The interaction between methylene blue and the cholinergic system

Anti-inflammatory, anti-oxidant and anti-ulcer activities of aqueous lyophilizate of Markhamia lutea (Bignoniaceae)

OBJECTIVES

This work was carried out with a view to determining the antioxidant, anti-inflammatory and anti-ulcer properties of the aqueous lyophilized extract of Markhamia lutea.

METHODS

In vitro proteinases inhibition, albumin denaturation, hemolysis of red blood cells by heat, inhibition of the proton pump H+/K+ATPase, FRAP (Ferric reducing antioxidant power) and DPPH (1,1-diphenyl-2-picrylhydrazyl) assays were performed. In vivo, cold water immersion-induced ulceration and methylene blue-induced ulceration was used to determine the anti-ulcer properties of the lyophilizate (100, 200 and 300 mg/kg).

RESULTS

In vitro, the lyophilizate (400 μg/mL) significantly inhibited protein denaturation (66.65 %), hemolysis of red blood cells (56.54 %), proteinase activity (69.22 %); then the IC50 was 26.31 μg/mL on proton pump activity. It has also developed a strong ferric reducing antioxidant power (EC50=52.96 mmol FeSO4/g) as well as free radicals scavenging activity (EC50=22.38 μg/mL). In vivo, the aqueous lyophilizate (200 and 300 mg/kg) protected the gastric mucosa (70.68 and 79.00 % protection respectively) and reduced (p<0.05) acetylcholine, calcium and corticosterone concentrations. A decrease in malondialdehyde level, an increased glutathione level and an increased in catalase and SOD activities were recorded. In the methylene blue test, it significantly increased gastric fluid pH, while reducing gastric volume and improving hematological parameters in ulcer animals. In addition, the histological sections show that the aqueous lyophilizate of M. lutea protected the gastric mucosa from the deleterious effects of stress.

CONCLUSIONS

The aqueous lyophilizate of M. lutea has anti-ulcer properties thanks to its anti-inflammatory, antioxidant and anti-secretory properties.

Anticholinergic Toxidrome as a Possible Explanation for Methylene Blue Toxicity

BACKGROUND Methylene blue has multiple uses in medicine. It is generally used to treat refractory vasoplegia and methemoglobin toxicity, and can be used as a dye to localize the parathyroid glands intra-operatively. In refractory vasoplegia, methylene blue inhibits endothelial nitric oxide and guanylate cyclase, causing vasoconstriction and potentially stabilizing blood pressure. Multiple complications have been associated with the use of methylene blue. These are related to either the sole effect of methylene blue or the combined effect of methylene blue and certain antidepressants, such as selective serotonin reuptake inhibitors (SSRIs). To the best of our knowledge, in the setting of post-cardiac surgery vasoplegia, there have been no reports of the neurological toxicity of methylene blue in the absence of SSRI use. In this case report, we describe the anticholinergic manifestations associated with the use of methylene blue in post-cardiac surgery vasoplegia. CASE REPORT A male patient in his mid-sixties with severe mitral regurgitation underwent elective mitral valve replacement. Postoperatively, he was hypotensive and required a high dose of vasopressors. Methylene blue was administered to treat refractory vasoplegia. The patient became anuric and febrile, with bilateral mydriasis. Internal cooling and continuous renal replacement therapy were initiated, and symptoms rapidly resolved. The patient was discharged after prolonged hospitalization with a permanent catheter for hemodialysis. CONCLUSIONS Anticholinergic toxidrome may explain the neurological adverse effects associated with high doses of methylene blue. Physicians should be cautious when using methylene blue in combination with other anticholinergic drugs and in conditions of renal failure. The development of methylene blue toxicity warrants the urgent discontinuation of the agent and early drug elimination.

Cardioprotection by methylene Blue Against Epinephrine-Induced Cardiac Arrhythmias and Myocardial Injury

Methylene blue is used in the treatment of vasoplegic syndrome after cardiac surgery, anaphylaxis, and septic shock refractory to epinephrine and fluid resuscitation. In this study, we investigated the potential protective effect of methylene blue on the development of cardiac arrhythmias after injection of epinephrine in rats. Methylene blue was given intraperitoneally at doses of 50 or 100 mg/kg. Cardiac arrhythmia was then induced with 10 μg/kg of epinephrine intravenously. In untreated, control rats, epinephrine caused bradycardia (96.48 ± 1.06 vs. 365.03 ± 0.68 beats/min), increased PR interval (0.54 ± 0.04 vs. 0.039 ± 0.004), RR interval (0.64 ± 0.003 vs. 0.16 ± 0.004 sec), shortened QTc interval (0.067 ± 0.05 vs. 0.1 ± 0.004 sec), increased QRS duration (0.048 ± 0.005 vs. 0.028 ± 0.002 sec), decreased R wave amplitude (0.3 ± 0.03 vs. 0.49 ± 0.04 mv), decreased the height of the ST segment (-0.0696 ± 0.004 vs. -0.0054 ± 0.003 mv), and caused ventricular extrasystoles (7.92 ± 0.56 vs. 0.5 ± 0.5). Methylene blue given at 50 or 100 mg/kg increased the heart rate, decreased RR interval, QRS duration and the drop in the ST height, increased duration of QTc interval and R wave amplitude and decreased the number of extrasystoles. The histological study showed that methylene blue protected against myocardial structural disorganization, cellular damage, necrosis, and haemorrhage between muscle fibres induced by epinephrine injection. We conclude that methylene blue dose-dependently prevented epinephrine-induced arrhythmias and cardiac muscle injury.

