Vagus nerve stimulation protects against ventricular fibrillation independent of muscarinic receptor activation

Aims The role of the vagus in the ventricle is controversial, although the vagus can protect against ventricular fibrillation (VF) via nitric oxide (NO). This study aims to determine whether the mechanisms involved are dependent on postganglionic release and muscarinic receptor activation. For this purpose, NO release and electrophysiological effects of vagus nerve stimulation (VNS) were evaluated in relation to acetylcholine and vasoactive intestinal peptide (VIP). In addition, the role of the coronary endothelium and afferent nerves was tested. Methods and results Using the isolated innervated rabbit heart, we measured ventricular NO release using 4,5-diaminofluorescein (DAF-2) fluorescence and ventricular fibrillation threshold (VFT) during VNS after muscarinic, ganglionic, and VIP inhibition [atropine, hexamethonium, and VIP (6-28), respectively] and after Triton-X endothelial functional dysfunction. The vagal-mediated increases in NO and VFT were not significantly affected (P > 0.05) during (i) atropine perfusion [increase in NO: 196.8 +/- 35.2 mV (control) vs. 156.1 +/- 20.3 mV (atropine) and VFT 3.1 +/- 0.5 mA (control) vs. 2.7 +/- 0.4 mA (atropine)], (ii) VIP inhibition-increase in NO: 243.0 +/- 42.4 mV (control) vs. 203.9 +/- 28.5 mV [VIP(6-28)] and VFT 3.3 +/- 0.3 mA (control) vs. 3.9 +/- 0.6 mA [VIP(6-28)], or (iii) after endothelial functional dysfunction [increase in NO: 127.7 +/- 31.7 mV (control) vs. 172.1 +/- 31.5 mV (Triton-X) and VFT 2.6 +/- 0.4 mA (control) vs. 2.5 +/- 0.5 mA (Triton-X)]. However, the vagal effects were inhibited during ganglionic blockade [increase in NO: 175.1 +/- 38.1 mV (control) vs. 0.6 +/- 25.3 mV (hexamethonium) and VFT 3.3 +/- 0.5 mA (control) vs. 20.3 +/- 0.3 mA (hexamethonium)]. Conclusions We show that the vagal anti-fibrillatory action in the rabbit ventricle occurs via post-ganglionic efferent nerve fibres, independent of muscarinic receptor activation, VIP, and the endothelium. Together with our previous publications, our data support the possibility of a novel ventricular nitrergic parasympathetic innervation and highlight potential for new therapeutic targets to treat ventricular dysrhythmias.