the role of Ito in cardiac nerve sprouting or denervation induced arrhythmias. We established a rat model of cardiac sympathetic nerve sprouting by subcutaneously injecting 4-Methylcatechol (4-MC), a strong stimulator of endogeneous NGF synthesis. We also established an rat model of cardiac sympathetic denervation by subcutaneous injection of 6-hydroxydopamine (6-OHDA, 100 mg/kg). The results showed that the current densities of Ito in the left ventricular (LV) myocytes decreased significantly and myocardial expression of Kv4.2 and KChIP2 in LV tissues were significantly decreased in rats with cardiac nerve sprouting or myocardial infarction (MI). A combination of nerve sprouting and MI further decreased the expresseions of Kv4.2 and Ito current densities. Nerve sprouting did not show significant influence on the Kv1.4 expression. We further found that nerve sprouting and MI decreased the levels of pERK1/2 and pCREB in cardiocyte, this may result in an downregulation of KChIP2 expression in cardiac tissues and increased the sudceptability to ventricular fibrillation (VF) induced by rapid pacing. Interestingly, we found that the effect of sympathetic denervation on the expressions of Kv4.2 and KChIP2 and Ito current densities is similar with nerve sprouting.The results in the near past in our laboratory showed that myocardial expression of NMDAR and AMPAR were significantly upregulated in rats with cardiac nerve sprouting or MI. Based on these findings, we hypothesized that ionotropic glutamate receptors may somehow affect Ito. We incubated cultured young rat (14-day old) cardiomyoctes with NMDA for 24h to stimulate NMDAR. The result showed that NMDA receptor overactivation significantly decreased the current densities of Ito and myocardial expressions of Kv4.2 and KChIP2. We further identified that subcellular localization of Kv4.2 and KChIP2 in the cultured cardiomycyoctes by confocal microscopy. We found that the trafficking of Kv4.2 and KChIP2 from endoplasmic reticulum to plasma membrane in the culture cardiomyocytes was decreased. Furthermore, NMDA increased the phosphorylation of serine 133 and serine 142 in CREB by elevating pERKl/2. As we have shown that NMDA induced an increase in [Ca2+]i in cardiocyte, we postulate that the phosphorylation of serine 142 in CREB by Ca2+-CamKII may lead to dissociation of the CREB dimer without impeding KChIP2 DNA binding capacity.Taken together, downregulation of Ito current density in heart with nerve sprouting or denervation may be an important contributor to the high susceptibility to ventricular tachyarrhythmias in healed MI. NMDAR is possibly involved in signaling of cardiac nerve remodeling induced Ito downregulation and arrhythmias. The ERK1/2-CREB signaling pathways may mediate at least in part the suppression of the trafficking of Kv4.2 and KChiP2 from endoplasmic to membrane by cardiac nerve remodeling. The interaction between iGlurR and Ito may play an important role in the pathogenesis of tachyarrhythmias and SCD in heart with nerve remodeling.