Adverse effects of methylene blue in peripheral neurons: An in vitro electrophysiology and cell culture study

Methylene blue (MB) is an effective treatment for methemoglobinemia, ifosfamide-induced encephalopathy, cyanide poisoning, and refractory vasoplegia. However, clinical case reports and preclinical studies indicate potentially neurotoxic activity of MB at certain concentrations. The exact mechanisms of MB neurotoxicity are not known, and while the effects of MB on neuronal tissue from different brain regions and myenteric ganglia have been examined, its effects on primary afferent neurons from dorsal root ganglia (DRG) have not been studied. Mouse DRG were exposed to MB (0.3-10 μM) in vitro to assess neurite outgrowth. Increasing concentrations of MB (0.3-10 μM) were associated with neurotoxicity as shown by a substantial loss of cells with neurite formation, particularly at 10 μM. In parallel experiments, cultured rat DRG neurons were treated with MB (100 μM) to examine how MB affects electrical membrane properties of small-diameter sensory neurons. MB decreased peak inward and outward current densities, decreased action potential amplitude, overshoot, afterhyperpolarization, increased action potential rise time, and decreased action potential firing in response to current stimulation. MB induced dose-dependent toxicity in peripheral neurons, in vitro. These findings are consistent with studies in brain and myenteric ganglion neurons showing increased neuronal loss and altered membrane electrical properties after MB application. Further research is needed to parse out the toxicity profile for MB to minimize damage to neuronal structures and reduce side effects in clinical settings.

Conjugates of Methylene Blue with Cycloalkaneindoles as New Multifunctional Agents for Potential Treatment of Neurodegenerative Disease

The development of multi-target-directed ligands (MTDLs) would provide effective therapy of neurodegenerative diseases (ND) with complex and nonclear pathogenesis. A promising method to create such potential drugs is combining neuroactive pharmacophoric groups acting on different biotargets involved in the pathogenesis of ND. We developed a synthetic algorithm for the conjugation of indole derivatives and methylene blue (MB), which are pharmacophoric ligands that act on the key stages of pathogenesis. We synthesized hybrid structures and performed a comprehensive screening for a specific set of biotargets participating in the pathogenesis of ND (i.e., cholinesterases, NMDA receptor, mitochondria, and microtubules assembly). The results of the screening study enabled us to find two lead compounds (4h and 4i) which effectively inhibited cholinesterases and bound to the AChE PAS, possessed antioxidant activity, and stimulated the assembly of microtubules. One of them (4i) exhibited activity as a ligand for the ifenprodil-specific site of the NMDA receptor. In addition, this lead compound was able to bypass the inhibition of complex I and prevent calcium-induced mitochondrial depolarization, suggesting a neuroprotective property that was confirmed using a cellular calcium overload model of neurodegeneration. Thus, these new MB-cycloalkaneindole conjugates constitute a promising class of compounds for the development of multitarget neuroprotective drugs which simultaneously act on several targets, thereby providing cognitive stimulating, neuroprotective, and disease-modifying effects.

Hydromethylthionine enhancement of central cholinergic signalling is blocked by rivastigmine and memantine

The prevention of tau protein aggregations is a therapeutic goal for the treatment of Alzheimer's disease (AD), and hydromethylthionine (HMT) (also known as leucomethylthioninium-mesylate [LMTM]), is a potent inhibitor of tau aggregation in vitro and in vivo. In two Phase 3 clinical trials in AD, HMT had greater pharmacological activity on clinical endpoints in patients not receiving approved symptomatic treatments for AD (acetylcholinesterase (AChE) inhibitors and/or memantine) despite different mechanisms of action. To investigate this drug interaction in an animal model, we used tau-transgenic L1 and wild-type NMRI mice treated with rivastigmine or memantine prior to adding HMT, and measured changes in hippocampal acetylcholine (ACh) by microdialysis. HMT given alone doubled hippocampal ACh levels in both mouse lines and increased stimulated ACh release induced by exploration of the open field or by infusion of scopolamine. Rivastigmine increased ACh release in both mouse lines, whereas memantine was more active in tau-transgenic L1 mice. Importantly, our study revealed a negative interaction between HMT and symptomatic AD drugs: the HMT effect was completely eliminated in mice that had been pre-treated with either rivastigmine or memantine. Rivastigmine was found to inhibit AChE, whereas HMT and memantine had no effects on AChE or on choline acetyltransferase (ChAT). The interactions observed in this study demonstrate that HMT enhances cholinergic activity in mouse brain by a mechanism of action unrelated to AChE inhibition. Our findings establish that the drug interaction that was first observed clinically has a neuropharmacological basis and is not restricted to animals with tau aggregation pathology. Given the importance of the cholinergic system for memory function, the potential for commonly used AD drugs to interfere with the treatment effects of disease-modifying drugs needs to be taken into account in the design of clinical trials.

