Abstract 605: Brain ERK1/2 Mitogen-Activated Protein Kinase Mediated Phosphorylation of Potassium Channel Subunit Kv4.2 Likely Contributes to Sympathetic Activation by Blood-borne TNF-α

Abstract

Blood-borne tumor necrosis factor (TNF)-α acts upon the central nervous system to augment sympathetic drive in normal and pathophysiological conditions including heart failure. We have found that brain extracellular signal-regulated kinase (ERK)1/2 mediates TNF-α induced sympatho-excitatory responses, but the precise mechanisms are not understood. ERK1/2 can rapidly phosphorylate the voltage gated potassium channel (Kv) subunit Kv4.2 in neurons to reduce the transient outward potassium current that normally restrains neuronal excitability in cardiovascular-related brain regions. We hypothesized that ERK1/2 phosphorylation of Kv4.2 in the hypothalamic paraventricular nucleus (PVN) may contribute to TNF-α-induced sympathetic activity. In anesthetized rats, a centrally directed intracarotid artery (ICA) injection of TNF-α (0.5 μg/kg) increased (*p<0.05, vs baseline) renal sympathetic nerve activity, mean blood pressure and heart rate beginning as early as 30 minutes. These responses were reduced by a 4-hour continuous intracerebroventricular (ICV) infusion of the ERK1/2 inhibitor PD98059 (0.4 μg/hour). Molecular studies revealed that TNF-α-induced phosphorylation of ERK1/2 and Kv4.2 was detectable at 30 minutes (0.51±0.03* vs 0.28±0.03 for ERK1/2, 0.19±0.02* vs 0.00±0.00 for Kv4.2, TNF-α vs control, corrected by β-actin) and increased with time (4 hours after TNF-α: 0.65±0.01* vs 0.25±0.04 for ERK1/2; 0.30±0.02* vs 0.00±0.00 for Kv4.2). Interestingly, mRNA expression for nuclear factor (NF)-κB p65 or its inhibitor IκBα was unchanged at 30 minutes. However, 4 hours after TNF-α injection NF-κB p65 mRNA (1.77±0.11* vs 1.00±0.02 fold change) was augmented and IκBα mRNA (0.48±0.03* vs 1.00±0.07 fold change) was decreased. ICV PD98059 normalized phosphorylation of ERK1/2 (0.30±0.01*) and reduced phosphorylation of Kv4.2 (0.16±0.02*) as well as activation of NF-κB (1.34±0.11* for NF-κB p65; 0.70±0.08* for IκBα) in the PVN 4 hours after TNF-α injection. These data suggest that the acute sympatho-excitatory responses elicited by blood-borne TNF-α are mediated, at least in part, by brain ERK1/2-induced phosphorylation of Kv4.2, reducing the transient outward potassium current and increasing neuronal excitation.