Direct evidence of nitric oxide release from neuronal nitric oxide synthase activation in the left ventricle as a result of cervical vagus nerve stimulation

Information regarding vagal innervation in the cardiac ventricle is limited and the direct effect of vagal stimulation on ventricular myocardial function is controversial. We have recently provided indirect evidence that the anti-fibrillatory effect of vagus nerve stimulation on the ventricle is mediated by nitric oxide (NO). The aim of this study was to provide direct evidence for the release of nitric oxide in the cardiac ventricle during stimulation of the efferent parasympathetic fibres of the cervical vagus nerve. The isolated innervated rabbit heart was employed with the use of the NO fluorescent indicator 4,5-diaminofluorescein diacetate (DAF-2 DA) during stimulation of the cervical vagus nerves and acetylcholine perfusion in the absence and presence of the non-specific NO synthase inhibitor N-G-nito-L-arginine (L-NNA) and the neuronal NO synthase selective inhibitor 1-(2-trifluormethylphenyl) imidazole (TRIM). Using the novel fluorescence method in the beating heart, we have shown that NO-dependent fluorescence is increased by 0.92 +/- 0.26, 1.20 +/- 0.30 and 1.91 +/- 0.27% (during low, medium and high frequency, respectively) in the ventricle in a stimulation frequency-dependent manner during vagus nerve stimulation, with comparable increases seen during separate stimulation of the left and right cervical vagus nerves. Background fluorescence is reduced during perfusion with L-NNA and the increase in fluorescence during high frequency vagal stimulation is inhibited during perfusion with both L-NNA (1.97 +/- 0.35% increase before L-NNA, 0.00 +/- 0.02% during L-NNA) and TRIM (1.78 +/- 0.18% increase before TRIM, -0.11 +/- 0.08% during TRIM). Perfusion with 0.1 mu m acetylcholine increased NO fluorescence by 0.76 +/- 0.09% which was blocked by L-NNA (change of 0.00 +/- 0.03%) but not TRIM (increase of 0.82 +/- 0.21%). Activation of cardiac parasympathetic efferent nerve fibres by stimulation of the cervical vagus is associated with NO production and release in the ventricle of the rabbit, via the neuronal isoform of nitric oxide synthase.