Purpose: Beta-blockers (BB) are the only anti-arrhythmic drugs that improve mortality in patients with ischaemic heart disease, but a significant risk of arrhythmia remains. During high-level sympathetic drive, such as during myocardial infarction, co-transmitters including neuropeptide Y (NPY) can be released with norepinephrine. We hypothesised that ventricular fibrillation threshold (VFT) would still be reduced following high-level sympathetic stimulation even in the presence of a BB. We also investigated whether NPY is independently pro-arrhythmic and decreases VFT, and if this mechanism is mediated by the Y1 or Y2 receptor.Methods: Hearts with intact stellate ganglia (SG) and sympathetic innervation were isolated from male Sprague Dawley rats (300-350g) and Langendorff perfused in constant flow mode. VFT was determined by pacing at a fixed cycle length (150ms x 20 beats) followed by a 5sec 50Hz burst at increasing current amplitude (mA) until sustained VF was induced. VF was cardioverted with 1ml of potassium chloride solution (50mmol/L). Control experiments demonstrated that VFT remained constant over 3 successive inductions (n=6). SG were stimulated at 10Hz, 1msec pulse width (above capture voltage). Drugs were perfused for 15 minutes before stimulation. Electrical restitution (RT) was derived from optically mapped action potentials from hearts loaded with RH237 and blebbistatin (10μM). All data are presented as mean ± standard error.Results: Direct right SG stimulation significantly (p<0.05) increased heart rate and left ventricular developed pressure (LVDP) and these changes were completely blocked by metoprolol (10μmol/L). However, 2 mins of RSG stimulation in the presence of metoprolol still significantly reduced VFT (2.0±0.0.45 vs. 1.08±0.24mA, n=6). A similar result was observed following 2 mins stimulation of the left SG. Perfusion with the NPY Y1 receptor antagonist BIBO3304 (1μM, n=4) in addition to metoprolol prevented the reduction in VFT seen with RSG stimulation. Perfusion with exogenous NPY (250nmol/L, n=6) also significantly reduced VFT by 50% (2.4±0.15 vs 1.2±0.12mA) and also increased coronary vascular resistance (CVR) and LVDP. The action of NPY on VFT, CVR and LVDP were abolished by the Y1 receptor antagonist BIBO3304 (1μM, n=6), but not by Y2 receptor blockade with BIIE0246 (1μM, n=6). Preliminary analysis of RT by optical mapping is suggestive of a steepening of the curve with NPY 250nM (0.27±0.04 vs NPY 0.42±0.12, n=4).Conclusion: Prolonged sympathetic stimulation remains pro-arrhythmic in the presence of BB, implicating sympathetic co-transmitters as a novel arrhythmic trigger. Addition of a NPY Y1 receptor antagonist to the BB could prevent the pro-arrhythmic action of sympathetic stimulation. NPY can directly decrease VFT via a Y1 receptor mediated mechanism and steepens the RT curve.