Activation of alpha2-adrenoceptors is necessary to induce nitric oxide release in isoprenaline-induced relaxation

Nitric oxide production contributes to Bacillus anthracis edema toxin-associated arterial hypotension and lethality: ex vivo and in vivo studies in the rat

We showed previously that Bacillus anthracis edema toxin (ET), comprised of protective antigen (PA) and edema factor (EF), inhibits phenylephrine (PE)-induced contraction in rat aortic rings and these effects are diminished in endothelial-denuded rings. Therefore, employing rat aortic ring and in vivo models, we tested the hypothesis that nitric oxide (NO) contributes to ET's arterial effects. Compared with rings challenged with PA alone, ET (PA + EF) reduced PE-stimulated maximal contractile force (MCF) and increased the PE concentration producing 50% MCF (EC50) (P < 0.0001). Compared with placebo, l-nitro-arginine methyl-ester (l-NAME), an NO synthase (NOS) inhibitor, reduced ET's effects on MCF and EC50 in patterns that approached or were significant (P = 0.06 and 0.03, respectively). In animals challenged with 24-h ET infusions, l-NAME (0.5 or 1.0 mg·kg(-1)·h(-1)) coadministration increased survival to 17 of 28 animals (60.7%) compared with 4 of 27 (14.8%) given placebo (P = 0.01). Animals receiving l-NAME but no ET all survived. Compared with PBS challenge, ET increased NO levels at 24 h and l-NAME decreased these increases (P < 0.0001). ET infusion decreased mean arterial blood pressure (MAP) in placebo and l-NAME-treated animals (P < 0.0001) but l-NAME reduced decreases in MAP with ET from 9 to 24 h (P = 0.03 for the time interaction). S-methyl-l-thiocitrulline, a selective neuronal NOS inhibitor, had effects in rings and, at a high dose in vivo models, comparable to l-NAME, whereas N'-[3-(aminomethyl)benzyl]-acetimidamide, a selective inducible NOS inhibitor, did not. NO production contributes to ET's arterial relaxant, hypotensive, and lethal effects in the rat.

Contractile function of smooth muscle retained after overnight storage

The functional responses of different overnight-stored in vitro tissues are not clearly described in any animal model. The influence of overnight storage in an animal model may vary between tissue types. We employed Sprague-Dawley rat as our animal model and investigated the functional changes of rat aorta, trachea, bronchus and bladder that were used (i) immediately after surgical removal (denoted as fresh) and (ii) after storage in aerated (95% O2, 5% CO2) Krebs-Ringer bicarbonate solution at 4 °C for 24 h (denoted as stored). The aorta ring was pre-contracted with phenylephrine, and the functional response of the tissue was investigated using isoprenaline, forskolin and carbachol. Carbachol was also used to increase the tone in trachea, bronchus rings and bladder strips. A clear reduced function of endothelium, with a minor if any effect in the smooth muscle function in rat aorta was observed after overnight storage. The contractile response of overnight-stored rat airway (trachea and bronchus) and bladder smooth muscles remained unchanged. Among all tested tissues, only bronchus showed a reduced response rate (only 40% responded) after storage. In vitro rat tissues that are stored in Krebs solution at 4 °C for 24 h can still be used to investigate smooth muscle responses, however, not endothelium-mediated responses for aorta. The influence of overnight storage on different tissues from an animal model (Sprague-Dawley rat in our study) also provides an insight in maximising the use of sacrificed animals.

Chronic stress improves NO- and Ca2+ flux-dependent vascular function: a pharmacological study

Background

Stress is associated with cardiovascular diseases.

Objective

This study aimed at assessing whether chronic stress induces vascular alterations, and whether these modulations are nitric oxide (NO) and Ca2+ dependent.

Methods

Wistar rats, 30 days of age, were separated into 2 groups: control (C) and Stress (St). Chronic stress consisted of immobilization for 1 hour/day, 5 days/week, 15 weeks. Systolic blood pressure was assessed. Vascular studies on aortic rings were performed. Concentration-effect curves were built for noradrenaline, in the presence of L-NAME or prazosin, acetylcholine, sodium nitroprusside and KCl. In addition, Ca²⁺ flux was also evaluated.

Results

Chronic stress induced hypertension, decreased the vascular response to KCl and to noradrenaline, and increased the vascular response to acetylcholine. L-NAME blunted the difference observed in noradrenaline curves. Furthermore, contractile response to Ca²⁺ was decreased in the aorta of stressed rats.

