Involvement of Potassium Channels and Calcium-Independent Mechanisms in Hydrogen Sulfide-Induced Relaxation of Rat Mesenteric Small Arteries.

Endogenous hydrogen sulfide (H2S) is involved in the regulation of vascular tone. We hypothesized that the lowering of calcium and opening of potassium (K) channels as well as calcium-independent mechanisms are involved in H2S-induced relaxation in rat mesenteric small arteries. Amperometric recordings revealed that free [H2S] after addition to closed tubes of sodium hydrosulfide (NaHS), Na2S, and GYY4137 [P-(4-methoxyphenyl)-P-4-morpholinyl-phosphinodithioic acid] were, respectively, 14%, 17%, and 1% of added amount. The compounds caused equipotent relaxations in isometric myographs, but based on the measured free [H2S], GYY4137 caused more relaxation in relation to released free H2S than NaHS and Na2S in rat mesenteric small arteries. Simultaneous measurements of [H2S] and tension showed that 15 µM of free H2S caused 61% relaxation in superior mesenteric arteries. Simultaneous measurements of smooth muscle calcium and tension revealed that NaHS lowered calcium and caused relaxation of NE-contracted arteries, while high extracellular potassium reduced NaHS relaxation without corresponding calcium changes. In NE-contracted arteries, NaHS (1 mM) lowered the phosphorylation of myosin light chain, while phosphorylation of myosin phosphatase target subunit 1 remained unchanged. Protein kinase A and G, inhibitors of guanylate cyclase, failed to reduce NaHS relaxation, whereas blockers of voltage-gated KV7 channels inhibited NaHS relaxation, and blockers of mitochondrial complex I and III abolished NaHS relaxation. Our findings suggest that low micromolar concentrations of free H2S open K channels followed by lowering of smooth muscle calcium, and by another mechanism involving mitochondrial complex I and III leads to uncoupling of force, and hence vasodilation.

Inhibition of KV7 Channels Protects the Rat Heart against Myocardial Ischemia and Reperfusion Injury.

The voltage-gated KV7 (KCNQ) potassium channels are activated by ischemia and involved in hypoxic vasodilatation. We investigated the effect of KV7 channel modulation on cardiac ischemia and reperfusion injury and its interaction with cardioprotection by ischemic preconditioning (IPC). Reverse-transcription polymerase chain reaction revealed expression of KV7.1, KV7.4, and KV7.5 in the left anterior descending rat coronary artery and all KV7 subtypes (KV7.1-KV7.5) in the left and right ventricles of the heart. Isolated hearts were subjected to no-flow global ischemia and reperfusion with and without IPC. Infarct size was quantified by 2,3,5-triphenyltetrazolium chloride staining. Two blockers of KV7 channels, XE991 [10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone] (10 µM) and linopirdine (10 µM), reduced infarct size and exerted additive infarct reduction to IPC. An opener of KV7 channels, flupirtine (10 µM) abolished infarct size reduction by IPC. Hemodynamics were measured using a catheter inserted in the left ventricle and postischemic left ventricular recovery improved in accordance with reduction of infarct size and deteriorated with increased infarct size. XE991 (10 µM) reduced coronary flow in the reperfusion phase and inhibited vasodilatation in isolated small branches of the left anterior descending coronary artery during both simulated ischemia and reoxygenation. KV7 channels are expressed in rat coronary arteries and myocardium. Inhibition of KV7 channels exerts cardioprotection and opening of KV7 channels abrogates cardioprotection by IPC. Although safety issues should be further addressed, our findings suggest a potential role for KV7 blockers in the treatment of ischemia-reperfusion injury.

Involvement of hydrogen sulfide in perivascular and hypoxia-induced inhibition of endothelin contraction in porcine retinal arterioles.

Perivascular retina has been shown to regulate retinal vascular tone. In the present study, we evaluated an ex vivo retina preparation, and investigated whether hydrogen sulfide (H2S) mediates an inhibitory effect of retina and/or hypoxia on arteriolar tone. In retina, immunolabeling showed an increase of glial fibrillary acidic protein, but not vimentin over time in Müller cells, and the presence of necrotic cells after 2 h and apoptotic cells after 8 h. Isometric tension recordings showed endothelin-1(ET-1) to induce concentration-dependent contractions, which were reduced in the presence of retina. In arterioles with retina no change was observed in ET-1 contractions after 5 h compared to 8 h. Hypoxia (1% O2) reduced ET-1 contraction in arterioles with and without retina. The H2S donor, GYY4137 and the salt, sodium hydrogen sulfide, induced concentration-dependent relaxations in ET-1 contracted retinal arterioles. Inhibition of the H2S producing enzymes, cystathionine ß-synthase (CBS) and cystathionine γ-lyase (CSE), with carboxymethoxylamine (AOA) and L-propargylglycine (PPG) enhanced ET-1 contractions. This effect was more pronounced in hypoxic conditions. However, even in the presence of AOA and PPG ET-1 induced less contraction in the presence of perivascular retina compared to isolated vessels. These findings suggest that both the presence of perivascular retina and hypoxia reduce arteriolar vasoconstriction and that both H2S and another factor mediate this effect. Finally, H2S donors, as well as endogenous H2S, can reduce retinal arteriolar tone, suggesting a potential therapeutic role for enhanced H2S bioavailability in the treatment of retinal disease.

