Nitric oxide‐cGMP pathway facilitates acetylcholine release and bradycardia during vagal nerve stimulation in the guinea‐pig in vitro

Abstract

We tested the hypothesis that nitric oxide (NO) augments vagal neurotransmission and bradycardia via phosphorylation of presynaptic calcium channels to increase vesicular release of acetylcholine. The effects of enzyme inhibitors and calcium channel blockers on the actions of the NO donor sodium nitroprusside (SNP) were evaluated in isolated guinea‐pig atrial‐right vagal nerve preparations. SNP (10 μm) augmented the heart rate response to vagal nerve stimulation but not to the acetylcholine analogue carbamylcholine (100 nm). SNP also increased the release of [3H]acetylcholine in response to field stimulation. No effect of SNP was observed on either the release of [3H] acetylcholine or the HR response to vagal nerve stimulation in the presence of the guanylyl cyclase inhibitor 1H‐(1,2,4)‐oxadiazolo‐(4,3‐a)‐quinoxalin‐1‐one (ODQ, 10 μm). The phosphodiesterase 3 (PDE 3) inhibitor milrinone (1 μm) increased the release of [3H] acetylcholine and the vagal bradycardia and prevented any further increase by SNP. SNP was still able to augment the vagal bradycardia in the presence of the protein kinase G inhibitor KT5823 (1 μm) but not after protein kinase A (PKA) inhibition with H‐89 (0.5 μm) or KT5720 (1 μm) had reduced the HR response to vagal nerve stimulation. Neither milrinone nor H‐89 changed the HR response to carbamylcholine. SNP had no effect on the magnitude of the vagal bradycardia after inhibition of N‐type calcium channels with ω‐conotoxin GVIA (100 nm). These results suggests that NO acts presynaptically to facilitate vagal neurotransmission via a cGMP‐PDE 3‐dependent pathway leading to an increase in cAMP‐PKA‐dependent phosphorylation of presynaptic N‐type calcium channels. This pathway may augment the HR response to vagal nerve stimulation by increasing presynaptic calcium influx and vesicular release of acetylcholine.

cGMP/Protein Kinase G-Dependent Inhibition of N-Type Ca2+Channels Induced by Nitric Oxide in Human Neuroblastoma IMR32 Cells

“…Moreover, the SNP/cGMP/PKG pathway has been considered responsible for inhibition of glutamate release in rat hippocampal nerve terminals (Sequeira et al, 1999). There is also evidence suggesting that NO can facilitate neurotransmitter release in some experimental models (Prast and Philippu, 1992;Herring and Paterson, 2001). It should be recalled, however, that in addition to its action on HVA Ca 2ϩ channels, NO also enhances calcium release from intracellular stores (Willmott et al, 1995;Stoyanovsky et al, 1997), and the net result of these potentially contrasting effects may well depend on the experimental model used.…”

Rosuvastatin Decreases Caveolin-1 and Improves Nitric Oxide–Dependent Heart Rate and Blood Pressure Variability in Apolipoprotein E −/− Mice In Vivo

“…Again, the effect of NOS inhibition on SBP ( Figure 4A) and SBP variability in the VLF band ( Figure 4C) was significantly attenuated in apoE Ϫ/Ϫ mice, consistent with constitutive impairment of their NO buffering system. In addition, because endogenous NO has been implicated in the modulation of the parasympathetic input to the heart at the presynaptic 19 and postsynaptic 20 -22 levels, respectively, a defective cardiac NOS may also participate in the loss of HR variability in the HF band (as shown above) irrespective of the vasculature.…”

Gastroprotective effects of Stachys Lavandulifolia extract on experimental gastric ulcer

“…On the other hand, it was showed that NO acts presynaptically to facilitate vagal neurotransmission via a pathway that ultimately leads to increased phosphorylation of presynaptic L-type Ca +2 channels. This pathway causes increased presynaptic calcium influx and vesicular release of acetylcholine (Herring and Paterson, 2001). Also, NO generated in parasympathetic ganglia may play a modulator role in facilitating the release of acetylcholine and the subsequent tissue response (Herring et al, 2002).…”