Effects of cyclic GMP and analogues on neurogenic transmission in the rat tail artery

Suppression of cyclic GMP-dependent protein kinase is essential to the Wnt/cGMP/Ca2+ pathway

Novel downstream effectors sensing changes in intracellular concentrations of Ca2+ and cyclic GMP in response to activation of the Wnt/Frizzled-2 pathway were sought. Activation of Frizzled-2 suppressed protein kinase G activity while activating NF-AT-dependent transcription. Each of these responses was abolished by pertussis toxin and by knock-down of the expression of either Galphat2 or Galphao. Activation of NF-AT-dependent transcription in response to Wnt5a stimulation was suppressed by activation of protein kinase G and by buffering intracellular Ca2+. Elevation of intracellular cyclic GMP either by inhibition of cyclic GMP phosphodiesterase or by addition of 8-bromocyclic GMP was shown to activate protein kinase G, to block Ca2+ mobilization, as well as to markedly attenuate activation of NF-AT-dependent transcription in response to Wnt5a stimulation. Chemical inhibition of protein kinase G by Rp-8-pCPT-cGMP, conversely, was shown to provoke increased NF-AT gene transcription and Ca2+ mobilization in the absence of Wnt stimulation. Protein kinase G is shown to be a critical downstream effector of the noncanonical Wnt-Frizzled-2/cGMP/Ca2+ pathway.

Low molecular mass dinitrosyl nonheme-iron complexes up-regulate noradrenaline release in the rat tail artery

BACKGROUND

Dinitrosyl nonheme-iron complexes can appear in cells and tissues overproducing nitric oxide. It is believed that due to their chemical nature these species may be implicated in certain pathophysiological events. We studied the possible role of low molecular mass dinitrosyl iron complexes in the control of noradrenaline release in electrically stimulated rat tail artery.

RESULTS

A model complex, dinitrosyl-iron-thiosulfate (at 1-10 microM) produced a concentration-dependent enhancement of electrical field stimulated [3H]noradrenaline release (up to 2 fold). At the same time, dinitrosyl-iron-thiosulfate inhibited neurogenic vasoconstriction, consistent with its nitric oxide donor properties. A specific inhibitor of cyclic GMP dependent protein kinase, Rp-8pCPT-cGMPS, partially inhibited the effect of dinitrosyl-iron-thiosulfate on neurogenic vasoconstriction, but not on [3H]noradrenaline release. Another model complex, dinitrosyl-iron-cysteine (at 3 microM) elicited similar responses as dinitrosyl-iron-thiosulfate. Conventional NO and NO+ donors such as sodium nitroprusside, S-nitroso-L-cysteine or S-nitroso-glutathione (at 10 microM) had no effect on [3H]noradrenaline release, though they potently decreased electrically-induced vasoconstriction. The "false complex", iron(II)-thiosulfate showed no activity.

CONCLUSIONS

Low molecular mass iron dinitrosyl complexes can up-regulate the stimulation-evoked release of vascular [3H]noradrenaline, apparently independently of their NO donor properties. This finding may have important implications in inflammatory tissues.

Nitric oxide-sensitive guanylyl cyclase inhibits acetylcholine release and excitatory motor transmission in the guinea-pig ileum

This study examined the mechanism through which nitric oxide inhibits the release of acetylcholine and excitatory motor neurotransmission in the guinea-pig ileum. The selective inhibitor of nitric oxide-sensitive guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), concentration-dependently enhanced both basal release (-log EC50: 6.8) and electrically (10 Hz)-evoked release (-log EC50: 6.0) of [3H]acetylcholine from longitudinal muscle-myenteric plexus preparations preincubated with [3H]choline. The increase by ODQ of basal release appeared to be exocytotic since it was prevented by tetrodotoxin (300 nM) and absence of calcium from the superfusion medium. In addition, ODQ (1 microM) increased the electrically-evoked tachykininergic and cholinergic muscle contractions as measured in the presence of scopolamine (100 nM) or of the neurokinin-1 receptor antagonist CP 99994 (100 nM), respectively. The nitric oxide synthase inhibitor L-N(G)-nitro-arginine (100 microM) behaved similar to ODQ and increased cholinergic and tachykininergic motor neurotransmission. The nitric oxide-independent activator of soluble guanylyl cyclase, 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole, concentration-dependently inhibited the electrically evoked acetylcholine release (-log EC50: 6.0) and longitudinal muscle contractions (-log EC50: 5.7). ODQ (10 microM) antagonized the effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole. The results suggest that endogenous nitric oxide tonically activates soluble guanylyl cyclase in myenteric neurons which leads to inhibition of the release of the excitatory transmitters acetylcholine and substance P. ODQ prevents the effects of nitric oxide and thus facilitates cholinergic and tachykininergic motor neurotransmission in the guinea-pig ileum.