Non-endothelial endothelin counteracts hypoxic vasodilation in porcine large coronary arteries.

BACKGROUND: The systemic vascular response to hypoxia is vasodilation. However, reports suggest that the potent vasoconstrictor endothelin-1 (ET-1) is released from the vasculature during hypoxia. ET-1 is reported to augment superoxide anion generation and may counteract nitric oxide (NO) vasodilation. Moreover, ET-1 was proposed to contribute to increased vascular resistance in heart failure by increasing the production of asymmetric dimethylarginine (ADMA). We investigated the role of ET-1, the NO pathway, the potassium channels and radical oxygen species in hypoxia-induced vasodilation of large coronary arteries. RESULTS: In prostaglandin F2α (PGF2α, 10 µM)-contracted segments with endothelium, gradual lowering of oxygen tension from 95 to 1% O2 resulted in vasodilation. The vasodilation to O2 lowering was rightward shifted in segments without endothelium at all O2 concentrations except at 1% O2. The endothelin receptor antagonist SB217242 (10 µM) markedly increased hypoxic dilation despite the free tissue ET-1 concentration in the arterial wall was unchanged in 1% O2 versus 95% O2. Exogenous ET-1 reversed hypoxic dilation in segments with and without endothelium, and the hypoxic arteries showed an increased sensitivity towards ET-1 compared to the normoxic controls. Without affecting basal NO, hypoxia increased NO concentration in PGF2α-contracted arteries, and an NO synthase inhibitor, L-NOARG,(300 µM, NG-nitro-L-Arginine) reduced hypoxic vasodilation. NO-induced vasodilation was reduced in endothelin-contracted preparations. Arterial wall ADMA concentrations were unchanged by hypoxia. Blocking of potassium channels with TEA (tetraethylammounium chloride)(10 µM) inhibited vasodilation to O2 lowering as well as to NO. The superoxide scavenger tiron (10 µM) and the putative NADPH oxidase inhibitor apocynin (10 µM) leftward shifted concentration-response curves for O2 lowering without changing vasodilation to 1% O2. PEG (polyethylene glycol) catalase (300 u/ml) inhibited H2O2 vasodilation, but failed to affect vasodilation to O2 lowering. Neither did PEG-SOD (polyethylene glycol superoxide dismutase)(70 u/ml) affect vasodilation to O2 lowering. The mitochondrial inhibitors rotenone (1 µM) and antimycin A (1 µM) both inhibited hypoxic vasodilatation. CONCLUSION: The present results in porcine coronary arteries suggest NO contributes to hypoxic vasodilation, probably through K channel opening, which is reversed by addition of ET-1 and enhanced by endothelin receptor antagonism. These latter findings suggest that endothelin receptor activation counteracts hypoxic vasodilation.

Current and Future Treatments for Persistent Pulmonary Hypertension in the Newborn.

Persistent pulmonary hypertension in newborn (PPHN) is a serious and possibly fatal syndrome characterized by sustained foetal elevation of pulmonary vascular resistance at birth. PPHN may manifest secondary to other conditions as meconium aspiration syndrome, infection and congenital diaphragmatic hernia. This MiniReview provides the reader with an overview of current and future treatment options for patients with PPHN without congenital diaphragmatic hernia. The study is based on systematic searches in the databases PubMed and Cochrane Library and registered studies on Clinicaltrials.gov investigating PPHN. Inhaled nitric oxide (iNO) is well documented for treatment of PPHN, but 30% fail to respond to iNO. Other current treatment options could be sildenafil, milrinone, prostaglandin analogues and bosentan. There are several ongoing trials with sildenafil, but evidence is lacking for the other treatments and/or for the combination with iNO. Currently, there is no evidence for effect in PPHN of other treatments, for example tadalafil, macitentan, ambrisentan, riociguat and selexipag used for pulmonary arterial hypertension in adults. Experimental studies in animal models for PPHN suggest effect of a series of approaches including recombinant human superoxide dismutase, L-citrulline, Rho-kinase inhibitors and peroxisome proliferator-activated receptor-γ agonists. We conclude that iNO is the most investigated and the only approved pulmonary vasodilator for infants with PPHN. In the iNO non-responders, sildenafil currently seems to be the best alternative either alone or in combination with iNO. Systematic and larger clinical studies are required for testing the other potential treatments of PPHN.

