Nitric oxide mediates the vagal protective effect on ventricular fibrillation via effects on action potential duration restitution in the rabbit heart

Abstract

We have previously shown that direct vagus nerve stimulation (VNS) reduces the slope of action potential duration (APD) restitution while simultaneously protecting the heart against induction of ventricular fibrillation (VF) in the absence of any sympathetic activity or tone. In the current study we have examined the role of nitric oxide (NO) in the effect of VNS. Monophasic action potentials were recorded from a left ventricular epicardial site on innervated, isolated rabbit hearts (n = 7). Standard restitution, effective refractory period (ERP) and VF threshold (VFT) were measured at baseline and during VNS in the presence of the NO synthase inhibitor N(G)-nitro-L-arginine (L-NA, 200 microm) and during reversing NO blockade with L-arginine (L-Arg, 1 mm). Data represent the mean +/- S.E.M. The restitution curve was shifted upwards and became less steep with VNS when compared to baseline. L-NA blocked the effect of VNS whereas L-Arg restored the effect of VNS. The maximum slope of restitution was reduced from 1.17 +/- 0.14 to 0.60 +/- 0.09 (50 +/- 5%, P < 0.0001) during control, from 0.98 +/- 0.14 to 0.93 +/- 0.12 (2 +/- 10%, P = NS) in the presence of L-NA and from 1.16 +/- 0.17 to 0.50 +/- 0.10 (41 +/- 9%, P = 0.003) with L-Arg plus L-NA. ERP was increased by VNS in control from 119 +/- 6 ms to 130 +/- 6 ms (10 +/- 5%, P = 0.045) and this increase was not affected by L-NA (120 +/- 4 to 133 +/- 4 ms, 11 +/- 3%, P = 0.0019) or L-Arg with L-NA (114 +/- 4 to 123 +/- 4 ms, 8 +/- 2%, P = 0.006). VFT was increased from 3.0 +/- 0.3 to 5.8 +/- 0.5 mA (98 +/- 12%, P = 0.0017) in control, 3.4 +/- 0.4 to 3.8 +/- 0.5 mA (13 +/- 12%, P = 0.6) during perfusion with L-NA and 2.5 +/- 0.4 to 6.0 +/- 0.7 mA (175 +/- 50%, P = 0.0017) during perfusion with L-Arg plus L-NA. Direct VNS increased VFT and flattened the slope of APD restitution curve in this isolated rabbit heart preparation with intact autonomic nerves. These effects were blocked using L-NA and reversed by replenishing the substrate for NO production with L-Arg. This is the first study to demonstrate that NO plays an important role in the anti-fibrillatory effect of VNS on the rabbit ventricle, possibly via effects on APD restitution.

Figure 1. Extrastimulus protocol to determine standard restitution

Left panel shows the last S₁ and S₂ monophasic action potentials (MAPs) during the standard restitution protocol (see text) at S₁–S₂ intervals of 140 ms (top trace) and 110 ms (bottom trace). Right panel shows the same traces superposed and aligned at S₁ to illustrate the difference in MAP durations with the different S₁–S₂ intervals.

Figure 2. Standard restitution curve and slope of restitution

A, plot of standard restitution curves (MAPD₉₀versus preceding diastolic interval (DI)) in a typical experiment at baseline and during vagus nerve stimulation (VNS) under control conditions and VNS during perfusion with N^G-nitro-l-arginine (l-NA) and l-NA together with l-arginine (l-NA +l-Arg). The curves were fitted to the exponential curve (MAPD₉₀ = MAPD_90max(1 − e^−DI/τ); see text for definitions) (continuous lines) and the maximum slopes (dashed lines) during restitution were calculated. B, plot of first derivative of the fitted curves in A to calculate the slope of the restitution curves. A horizontal dashed line is plotted at slope = 1.

Figure 3. Maximum slope of restitution

Plot of the maximum slope of the restitution curve at baseline and during vagus nerve stimulation (VNS) during control and during perfusion with N^G-nitro-l-arginine (l-NA) and l-NA together with l-arginine (l-NA +l-Arg) and during washout. *P < 0.001, NS P > 0.05 versus baseline (n = 7).

Figure 4. Ventricular effective refractory period

Plot of ventricular effective refractory period (ERP) at baseline and during vagus nerve stimulation (VNS) during control and during perfusion with N^G-nitro-l-arginine (l-NA) and l-NA together with l-arginine (l-NA +l-Arg) and during washout. *P < 0.05 versus baseline (n = 7).

Figure 5. Ventricular fibrillation threshold

Plot of ventricular fibrillation threshold (VFT) at baseline and during vagus nerve stimulation (VNS) during control and during perfusion with N^G-nitro-l-arginine (l-NA) and l-NA together with l-arginine (l-NA +l-Arg) and during washout. *P < 0.001, NS P > 0.05 versus baseline (n = 7).

Figure 6. Percentage change in electrophysiological parameters

Plot of percentage change in maximum slope of standard restitution, ventricular effective refractory period (ERP) and ventricular fibrillation threshold (VFT) during vagus nerve stimulation (VNS) during control and during perfusion with N^G-nitro-l-arginine (l-NA) and l-NA together with l-arginine (l-NA +l-Arg) and during washout (n = 7).