Localization of heme oxygenase-2 immunoreactivity to parasympathetic ganglia of human and guinea-pig airways

Carbon monoxide (CO), an activator of soluble guanylate cyclase and generated enzymatically by heme oxygenase-2 (HO-2), is thought to function as an intra- and intercellular neurotransmitter in the central and peripheral nervous system. In the present study, the distribution of HO-2 in airway nerves from both humans and guinea pigs was assessed. HO-2 was found in all neuronal perikarya of the intrinsic ganglia of guinea-pig airways and in all ganglion nerve cell bodies localized to the trachea and bronchi of humans. By contrast, nerve fibers innervating the smooth muscle, lamina propria, and epithelium of the airways in both species were devoid of HO-2 immunoreactivity. HO-1, the inducible isoform of heme oxygenase, was not found in airway nerves. The pattern of distribution of HO-2 observed suggests that CO might serve as a modulator of synaptic neurotransmission in the lung and airways rather than as a bona fide neurotransmitter in the smooth muscle, vasculature, or glands. Consistent with this hypothesis, 8-bromo-cyclic guanosine monophosphate (cGMP) (30 microM), a stable, pharmacologically active analog of cGMP, markedly inhibited vagally-mediated cholinergic contractions of the isolated guinea-pig trachea. In subsequent studies, however, neither inhibiting heme oxygenase with zinc protoporphyrin-IX (30 microM) nor inhibiting the soluble isoform of guanylate cyclase with ODQ (3 microM) had measurable effects on vagally-mediated cholinergic contractions of the trachea. These results indicate that CO could play a modulatory role in efferent (parasympathetic) synaptic neurotransmission in the airways, but under normal conditions may not be activated to an appreciable extent during periods of elevated vagal activity.

Involvement of cyclic nucleotide-dependent protein kinases in cyclic AMP-mediated vasorelaxation

1. The involvement of cyclic AMP-dependent protein kinase (PKA) and cyclic GMP-dependent protein kinase (PKC) in the effects of cyclic AMP-elevating agents on vascular smooth muscle relaxation, cyclic nucleotide dependent-protein kinase activities and ATP-induced calcium signalling ([Ca2+]i was studied in rat aorta. Cyclic AMP-elevating agents used were a beta-adrenoceptor agonist (isoprenaline), a phosphodiesterase 3 (PDE3) inhibitor (SK&F 94120) and a PDE4 inhibitor (rolipram). 2. In rat intact aorta, the relaxant effect induced by isoprenaline (0.01-0.03 microM) was decreased by a specific inhibitor of PKA, H-89, whereas a specific inhibitor of PKG, Rp-8-Br-cyclic GMPs, was without effect. NO significant difference in PKA and PKG activity ratios was detected in aortic rings when isoprenaline 10 microM was used. At the same concentration, isoprenaline did not modify ATP-induced changes in [Ca2+]i in smooth muscle cells. Neither H-89 nor Rp-8-Br-cyclic GMPs modified this response. These findings suggest that PKA is only involved in the relaxant effect induced by low concentrations of isoprenaline (0.01-0.3 microM), whereas for higher concentrations, other mechanisms independent of PKA and PKG were involved. 3. The relaxant effects induced by SK&F 94120 and rolipram were inhibited by Rp-8-Br-cyclic GMPS with no significant effect of H-89. Neither SK&F 94120, nor rolipram at 30 microM significantly modified the activity ratios of PKA and PKG. Rolipram inhibited the ATP-induced transient increase in [Ca2+]i. This decrease was abolished by Rp-8-Br-cyclic GMPS whereas H-89 had no significant effect. These results suggests that PKG is involved in the vascular effects induced by the inhibitors of PDE3 and PDE4. Moreover, since it was previously shown that PDE3 and PDE4 inhibitors only increased cyclic AMP levels with no change in cyclic GMP level, these data also suggest a cross-activation of PKG by cyclic AMP in rat aorta. 4. The combinations of 5 microM SK&F 94120 with rolipram markedly potentiated the relaxant effect of rolipram. This relaxation was decreased by H-89 and not significantly modified by Rp-8-Br-cyclic GMPS. Moreover, the association of the two PDE inhibitors significantly increased the activity ratio of PKA without changing the PKG ratio. The present findings show that PKA rather than PKG is involved in this type of vasorelaxation. The differences in the participation of PKA vs PKG observed when inhibitors of PDE3 and PDE4 were used alone or together could be due to differences in the degree of accumulation of cyclic AMP, resulting in the activation of PKA or PKG which are differently localized in the cell. 5. These findings support for both PKA and PKG in cyclic AMP-mediated relaxation in raT aorta. Their involvement depends on the cellular pathway used to increase the cyclic AMP level.