Associations of Plasma Nitrite, L-Arginine and Asymmetric Dimethylarginine With Morbidity and Mortality in Patients With Necrotizing Soft Tissue Infections.

BACKGROUND: The nitric oxide system could play an important role in the pathophysiology related to necrotizing soft tissue infection (NSTI). Accordingly, we investigated the association between plasma nitrite level at admission and the presence of septic shock in patients with NSTI. We also evaluated the association between nitrite, asymmetric dimethylarginine (ADMA), L-arginine, L-arginine/ADMA ratio, and outcome. METHODS: We analyzed plasma from 141 NSTI patients taken upon hospital admission. The severity of NSTI was assessed by the presence of septic shock, Simplified Acute Physiology Score (SAPS) II, Sepsis-Related Organ Failure Assessment (SOFA) score, use of renal replacement therapy (RRT), amputation, and 28-day mortality. RESULTS: No difference in nitrite levels was found between patients with and without septic shock (median 0.82 µmol/L [interquartile range (IQR) 0.41-1.21] vs. 0.87 µmol/L (0.62-1.24), P = 0.25). ADMA level was higher in patients in need of RRT (0.64 µmol/L (IQR 0.47-0.90) vs. (0.52 µmol/L (0.34-0.70), P = 0.028), and ADMA levels correlated positively with SAPS II (rho = 0.32, P = 0.0002) and SOFA scores (rho = 0.22, P = 0.01). In a logistic regression analysis, an L-arginine/ADMA ratio below 101.59 was independently associated with 28-day mortality, odds ratio 6.03 (95% confidence interval, 1.41-25.84), P = 0.016. None of the other analyses indicated differences in the NO system based on differences in disease severity. CONCLUSIONS: In patients with NSTI, we found no difference in baseline nitrite levels according to septic shock. High baseline ADMA level was associated with the use of RRT and patients with a low baseline L-arginine/ADMA ratio were at higher risk of dying within 28 days after hospital admission.

Ivabradine: Current and Future Treatment of Heart Failure.

In heart failure (HF), the heart cannot pump blood efficiently and is therefore unable to meet the body's demands of oxygen, and/or there is increased end-diastolic pressure. Current treatments for HF with reduced ejection fraction (HFrEF) include angiotensin-converting enzyme (ACE) inhibitors, angiotension receptor type 1 (AT1 ) antagonists, ß-adrenoceptor antagonists, aldosterone receptor antagonists, diuretics, digoxin and a combination drug with AT1 receptor antagonist and neprilysin inhibitor. In HF, the risk of readmission for hospital and mortality is markedly higher with a heart rate (HR) above 70 bpm. Here, we review the evidence regarding the use of ivabradine for lowering HR in HF. Ivabradine is a blocker of an I funny current (I(f)) channel and causes rate-dependent inhibition of the pacemaker activity in the sinoatrial node. In clinical trials of HFrEF, treatment with ivabradine seems to improve clinical outcome, for example improved ejection fraction (EF) and less readmission for hospital, but the effect appears most pronounced in patients with HRs above 70 bpm, while the effect on cardiovascular death appears less consistent. The adverse effects of ivabradine include bradycardia, atrial fibrillation and visual disturbances, but ivabradine avoids the negative inotrope effects observed with ß-adrenoceptor antagonists. In conclusion, in patients with stable HFrEF with EF<35% and HR above 70 bpm, ivabradine improves the outcome and might be a first choice of therapy, if beta-adrenoceptor antagonists are not tolerated. Further studies must show whether that can be extended to HF patients with preserved EF.

Small and Intermediate Calcium-Activated Potassium Channel Openers Improve Rat Endothelial and Erectile Function.