Mechanisms of Anticholinesterase Interference with Tau Aggregation Inhibitor Activity in a Tau-Transgenic Mouse Model

Background

Symptomatic treatments of Alzheimer’s Disease (AD) with cholinesterase inhibitors and/or memantine are relatively ineffective and there is a need for new treatments targeting the underlying pathology of AD. In most of the failed disease-modifying trials, patients have been allowed to continue taking symptomatic treatments at stable doses, under the assumption that they do not impair efficacy. In recently completed Phase 3 trials testing the tau aggregation inhibitor leuco-methylthioninium bis (hydromethane-sulfonate) (LMTM), we found significant differences in treatment response according to whether patients were taking LMTM either as monotherapy or as an add-on to symptomatic treatments.

Methods

We have examined the effect of either LMTM alone or chronic rivastigmine prior to LMTM treatment of tau transgenic mice expressing the short tau fragment that constitutes the tangle filaments of AD. We have measured acetylcholine levels, synaptosomal glutamate release, synaptic proteins, mitochondrial complex IV activity, tau pathology and Choline Acetyltransferase (ChAT) immunoreactivity.

Results

LMTM given alone increased hippocampal Acetylcholine (ACh) levels, glutamate release from synaptosomal preparations, synaptophysin levels in multiple brain regions and mitochondrial complex IV activity, reduced tau pathology, partially restored ChAT immunoreactivity in the basal forebrain and reversed deficits in spatial learning. Chronic pretreatment with rivastigmine was found to reduce or eliminate almost all these effects, apart from a reduction in tau aggregation pathology. LMTM effects on hippocampal ACh and synaptophysin levels were also reduced in wild-type mice.

Conclusion

The interference with the pharmacological activity of LMTM by a cholinesterase inhibitor can be reproduced in a tau transgenic mouse model and, to a lesser extent, in wild-type mice. Long-term pretreatment with a symptomatic drug alters a broad range of brain responses to LMTM across different transmitter systems and cellular compartments at multiple levels of brain function. There is, therefore, no single locus for the negative interaction. Rather, the chronic neuronal activation induced by reducing cholinesterase function produces compensatory homeostatic downregulation in multiple neuronal systems. This reduces a broad range of treatment responses to LMTM associated with a reduction in tau aggregation pathology. Since the interference is dictated by homeostatic responses to prior symptomatic treatment, it is likely that there would be similar interference with other drugs tested as add-on to the existing symptomatic treatment, regardless of the intended therapeutic target or mode of action. The present findings outline key results that now provide a working model to explain interference by symptomatic treatment.

Hereditary Spastic Paraplegia and Future Therapeutic Directions: Beneficial Effects of Small Compounds Acting on Cellular Stress

Hereditary spastic paraplegia (HSP) is a group of inherited neurodegenerative conditions that share a characteristic feature of degeneration of the longest axons within the corticospinal tract, which leads to progressive spasticity and weakness of the lower limbs. Mutations of over 70 genes produce defects in various biological pathways: axonal transport, lipid metabolism, endoplasmic reticulum (ER) shaping, mitochondrial function, and endosomal trafficking. HSPs suffer from an adequate therapeutic plan. Currently the treatments foreseen for patients affected by this pathology are physiotherapy, to maintain the outgoing tone, and muscle relaxant therapies for spasticity. Very few clinical studies have been conducted, and it's urgent to implement preclinical animal studies devoted to pharmacological test and screening, to expand the rose of compounds potentially attractive for clinical trials. Small animal models, such as Drosophila melanogaster and zebrafish, have been generated, analyzed, and used as preclinical model for screening of compounds and their effects. In this work, we briefly described the role of HSP-linked proteins in the organization of ER endomembrane system and in the regulation of ER homeostasis and stress as a common pathological mechanism for these HSP forms. We then focused our attention on the pharmacodynamic and pharmacokinetic features of some recently identified molecules with antioxidant property, such as salubrinal, guanabenz, N-acetyl cysteine, methylene blue, rapamycin, and naringenin, and on their potential use in future clinical studies. Expanding the models and the pharmacological screening for HSP disease is necessary to give an opportunity to patients and clinicians to test new molecules.

