Phosphorylation of TASK-1 and its role in atrial arrhythmias

Atrial fibrillation (AF) is the most common sustained arrhythmia in human patients, and is associated with an increased risk of stroke and other morbidity. Acute-onset AF is frequently associated with cardiothoracic surgery, and its initiation is thought to involve proinflammatory signaling. Activated neutrophils are known to be arrhythmogenic, and one of the inflammatory mediators released from neutrophils that contributes to arrhythmogenicity is the phospholipid platelet-activating factor (PAF). PAF acts via a G-protein coupled receptor, present in myocytes, to activate downstream signaling cascades. Data from our lab indicates that inhibition of the two-pore domain potassium channel TASK-1 may contribute to the arrhythmogenic effect of PAF. This study describes the identification of a novel phosphorylation site in the human TASK-1 carboxyl terminus, T383, and has explored the association between phosphorylation at this site and the development or maintenance of various models of atrial fibrillation. Signaling downstream of PAF was previously shown to inhibit the two pore-domain potassium channel TASK-1. This inhibition required the activity of the epsilon isoform of protein kinase C, and a putative phosphorylation site was identified in the murine TASK-1 channel. In this thesis, I have identified a homologous phosphorylation site in the human TASK-1 channel, T383, and have demonstrated that this site is targeted in native atrial myocardium. The known pro-inflammatory effects of PAF, as well as the action potential abnormalities observed in previous studies after direct TASK-1 antagonism in isolated mouse ventricular myocytes, suggested that inhibitory phosphorylation at T383 could play a role in arrhythmias, particularly those with an inflammatory component to their etiology. Therefore, an aim of this thesis was to investigate the relationship between inhibitory TASK-1 phosphorylation and peri-operative atrial fibrillation (POAF) in a canine model. POAF is an acute onset of atrial fibrillation, with a known inflammatory component. This thesis describes the association of POAF with increased infiltration of neutrophils into atrial myocardium and the phosphorylation-dependent inhibition of TASK-1. Furthermore, this thesis has demonstrated that phosphorylation-dependent inhibition of TASK-1 is also associated with sustained atrial fibrillation (AF), in a canine model of chronic AF and in human patients. However, in chronic AF models, phosphorylation at T383 is not responsible for inhibition of the channel, and TASK-1 inhibition is caused by phosphorylation of another as yet unidentified site. Since TASK-1 inhibition is associated with AF in both animal models and in human patients, and in both the acute and chronic disease, TASK-1 could be a new drug target for improved pharmacotherapy in the prevention and treatment of atrial fibrillation

Increased late sodium current contributes to the electrophysiological effects of chronic, but not acute, dofetilide administration

Background - Drugs are screened for delayed rectifier potassium current (I Kr) blockade to predict long QT syndrome prolongation and arrhythmogenesis. However, single-cell studies have shown that chronic (hours) exposure to some I Kr blockers (eg, dofetilide) prolongs repolarization additionally by increasing late sodium current (I Na-L) via inhibition of phosphoinositide 3-kinase. We hypothesized that chronic dofetilide administration to intact dogs prolongs repolarization by blocking I Kr and increasing I Na-L. Methods and Results - We continuously infused dofetilide (6-9 μg/kg bolus+6-9 μg/kg per hour IV infusion) into anesthetized dogs for 7 hours, maintaining plasma levels within the therapeutic range. In separate experiments, myocardial biopsies were taken before and during 6-hour intravenous dofetide infusion, and the level of phospho-Akt was determined. Acute and chronic dofetilide effects on action potential duration (APD) were studied in canine left ventricular subendocardial slabs using microelectrode techniques. Dofetilide monotonically increased QTc and APD throughout 6.5-hour exposure. Dofetilide infusion during ≥210 minutes inhibited Akt phosphorylation. I Na-L block with lidocaine shortened QTc and APD more at 6.5 hours than at 50 minutes (QTc) or 30 minutes (APD) dofetilide administration. In comparison, moxifloxacin, an I Kr blocker with no effects on phosphoinositide 3-kinase and I Na-L prolonged APD acutely but no additional prolongation occurred on chronic superfusion. Lidocaine shortened APD equally during acute and chronic moxifloxacin superfusion. Conclusions - Increased I Na-L contributes to chronic dofetilide effects in vivo. These data emphasize the need to include time and I Na-L in evaluating the phosphoinositide 3-kinase inhibition-derived proarrhythmic potential of drugs and provide a mechanism for benefit from lidocaine administration in clinical acquired long QT syndrome