Abstract 1413: Vagus Nerve Stimulation Releases Nitric Oxide in the Cardiac Ventricle: Direct Evidence from Fluorescence Studies in the Innervated Isolated Heart
We have previously shown that vagus nerve stimulation (VNS) decreases the susceptibility of the heart to ventricular fibrillation and that this protective effect is blocked by inhibiting Nitric Oxide (NO) production suggesting that NO is involved. Direct evidence that VNS releases NO is lacking.
Methods: We have developed a new technique to measure NO using fluorescence dye, 4–5 Diaminofluorescein (DAF-2) in a novel innervated isolated heart preparation. Adult rabbits (2–2.5kg, n=5) were sacrificed and vagus nerves isolated at the cervical region with the isolated heart preparation perfused. Epicardial fluorescence (F) was measured with excitation at 480±10, 490±10 and 500±10 nm and emission collected at 535nm (50nm bandpass) from the left ventricular surface using a bifurcated light guide monochromator system. DAF-2 was loaded into hearts in its diacetate form (DAF-2DA) with a bolus injection (1 μmol) into the perfusate. Fluorescence was measured and the effects of right VNS (7.4±1.1V) at Low (9Hz), Medium (15Hz) and High (20Hz) frequency were observed before and during infusion with NO synthase inhibitor, N-nitro-L-arginine (LNA, 200μM).
Results (mean=SEM, Table⇓): VNS decreased heart rate from 166.1±13.6 to 97.6±1.2 (L), 76.6±3.9 (M) and 57.2±2.6 (H) bpm. Changes in F490 were most prominent. During constant ventricular pacing at 200 bpm, VNS caused a significant increase in F490 of 1.4±0.9 (L), 1.7±0.5 (M) and 2.9±0.5 (H) % (p<0.04). There was no significant increase in F490 in the presence of LNA with any frequency of VNS.
Conclusions: These are the first data to demonstrate that vagus nerve stimulation causes a frequency dependent increase in NO measured in the beating heart. Further work will allow the study of the types of NO synthase isoforms involved in the release of NO from vagus nerve stimulation.