Mitochondria as a promising target for developing novel agents for treating Alzheimer's disease

The mitochondria-targeting drugs can be conventionally divided into the following groups: those compensating for the energy deficit involved in neurodegeneration, including stimulants of mitochondrial bioenergetics and activators of mitochondrial biogenesis; and neuroprotectors, that are compounds increasing the resistance of mitochondria to opening of mitochondrial permeability transition (MPT) pores. Although compensating for the energy deficit and inhibition of MPT are obvious targets for drugs used in the very early stages of Alzheimer-like pathology, but their use as the monotherapy for patients with severe symptoms is unlikely to be sufficiently effective. It would be optimal to combine targets that would provide the cognitive-stimulating, the neuroprotective effects and the ability to affect specific disease-forming mechanisms. In the design of such drugs, assessment of their potential mitochondrial-targeted effects is of particular importance. The possibility of targeted drug design for simultaneous action on mitochondrial and neurotransmitter's receptors targets is, in particularly, based on the known interplay of various cellular pathways and the presence of common structural components. Of particular interest is directed search for multitarget drugs that would act simultaneously on mitochondrial calcium-dependent functions, the targets (receptors, enzymes, etc.) facilitating neurotransmission, and the molecular targets related to the action of so-called disease-modifying factors, in particular, the formation and overcoming of the toxicity of β-amyloid or hyperphosphorylated tau protein. The examples of such approaches realized on the level of preclinical and clinical trials are presented below.

Azure B affects amyloid precursor protein metabolism in PS70 cells

Alzheimer's disease (AD), the most common form of dementia, is characterized by abundant deposition of amyloid-β (Aβ) peptide that is the result of sequential cleavage of amyloid precursor protein (APP) by β-secretase and γ-secretase. Several studies have documented that inhibition of Aβ peptide synthesis or facilitating its degradation is one of the attractive therapeutic strategies in AD. Methylene blue (MethB), which has recently been investigated in Phase II clinical trials, is a prominent inhibitor in reducing Aβ oligomers. Herein, we wonder whether the mitigating effects of MethB on amyloid metabolism are related to the activity of its major metabolite, azure B. The goal of this study was to investigate the effects of azure B, which is also a cholinesterase inhibitor, on APP processing by using Chinese hamster ovary cells stably expressing human wild-type APP and presenilin 1 (PS70). Azure B significantly decreased the levels of secreted APPα (sAPPα) and Aβ_40/42 in culture medium with a dose-dependent manner. A significant decrease was also observed in the levels of intracellular APP without affecting the cell viability. In parallel with the decrease of APP and APP metabolites, the activity of β-secretase 1 (BACE1) was significantly attenuated compared to control. Overall, our results show that azure B has a large contribution for the pharmacological profile of MethB in APP metabolism.

Reversible cholinesterase inhibitors as pretreatment for exposure to organophosphates. A review

Organophosphorus compounds (OPCs), inhibitors of acetylcholinesterase (AChE), are useful agents as pesticides, but also represent a serious health hazard. Standard therapy with atropine and established oxime-type enzyme reactivators (pralidoxime, obidoxime) is unsatisfactory. Better therapeutic results are obtained, when reversible AChE inhibitors are administered before OPC exposure. This review summarizes the history of such a pretreatment approach and sums up a set of experiments undertaken in search of compounds that are efficacious when given before a broad range of OPCs. The prophylactic efficacy of 10 known AChE inhibitors, either already used clinically for different indications (physostigmine, pyridostigmine, ranitidine, tiapride, tacrine, amiloride, metoclopramide, methylene blue) or developed for possible therapeutic use in the future (7-methoxytacrine, K-27) was compared, when administered before exposure to six chemically diverse OPCs in the same experimental setting: ethyl-paraoxon, methyl-paraoxon, diisopropylfluorophosphate, terbufos sulfone, azinphos-methyl and dicrotophos. The experimental oxime K-27 was the most efficacious compound, affording best protection, when administered before terbufos sulfone, azinphos-methyl and dicrotophos, second best before ethyl- and methyl-paraoxon exposure and third best before diisopropylfluorophosphate administration. This ranking was similar to that of physostigmine, which was superior to the Food and Drug Administration-approved pretreatment for soman with pyridostigmine. Tiapride, amiloride, metoclopramide, methylene blue and 7-methoxytacrine did not achieve protection. No correlation was observed between the IC₅₀ of the reversible AChE inhibitors and their protective efficacy. These studies indicate that K-27 can be considered a very promising broad-spectrum prophylactic agent in case of imminent organophosphate exposure, which may be related to its AChE reactivating activity rather than its AChE inhibition.