Inhibition of cyclic GMP-dependent protein kinase-mediated effects by (Rp)-8-bromo-PET-cyclic GMPS

1. The modulation of the guanosine 3':5'-cyclic monophosphate (cyclic GMP)- and adenosine 3':5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase activities by the diastereomers of 8-bromo-beta phenyl-1, N2-ethenoguanosine 3':5'-cyclic monophosphorothioate, ((Rp)- and (Sp)-8-bromo-PET-cyclic GMPS) was investigated by use of purified protein kinases. In addition, the effects of (Rp)-8-bromo-PET-cyclic GMPS on protein phosphorylation in intact human platelets and on [3H]-noradrenaline release and neurogenic vasoconstriction in electrical field stimulated rat tail arteries were also studied. 2. Kinetic analysis with purified cyclic GMP-dependent protein kinase (PKG) type I alpha and I beta, which are expressed in the rat tail artery, revealed that (Rp)-8-bromo-PET-cyclic GMPS is a competitive inhibitor with an apparent Ki of 0.03 microM. The activation of purified cyclic AMP-dependent protein kinase (PKA) type II was antagonized with an apparent Ki of 10 microM. 3. In human platelets, (Rp)-8-bromo-PET-cyclic GMPS (0.1 mM) antagonized the activation of the PKG by the selective activator 8-(4-chlorophenylthio)-guanosine 3':5'-cyclic monophosphate (8-pCPT-cyclic GMP; 0.2 mM) without affecting the activation of PKA by (Sp)-5, 6-dichloro-1-beta-D-ribofurano-sylbenzimidazole- 3':5'-cyclic monophosphorothioate ((Sp)-5,6-DCl-cyclic BiMPS; 0.1 mM). 4. (Rp)-8-bromo-PET-cyclic GMPS was not hydrolysed by the cyclic GMP specific phosphodiesterase (PDE) type V from bovine aorta but potently inhibited this PDE. 5. The corresponding sulphur free cyclic nucleotide of the two studied phosphorothioate derivatives, 8-bromo-beta-phenyl-1, N2-ethenoguanosine-3':5'-cyclic monophosphate (8-bromo-PET-cyclic GMP), had no effect on electrically-induced [3H]-noradrenaline release but concentration-dependently decreased the stimulation-induced vasoconstriction. (Rp)-8-bromo-PET-cyclic GMPS (3 microM) shifted the vasoconstriction response to the right without affecting stimulation evoked tritium overflow. 6. The NO donor, 3-morpholinosydnonimine (SIN-1) relaxed rat tail arteries precontracted with phenylephrine (1 microM). The SIN-1 concentration-relaxation curve was shifted in a parallel manner to the right by (Rp)-8-bromo-PET-cyclic GMPS, suggesting that the relaxation was mediated by a cyclic GMP/PKG-dependent mechanism. 7. The [3H]-noradrenaline release-enhancing effect and stimulation-induced decrease in vasoconstriction of forskolin were unaffected by (Rp)-8-bromo-PET-cyclic GMPS. Moreover, the forskolin concentration-relaxation curve was not changed in the presence of the PKG inhibitor, suggesting a high selectivity in intact cells for PKG- over PKA-mediated effects. 8. The results obtained indicate that (Rp)-8-bromo-PET-cyclic GMPS presently is the most potent and selective inhibitor of PKG and is helpful in distinguishing between cyclic GMP and cyclic AMP messenger pathways activation. Therefore, this phosphorothioate stereomer may be a useful tool for studying the role of cyclic GMP in vitro.