2 * Beta-blockers do not prevent the pro-arrhythmic action of high-level sympathetic stimulation: a role for neuropeptide Y?

Abstract

Introduction: 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. NPY is a potent vasoconstrictor that can also alter cardiac electrophysiology. 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 receptor. Methods: Hearts with intact stellate ganglia (SG) and sympathetic innervation were isolated from male Sprague Dawley rats (250-300g) 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). A control experiment 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 (RH237, 15 µL of 5mg/mL) in the presence of blebbistatin (10 µmol/L). All data are presented as mean±standard error. Results: Direct SG stimulation significantly (p<0.05) increased heart rate and left ventricular developed pressure (LVDP) and these changes were completely blocked by metoprolol (10 µmol/L). However, 2 mins of right SG stimulation in the presence of metoprolol still significantly reduced VFT (2.2±0.0.49 vs. 1.2±.0.26mA, n=6). VFT was also decreased by norepinephrine (1 µmol/L), however this effect was abolished by metoprolol (n=4). NPY (250nmol/L, n=6) mimicked the effect of SG stimulation and significantly reduced VFT by 50% (2.4±0.15 vs 1.2±0.12mA) and increased coronary vascular resistance and LVDP. The action of NPY on VFT, CVR and LVDP were abolished by the Y1 receptor antagonist BIBO3304 (1 µM, 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). The Y1 receptor antagonist BIBO3304 (1 µM, n=6) also prevented the reduction in VFT in response to SG stimulation in the presence of metoprolol. Conclusion: Prolonged sympathetic stimulation remains pro-arrhythmic in the presence of BB, implicating sympathetic co-transmitters as a novel arrhythmic trigger. NPY can directly decrease VFT via a Y1 receptor mediated mechanism that steepens the RT curve. A combination of BB and Y1 receptor blockade is able to fully prevent the pro-arrhythmic action of prolonged sympathetic stimulation.