Methylene Blue (Tetramethylthionine Chloride) Influences the Mobility of Adult Neural Stem Cells: A Potentially Novel Therapeutic Mechanism of a Therapeutic Approach in the Treatment of Alzheimer's Disease

An interest in neurogenesis in the adult human brain as a relevant and targetable process has emerged as a potential treatment option for Alzheimer's disease and other neurodegenerative conditions. The aim of this study was to investigate the effects of tetramethylthionine chloride (methylene blue, MB) on properties of adult murine neural stem cells. Based on recent clinical studies, MB has increasingly been discussed as a potential treatment for Alzheimer's disease. While no differences in the proliferative capacity were identified, a general potential of MB in modulating the migratory capacity of adult neural stem cells was indicated in a cell mobility assay. To our knowledge, this is the first time that MB could be associated with neural mobility. The results of this study add insight to the spectrum of features of MB within the central nervous system and may be helpful for understanding the molecular mechanisms underlying a potential therapeutic effect of MB.

Methylene blue and its analogues as antidepressant compounds

Methylene Blue (MB) is considered to have diverse medical applications and is a well-described treatment for methemoglobinemias and ifosfamide-induced encephalopathy. In recent years the focus has shifted to MB as an antimalarial agent and as a potential treatment for neurodegenerative disorders such as Alzheimer's disease. Of interest are reports that MB possesses antidepressant and anxiolytic activity in pre-clinical models and has shown promise in clinical trials for schizophrenia and bipolar disorder. MB is a noteworthy inhibitor of monoamine oxidase A (MAO-A), which is a well-established target for antidepressant action. MB is also recognized as a non-selective inhibitor of nitric oxide synthase (NOS) and guanylate cyclase. Dysfunction of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) cascade is strongly linked to the neurobiology of mood, anxiety and psychosis, while the inhibition of NOS and/or guanylate cyclase has been associated with an antidepressant response. This action of MB may contribute significantly to its psychotropic activity. However, these disorders are also characterised by mitochondrial dysfunction and redox imbalance. By acting as an alternative electron acceptor/donor MB restores mitochondrial function, improves neuronal energy production and inhibits the formation of superoxide, effects that also may contribute to its therapeutic activity. Using MB in depression co-morbid with neurodegenerative disorders, like Alzheimer's and Parkinson's disease, also represents a particularly relevant strategy. By considering their physicochemical and pharmacokinetic properties, analogues of MB may provide therapeutic potential as novel multi-target strategies in the treatment of depression. In addition, low MAO-A active analogues may provide equal or improved response with a lower risk of adverse effects.

Tau-directed approaches for the treatment of Alzheimer's disease: focus on leuco-methylthioninium

Small molecular weight compounds able to inhibit formation of tau oligomers and fibrils have already been tested for Alzheimer's disease (AD) treatment. The most advanced tau aggregation inhibitor (TAI) is methylthioninium (MT), a drug existing in equilibrium between a reduced (leuco-methylthioninium) and oxidized form (MT(+)). MT chloride (also known as methylene blue) was investigated in a 24-week Phase II study in 321 mild-to-moderate AD patients at the doses of 69, 138, and 228 mg/day. This trial failed to show significant positive effects of MT in the overall patient population. The dose of 138 mg/day showed potential benefits on cognitive performance of moderately affected patients and cerebral blood flow in mildly affected patients. A follow-up compound (TRx0237) claimed to be more bioavailable and less toxic than MT, is now being developed. Phase III clinical trials on this novel TAI in AD and in the behavioral variant of frontotemporal dementia are underway.

Preservation of neuromuscular function in symptomatic SOD1-G93A mice by peripheral infusion of methylene blue

In mutant superoxide dismutase 1 (SOD1) mouse models of familial amyotrophic lateral sclerosis (fALS) some of the earliest signs of morphological and functional damage occur in the motor nerve terminals that innervate fast limb muscles. This study tested whether localized peripheral application of a protective drug could effectively preserve neuromuscular junctions in late-stage disease. Methylene blue (MB), which has mitochondria-protective properties, was infused via an osmotic pump into the anterior muscle compartment of one hind limb of late pre- symptomatic SOD1-G93A mice for ≥3weeks. When mice reached end-stage disease, peak twitch and tetanic contractions evoked by stimulation of the muscle nerve were measured in two anterior compartment muscles (tibialis anterior [TA] and extensor digitorum longus [EDL], both predominantly fast muscles). With 400μM MB in the infusion reservoir, muscles on the MB-infused side exhibited on average a ~100% increase in nerve-evoked contractile force compared to muscles on the contralateral non-infused side (p<0.01 for both twitch and tetanus in EDL and TA). Pairwise comparisons of endplate innervation also revealed a beneficial effect of MB infusion, with an average of 65% of endplates innervated in infused EDL, compared to only 35% on the non-infused side (p<0.01). Results suggested that MB's protective effects required an extracellular [MB] of ~1μM, were initiated peripherally (no evidence of retrograde transport into the spinal cord), and involved MB's reduced form. Thus peripherally-initiated actions of MB can help preserve neuromuscular structure and function in SOD1-G93A mice, even at late stages of disease.