Conclusion

Our data suggest that the vascular response to chronic stress is an adaptation to its deleterious effects, such as hypertension. In addition, this adaptation is NO- and Ca²⁺-dependent. These data help to clarify the contribution of stress to cardiovascular abnormalities. However, further studies are necessary to better elucidate the mechanisms involved in the cardiovascular dysfunction associated with stressors. (Arq Bras Cardiol. 2014; [online].ahead print, PP.0-0)

B. anthracis edema toxin increases cAMP levels and inhibits phenylephrine-stimulated contraction in a rat aortic ring model

B. anthracis edema toxin (ET) and lethal toxin (LT) are each composed of protective antigen (PA), necessary for toxin uptake by host cells, and their respective toxic moieties, edema factor (EF) and lethal factor (LF). Although both toxins likely contribute to shock during infection, their mechanisms are unclear. To test whether ET and LT produce arterial relaxation, their effects on phenylephrine (PE)-stimulated contraction in a Sprague-Dawley rat aortic ring model were measured. Rings were prepared and connected to pressure transducers. Their viability was confirmed, and peak contraction with 60 mM KCl was determined. Compared with PA pretreatment (control, 60 min), ET pretreatment at concentrations similar to those noted in vivo decreased the mean (±SE) maximum contractile force (MCF; percent peak contraction) in rings generated during stimulation with increasing PE concentrations (96.2 ± 7.0 vs. 57.3 ± 9.1) and increased the estimated PE concentration producing half the MCF (EC50; 10(-7) M, 1.1 ± 0.3 vs. 3.7 ± 0.8, P ≤ 0.002). ET inhibition with PA-directed monoclonal antibodies, selective EF inhibition with adefovir, or removal of the ring endothelium inhibited the effects of ET on MCF and EC50 (P ≤ 0.02). Consistent with its adenyl cyclase activity, ET increased tissue cAMP in endothelium-intact but not endothelium-denuded rings (P < 0.0001 and 0.25, respectively). LT pretreatment, even in high concentrations, did not significantly decrease MCF or increase EC50 (all P > 0.05). In rings precontracted with PE compared with posttreatment with PA (90 min), ET posttreatment produced progressive reductions in contractile force and increases in relaxation in endothelium-intact rings (P < 0.0001) but not endothelium-denuded rings (P = 0.51). Thus, ET may contribute to shock by producing arterial relaxation.

Increased vascular contractility and oxidative stress in β₂-adrenoceptor knockout mice: the role of NADPH oxidase

BACKGROUND/AIMS

β(2)-adrenoceptor (β(2)-AR) activation induces smooth muscle relaxation and endothelium-derived nitric oxide (NO) release. However, whether endogenous basal β(2)-AR activity controls vascular redox status and NO bioavailability is unclear. Thus, we aimed to evaluate vascular reactivity in mice lacking functional β(2)-AR (β(2)KO), focusing on the role of NO and superoxide anion.

METHODS AND RESULTS

Isolated thoracic aortas from β(2)KO and wild-type mice (WT) were studied. β(2)KO aortas exhibited an enhanced contractile response to phenylephrine compared to WT. Endothelial removal and L-NAME incubation increased phenylephrine-induced contraction, abolishing the differences between β(2)KO and WT mice. Basal NO availability was reduced in aortas from β(2)KO mice. Incubation of β(2)KO aortas with superoxide dismutase or NADPH inhibitor apocynin restored the enhanced contractile response to phenylephrine to WT levels. β(2)KO aortas exhibited oxidative stress detected by enhanced dihydroethidium fluorescence, which was normalized by apocynin. Protein expression of eNOS was reduced, while p47(phox) expression was enhanced in β(2)KO aortas.

CONCLUSIONS

The present results demonstrate for the first time that enhanced NADPH-derived superoxide anion production is associated with reduced NO bioavailability in aortas of β(2)KO mice. This study extends the knowledge of the relevance of the endogenous activity of β(2)-AR to the maintenance of the vascular physiology.

Inhibition of nitric oxide-guanylate cyclase-dependent and -independent signaling contributes to impairment of beta-adrenergic vasorelaxations by cyclosporine

