Abstract
Key points Animal studies suggest an anti‐fibrillatory action of the vagus nerve on the ventricle, although the exact mechanism is controversial.Using a Langendorff perfused rat heart, we show that the acetylcholine analogue carbamylcholine raises ventricular fibrillation threshold (VFT) and flattens the electrical restitution curve.The anti‐fibrillatory action of carbamylcholine was prevented by the nicotinic receptor antagonist mecamylamine, inhibitors of neuronal nitric oxide synthase (nNOS) and soluble guanylyl cyclase (sGC), and can be mimicked by the nitric oxide (NO) donor sodium nitroprusside.Carbamylcholine increased NO metabolite content in the coronary effluent and this was prevented by mecamylamine.The anti‐fibrillatory action of both carbamylcholine and sodium nitroprusside was ultimately dependent on muscarinic receptor stimulation as all effects were blocked by atropine.These data demonstrate a protective effect of carbamylcholine on VFT that depends upon both muscarinic and nicotinic receptor stimulation, where the generation of NO is likely to be via a neuronal nNOS–sGC dependent pathway. Implantable cardiac vagal nerve stimulators are a promising treatment for ventricular arrhythmia in patients with heart failure. Animal studies suggest the anti‐fibrillatory effect may be nitric oxide (NO) dependent, although the exact site of action is controversial. We investigated whether a stable analogue of acetylcholine could raise ventricular fibrillation threshold (VFT), and whether this was dependent on NO generation and/or muscarinic/nicotinic receptor stimulation. VFT was determined in Langendorff perfused rat hearts by burst pacing until sustained VF was induced. Carbamylcholine (CCh, 200 nmol l–1, n = 9) significantly (P < 0.05) reduced heart rate from 292 ± 8 to 224 ± 6 b.p.m. Independent of this heart rate change, CCh caused a significant increase in VFT (control 1.5 ± 0.3 mA, CCh 2.4 ± 0.4 mA, wash 1.1 ± 0.2 mA) and flattened the restitution curve (n = 6) derived from optically mapped action potentials. The effect of CCh on VFT was abolished by a muscarinic (atropine, 0.1 μmol l−1, n = 6) or a nicotinic receptor antagonist (mecamylamine, 10 μmol l−1, n = 6). CCh significantly increased NOx content in coronary effluent (n = 8), but not in the presence of mecamylamine (n = 8). The neuronal nitric oxide synthase inhibitor AAAN (N‐(4S)‐4‐amino‐5‐[aminoethyl]aminopentyl‐N′‐nitroguanidine; 10 μmol l−1, n = 6) or soluble guanylate cyclase (sGC) inhibitor ODQ (1H‐[1,2,4]oxadiazolo[4,3‐a]quinoxalin‐1‐one; 10 μmol l−1, n = 6) prevented the rise in VFT with CCh. The NO donor sodium nitrprusside (10 μmol l–1, n = 8) mimicked the action of CCh on VFT, an effect that was also blocked by atropine (n = 10). These data demonstrate a protective effect of CCh on VFT that depends upon both muscarinic and nicotinic receptor stimulation, where the generation of NO is likely to be via a neuronal nNOS/sGC‐dependent pathway.
Highlights
The anti-arrhythmic action of the vagus on the ventricle is well established (Einbrodt, 1859; Kent et al 1973; Kolman et al 1975; De Ferrari et al 1991; Nash et al 2001)
A series of time control experiments demonstrated that ventricular fibrillation threshold (VFT) remained constant over successive inductions (VFT1 2.21 ± 0.39 vs. VFT2 2.21 ± 0.40 vs. VFT3 2.35 ± 0.45 mA, n = 7) with no deterioration in Langendorff physiology or epicardial scarring (n = 7)
The anti-fibrillatory action of CCh was dependent upon stimulation of nicotinic receptors, and the generation of nitric oxide (NO) from neuronal nitric oxide synthase (nNOS)
Summary
The anti-arrhythmic action of the vagus on the ventricle is well established (Einbrodt, 1859; Kent et al 1973; Kolman et al 1975; De Ferrari et al 1991; Nash et al 2001). There are a range of mechanisms by which stimulation of the vagus may prevent the initiation of ventricular ectopics, re-entry and subsequent wavebreak that leads to VF These include reducing myocyte calcium load (Levy & Zieske, 1969; Levy & Blattberg, 1976), prolonging action potential duration (APD) and reducing dynamic APD shortening (electrical restitution) (Ng et al 2001), prolonging refractory period (Martins & Zipes, 1980a,b, Ito & Zipes, 1994), increasing conduction velocity (Ando et al 2005; Sabbah, 2011), and reducing the spatial dispersion of these variables (Levy & Zieske, 1969; Martin et al 1969). Chronic vagal stimulation improves conduction velocity by modifying gap junction expression (Ando et al 2005)
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