Tau-Centric Targets and Drugs in Clinical Development for the Treatment of Alzheimer's Disease

The failure of several Phase II/III clinical trials in Alzheimer's disease (AD) with drugs targeting β-amyloid accumulation in the brain fuelled an increasing interest in alternative treatments against tau pathology, including approaches targeting tau phosphatases/kinases, active and passive immunization, and anti-tau aggregation. The most advanced tau aggregation inhibitor (TAI) is methylthioninium (MT), a drug existing in equilibrium between a reduced (leuco-methylthioninium) and oxidized form (MT⁺). MT chloride (methylene blue) was investigated in a 24-week Phase II clinical trial in 321 patients with mild to moderate AD that failed to show significant positive effects in mild AD patients, although long-term observations (50 weeks) and biomarker studies suggested possible benefit. The dose of 138 mg/day showed potential benefits on cognitive performance of moderately affected AD patients and cerebral blood flow in mildly affected patients. Further clinical evidence will come from the large ongoing Phase III trials for the treatment of AD and the behavioral variant of frontotemporal dementia on a new form of this TAI, more bioavailable and less toxic at higher doses, called TRx0237. More recently, inhibitors of tau acetylation are being actively pursued based on impressive results in animal studies obtained by salsalate, a clinically used derivative of salicylic acid.

A review on Alzheimer's disease pathophysiology and its management: an update

Alzheimer's disease acknowledged as progressive multifarious neurodegenerative disorder, is the leading cause of dementia in late adult life. Pathologically it is characterized by intracellular neurofibrillary tangles and extracellular amyloidal protein deposits contributing to senile plaques. Over the last two decades, advances in the field of pathogenesis have inspired the researchers for the investigation of novel pharmacological therapeutics centered more towards the pathophysiological events of the disease. Currently available treatments i.e. acetylcholinesterase inhibitors (rivastigmine, galantamine, donepezil) and N-methyl d-aspartate receptor antagonist (memantine) contribute minimal impact on the disease and target late aspects of the disease. These drugs decelerate the progression of the disease, provide symptomatic relief but fail to achieve a definite cure. While the neuropathological features of Alzheimer's disease are recognized but the intricacies of the mechanism have not been clearly defined. This lack of understanding regarding the pathogenic process may be the likely reason for the non-availability of effective treatment which can prevent onset and progression of the disease. Owing to the important progress in the field of pathophysiology in the last couple of years, new therapeutic targets are available that should render the underlying disease process to be tackled directly. In this review, authors will discusses the different aspects of pathophysiological mechanisms behind Alzheimer's disease and its management through conventional drug therapy, including modern investigational therapeutic strategies, recently completed and ongoing.

Tau-aggregation inhibitor therapy for Alzheimer's disease

Many trials of drugs aimed at preventing or clearing β-amyloid pathology have failed to demonstrate efficacy in recent years and further trials continue with drugs aimed at the same targets and mechanisms. The Alzheimer neurofibrillary tangle is composed of tau and the core of its constituent filaments are made of a truncated fragment from the repeat domain of tau. This truncated tau can catalyse the conversion of normal soluble tau into aggregated oligomeric and fibrillar tau which, in turn, can spread to neighbouring neurons. Tau aggregation is not a late-life process and onset of Braak stage 1 peaks in people in their late 40s or early 50s. Tau aggregation pathology at Braak stage 1 or beyond affects 50% of the population over the age of 45. The initiation of tau aggregation requires its binding to a non-specific substrate to expose a high affinity tau-tau binding domain and it is self-propagating thereafter. The initiating substrate complex is most likely formed as a consequence of a progressive loss of endosomal-lysosomal processing of neuronal proteins, particularly of membrane proteins from mitochondria. Mutations in the APP/presenilin membrane complex may simply add to the age-related endosomal-lysosomal processing failure, bringing forward, but not directly causing, the tau aggregation cascade in carriers. Methylthioninium chloride (MTC), the first identified tau aggregation inhibitor (TAI), offers an alternative to the amyloid approach. Phase 3 trials are underway with a novel stabilized reduced form of methylthioninium (LMTX) that has improved tolerability and absorption.