This study investigated the role of endothelium- and smooth muscle-dependent mechanisms in the interaction of cyclosporine (CyA), an immunosuppressant drug, with beta-adrenoceptor (isoprenaline)-mediated relaxations in isolated rat aortas precontracted with phenylephrine. CyA effects were assessed in the absence and presence of NG-nitro-L-arginine methyl ester (L-NAME, nitric oxide synthase inhibitor), methylene blue (guanylate cyclase inhibitor), or propranolol (beta-adrenoceptor antagonist). In aortas with intact endothelium (E+), pretreatment with L-NAME or methylene blue significantly reduced isoprenaline (1 x 10(-9) to 1 x 10(-7)M) relaxations in contrast to no effect for tetraethylammonium (K+ channel blocker), or diclophenac (cyclooxygenase inhibitor), suggesting a major role for the nitric oxide-guanylate cyclase (NO-GC) pathway, but not endothelial hyperpolarizing factor or vasodilator prostanoids, in isoprenaline responses. Isoprenaline relaxations were still evident, though significantly attenuated, in endothelium-denuded aortas (E-) and were resistant to L-NAME or methylene blue. Acute exposure to CyA (2 microM) caused propranolol-sensitive reductions in isoprenaline responses in E+ and E- aortas. The CyA-induced attenuation of isoprenaline responses in E+ aortas largely disappeared in L-NAME-treated aortas and after supplementation with L-arginine, the substrate of nitric oxide. CyA also reduced the endothelium-independent, GC-dependent aortic relaxations evoked by sodium nitroprusside, an effect that was virtually abolished by methylene blue. We conclude that: (i) endothelial and smooth muscle mechanisms contribute to aortic beta-adrenoceptor relaxations and both components are negatively influenced by CyA, and (ii) NO-GC signaling plays an integral role in the vascular CyA-beta-adrenoceptor interaction. The clinical relevance of the present study is warranted given the established role of impaired vascular function in CyA toxicity.

β2-Adrenoceptor Genotype and Function Affect Hemodynamic Profile Heterogeneity in Postural Tachycardia Syndrome

Previous studies suggest that the β2-adrenoceptor functions abnormally in patients with postural tachycardia syndrome (POTS) and may contribute to their altered hemodynamic profile. To test the hypothesis that the β2-adrenoceptor response is decreased in POTS, we studied: (1) the arterial vasodilation response to the β agonist, isoproterenol, and (2) the distribution of common polymorphisms (codons 16 and 27) of the gene coding the receptor (β 2 -AR) in a large population with POTS. We measured plasma catecholamines and monitored hemodynamics and changes in forearm and leg blood flow to incremental doses of intraarterial isoproterenol in 9 patients with POTS compared with 8 healthy subjects. For polymorphism assessment we collected DNA from 57 patients with POTS and compared with 67 age-sex matched healthy subjects. Circulating catecholamines were significantly higher in POTS subjects compared with controls. Intrabrachial and intrafemoral isoproterenol infusion elicited a dose-dependent increase in blood flow. In healthy subjects, blood flow increased (mean±SEM) 400±70% in the forearm and 170±40% in the leg, but only 280±60% in forearms and 120±20% in legs of patients with POTS (ANOVA for both P<0.001). The genotype and allele distributions for codons 16 and 27 β 2 -AR variants were not different in the 2 groups. However, the blood pressure and plasma norepinephrine levels diverged in patients according to their genotype. Patients with Gly16Gly and patients with Glu27Glu had lower plasma catecholamines and higher supine and upright blood pressure, compared with other genotypes. Therefore, both decreased β2-adrenoceptor-related vasodilation and β 2 -AR polymorphisms may contribute to the hemodynamic diversity of patients with POTS.

Extracellular ATP stimulates NO production in rat thick ascending limb

NO produced by NO synthase (NOS) 3 acts as an autacoid to regulate NaCl absorption in the thick ascending limb. ATP induces NO production by NOS 3 in endothelial cells. We hypothesized that extracellular ATP activates NOS in thick ascending limbs through P2 receptors. To test this, we measured intracellular NO production using the NO-selective fluorescent dye DAF-2 in suspensions of rat medullary thick ascending limbs. We found that ATP increased DAF-2 fluorescence in a concentration-dependent manner, reaching saturation at &200 micromol/L with an EC50 of 37 micromol/L. The increase was blunted by 74% by the nonselective NOS inhibitor L-omega-nitro-arginine-methyl-ester (2 mmol/L; 60+/-7 versus 16+/-6 arbitrary fluorescence units; P<0.02; n=5). In the presence of the P2 receptor antagonist suramin (300 micromol/L), ATP-induced NO production was reduced by 64% (101+/-11 versus 37+/-5 arbitrary fluorescence units; P<0.002; n=5). Blocking ATP hydrolysis with a 5'-ectonucleotidase inhibitor, ARL67156 (30 micromol/L) enhanced the response to ATP and shifted the EC(50) to 0.8 micromol/L. In the presence of ARL67156, the EC50 of the P2X-selective agonist beta,gamma-methylene-adenosine 5'-triphosphate was 4.8 micromol/L and the EC50 for the P2Y-selective agonist UTP was 40.4 micromol/L. The maximal responses for both agonists were similar. Taken together, these data indicate that ATP stimulates NO production in the thick ascending limb primarily through P2X receptor activation and that ATP hydrolysis may regulate NO production.