Modulation of endothelial calcium-activated potassium (KCa) channels has been proposed as an approach to restore endothelial function. The present study investigated whether novel openers of KCa channels with small (KCa2.x) and intermediate (KCa3.1) conductance, NS309 and NS4591, improve endothelium-dependent relaxation and erectile function. Rat corpus cavernosum (CC) strips were mounted for isometric tension recording and processed for immunoblotting. Mean arterial pressure (MAP), intracavernosal pressure (ICP), and electrocardiographic (ECG) measurements were conducted in anesthetized rats. Immunoblotting revealed the presence of KCa2.3 and large KCa conductance (KCa1.1) channels in the corpus cavernosum. NS309 and NS4591 increased current in CC endothelial cells in whole cell patch clamp experiments. Relaxation induced by NS309 (<1 µM) was inhibited by endothelial cell removal and high extracellular potassium. An inhibitor of nitric oxide (NO) synthase, and blockers of KCa2.x and KCa1.1 channels, apamin and iberiotoxin also inhibited NS309 relaxation. Incubation with NS309 (0.5 µM) markedly enhanced acetylcholine relaxation. Basal erectile function (ICP/MAP) increased during administration of NS309. Increases in ICP/MAP after cavernous nerve stimulation with NS309 were unchanged, whereas NS4591 significantly improved erectile function. Administration of NS309 and NS4591 caused small changes in the electrocardiogram, but neither arrhythmic events nor prolongation of the QTc interval were observed. The present study suggests that openers of KCa2.x and KCa3.1 channels improve endothelial and erectile function. The effects of NS309 and NS4591 on heart rate and ECG are small, but will require additional safety studies before evaluating whether activation of KCa2.3 channels has a potential for treatment of erectile dysfunction.

Down-regulation of KCa2.3 channels causes erectile dysfunction in mice.

Modulation of endothelial calcium-activated K+ channels has been proposed as an approach to restore arterial endothelial cell function in disease. We hypothesized that small-conductance calcium-activated K+ channels (KCa2.3 or SK3) contributes to erectile function. The research was performed in transgenic mice with overexpression (KCa2.3 T/T(-Dox)) or down-regulation (KCa2.3 T/T(+Dox)) of the KCa2.3 channels and wild-type C57BL/6-mice (WT). QPCR revealed that KCa2.3 and KCa1.1 channels were the most abundant in mouse corpus cavernosum. KCa2.3 channels were found by immunoreactivity and electron microscopy in the apical-lateral membrane of endothelial cells in the corpus cavernosum. Norepinephrine contraction was enhanced in the corpus cavernosum of KCa2.3 T/T(+Dox) versus KCa2.3 T/T(-Dox) mice, while acetylcholine relaxation was only reduced at 0.3 µM and relaxations in response to the nitric oxide donor sodium nitroprusside were unaltered. An opener of KCa2 channels, NS309 induced concentration-dependent relaxations of corpus cavernosum. Mean arterial pressure was lower in KCa2.3 T/T(-Dox) mice compared with WT and KCa2.3 T/T(+Dox) mice. In anesthetized mice, cavernous nerve stimulation augmented in frequency/voltage dependent manner erectile function being lower in KCa2.3 T/T(+Dox) mice at low frequencies. Our findings suggest that down-regulation of KCa2.3 channels contributes to erectile dysfunction, and that pharmacological activation of KCa2.3 channels may have the potential to restore erectile function.

Mechanisms involved in increased sensitivity to adenosine A(2A) receptor activation and hypoxia-induced vasodilatation in porcine coronary arteries.

Hypoxia-induced coronary vasorelaxation is a compensatory mechanism increasing blood flow. We hypothesized that hypoxia shares pathways with adenosine and causes vasorelaxation through the adenosine A(2A) receptor and force suppression by increasing cAMP and phosphorylated heat shock protein (HSP)20. Adenosine receptors in porcine left anterior descending coronary arteries (LAD) were examined by RT-PCR and isometric tension recording in myographs. Vasorelaxation was induced by adenosine, 1% oxygen, or both in the absence or presence of ZM241385, an adenosine A(2A) receptor antagonist. cAMP was determined by ELISA and p-HSP20/HSP20 and p-MLC/MLC were determined by immunoblotting and densitometric analyses. In coronary arteries exposed to 1% oxygen, there was increased sensitivity to adenosine, the adenosine A2 selective agonist NECA, and the adenosine A(2A) selective receptor agonist CGS21680. ZM241385 shifted concentration-response curves for CGS21680 to the right, whereas the adenosine A1 antagonist DPCPX, the adenosine A2B receptor antagonist MRS1754 and the adenosine A3 receptor antagonist MRS1523 failed to reduce vasodilatation induced by CGS21680. 1% oxygen or adenosine increased cAMP accumulation and HSP20 phosphorylation without changing T850-MYPT1 and MLC phosphorylation. ZM241385 failed to change 1% oxygen-induced vasodilation, cAMP accumulation, HSP20 phosphorylation and MLC phosphorylation. The PKA inhibitor Rp-8-CPT-cAMPS significantly reduced vasorelaxation induced by 1% oxygen or CGS21680. Our findings suggest that the increased sensitivity to adenosine, NECA, and CGS21680 at 1% oxygen involves adenosine A(2A) receptors. Adenosine and 1% oxygen induce vasorelaxation in PGF2α-contracted porcine coronary arteries partly by force suppression caused by increased cAMP and phosphorylation of HSP20.