Therapeutics of Alzheimer's disease: Past, present and future

Alzheimer's disease (AD) is the most common cause of dementia worldwide. The etiology is multifactorial, and pathophysiology of the disease is complex. Data indicate an exponential rise in the number of cases of AD, emphasizing the need for developing an effective treatment. AD also imposes tremendous emotional and financial burden to the patient's family and community. The disease has been studied over a century, but acetylcholinesterase inhibitors and memantine are the only drugs currently approved for its management. These drugs provide symptomatic improvement alone but do less to modify the disease process. The extensive insight into the molecular and cellular pathomechanism in AD over the past few decades has provided us significant progress in the understanding of the disease. A number of novel strategies that seek to modify the disease process have been developed. The major developments in this direction are the amyloid and tau based therapeutics, which could hold the key to treatment of AD in the near future. Several putative drugs have been thoroughly investigated in preclinical studies, but many of them have failed to produce results in the clinical scenario; therefore it is only prudent that lessons be learnt from the past mistakes. The current rationales and targets evaluated for therapeutic benefit in AD are reviewed in this article. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

In Vivo Models Used for Evaluation of Potential Antigastroduodenal Ulcer Agents

Peptic ulcer is among the most serious gastrointestinal diseases in the world. Several orthodox drugs are employed for the treatment of the disease. Although these drugs are effective, they produce many adverse effects thus limiting their use. In recent years, there has been a growing interest in alternative therapies, especially those from plants due to their perceived relative lower side effects, ease of accessibility, and affordability. Plant medicines with ethnomedicinal use in peptic ulcer management need to be screened for their effectiveness and possible isolation of lead compounds. This requires use of appropriate animal models of various ulcers. The limited number of antiulcer models for drug development against gastric and duodenal ulcer studies has hindered the progress of targeted therapy in this field. It is, therefore, necessary to review the literature on experimental models used to screen agents with potential antigastroduodenal ulcer activity and explain their biochemical basis in order to facilitate their use in the development of new preventive and curative antiulcer drugs. Clinical trials can then be carried out on agents/drugs that show promise. In this paper, current in vivo animal models of ulcers and the pathophysiological mechanisms underlying their induction, their limitations, as well as the challenges associated with their use have been discussed.

Neurometabolic mechanisms for memory enhancement and neuroprotection of methylene blue

This paper provides the first review of the memory-enhancing and neuroprotective metabolic mechanisms of action of methylene blue in vivo. These mechanisms have important implications as a new neurobiological approach to improve normal memory and to treat memory impairment and neurodegeneration associated with mitochondrial dysfunction. Methylene blue's action is unique because its neurobiological effects are not determined by regular drug-receptor interactions or drug-response paradigms. Methylene blue shows a hormetic dose-response, with opposite effects at low and high doses. At low doses, methylene blue is an electron cycler in the mitochondrial electron transport chain, with unparalleled antioxidant and cell respiration-enhancing properties that affect the function of the nervous system in a versatile manner. A major role of the respiratory enzyme cytochrome oxidase on the memory-enhancing effects of methylene blue is supported by available data. The memory-enhancing effects have been associated with improvement of memory consolidation in a network-specific and use-dependent fashion. In addition, low doses of methylene blue have also been used for neuroprotection against mitochondrial dysfunction in humans and experimental models of disease. The unique auto-oxidizing property of methylene blue and its pleiotropic effects on a number of tissue oxidases explain its potent neuroprotective effects at low doses. The evidence reviewed supports a mechanistic role of low-dose methylene blue as a promising and safe intervention for improving memory and for the treatment of acute and chronic conditions characterized by increased oxidative stress, neurodegeneration and memory impairment.

Cellular and molecular actions of Methylene Blue in the nervous system

Methylene Blue (MB), following its introduction to biology in the 19th century by Ehrlich, has found uses in various areas of medicine and biology. At present, MB is the first line of treatment in methemoglobinemias, is used frequently in the treatment of ifosfamide-induced encephalopathy, and is routinely employed as a diagnostic tool in surgical procedures. Furthermore, recent studies suggest that MB has beneficial effects in Alzheimer's disease and memory improvement. Although the modulation of the cGMP pathway is considered the most significant effect of MB, mediating its pharmacological actions, recent studies indicate that it has multiple cellular and molecular targets. In the majority of cases, biological effects and clinical applications of MB are dictated by its unique physicochemical properties including its planar structure, redox chemistry, ionic charges, and light spectrum characteristics. In this review article, these physicochemical features and the actions of MB on multiple cellular and molecular targets are discussed with regard to their relevance to the nervous system.

"Lest we forget you--methylene blue..."

Methylene blue (MB), the first synthetic drug, has a 120-year-long history of diverse applications, both in medical treatments and as a staining reagent. In recent years there was a surge of interest in MB as an antimalarial agent and as a potential treatment of neurodegenerative disorders such as Alzheimer's disease (AD), possibly through its inhibition of the aggregation of tau protein. Here we review the history and medical applications of MB, with emphasis on recent developments.

Pretreatment for acute exposure to diisopropylfluorophosphate: in vivo efficacy of various acetylcholinesterase inhibitors

Prophylactic administration of reversible acetylcholinesterase (AChE) inhibitors before exposure to organophosphorus compounds (OPCs) can reduce OPC-induced mortality. Pyridostigmine is the only FDA-approved substance for such use. The AChE-inhibitory activity of known AChE inhibitors was quantified in vitro and their in vivo mortality-reducing efficacy was compared, when given prophylactically before the exposure to the OPC diisopropylfluorophosphate (DFP). The IC50 was measured in vitro for the known AChE inhibitors pyridostigmine, physostigmine, ranitidine, tiapride, tacrine, 7-methoxytacrine, amiloride, metoclopramide, methylene blue and the experimental oxime K-27. Their in vivo efficacy, when given as pretreatment, to protect rats from DFP-induced mortality was quantified by determining the relative risk of death (RR) by Cox analysis, with RR = 1 for animals given only DFP, but no pretreatment. Physostigmine was the strongest in vitro AChE-inhibitor (IC50 = 0.012 µ m), followed by 7-methoxytacrine, tacrine, pyridostigmine and methylene blue. Ranitidine (IC50 = 2.5 µ m), metoclopramide and amiloride were in the mid-range. Tiapride (IC50 = 256 µ m) and K-27 (IC50 = 414 µ m) only weakly inhibited RBC AChE activity. Best in vivo protection from DFP-induced mortality was achieved when physostigmine (RR = 0.02) or tacrine (RR = 0.05) was given before DFP exposure, which was significantly superior to the pretreatment with all other tested compounds, except K-27 (RR = 0.18). The mortality-reducing effect of pyridostigmine, ranitidine and 7-methoxytacrine was inferior, but still significant. Tiapride, methylene blue, metoclopramide and amiloride did not significantly improve DFP-induced mortality. K-27 may be a more efficacious alternative to pyridostigmine, when passage into the brain precludes administration of physostigmine or tacrine.

Methylene blue fails to inhibit Tau and polyglutamine protein dependent toxicity in zebrafish

Methylene blue is an FDA approved compound with a variety of pharmacologic activities. It inhibits aggregation of several amyloidogenic proteins known to be deposited in neurodegenerative diseases. Recently, it has been proposed that methylene blue shows significant beneficial effects in a phase 2 clinical trial by slowing cognitive decline in Alzheimer's disease patients. To analyze its therapeutic potential, we investigated the effect of methylene blue on neurotoxicity in a zebrafish model for tauopathies. Transgenic expression of the frontotemporal dementia associated Tau-P301L mutation recapitulates a number of the pathological features observed in humans including abnormal phosphorylation and folding of Tau, tangle formation and Tau dependent neuronal loss. Upon incubation of zebrafish larvae with methylene blue, neither abnormal phosphorylation nor neuronal cell loss, reduced neurite outgrowth or a swimming defect were rescued. Methylene blue is biologically active in zebrafish since it reduced aggregation of a huntingtin variant containing a stretch of 102 glutamine residues. However, although huntingtin aggregation was largely prevented by methylene blue, huntingtin-dependent toxicity was unaffected. Our findings are consistent with the hypothesis that toxicity is not necessarily associated with deposition of insoluble amyloid proteins.

Methylthioninium chloride reverses cognitive deficits induced by scopolamine: comparison with rivastigmine

RATIONALE

The cholinergic system is involved in cognition as well as in age-related cognitive decline and Alzheimer disease (AD). Cholinergic enhancers ameliorate AD symptoms and represent the main current therapy for AD. MTC (Methylthioninium chloride), an antioxidant with metabolism-enhancing properties may be a novel candidate with pro-cognitive capacities.

OBJECTIVES

This study was performed: (1) to assess the pro-cognitive efficacy of MTC and establish its dose-response; (2) to compare the efficacy of MTC with rivastigmine and (3) to determine the potential for combination therapy by co-administration of MTC and rivastigmine.

METHODS

Spatial cognition of female NMRI mice was tested in a reference memory water maze task. Subjects received intra-peritoneal injections of scopolamine (0.5 mg/kg) followed by vehicle, and/or MTC and/or rivastigmine (0.15-4 mg/kg MTC; 0.1-0.5 mg/kg rivastigmine) in mono or combination treatment.

RESULTS

Scopolamine treatment prevented spatial learning in NMRI female mice and the deficit was reversed by both rivastigmine and MTC in a dose-dependent manner. Mono-therapy with high doses of rivastigmine (>0.5 mg/kg) caused severe side effects but MTC was safe up to 4 mg/kg. Co-administration of sub-effective doses of both drugs acted synergistically in reversing learning deficits and scopolamine-induced memory impairments.

CONCLUSIONS

In our model, MTC reversed the spatial learning impairment. When combined with the ChEI rivastigmine, the effect of MTC appeared to be amplified indicating that combination therapy could potentially improve not only symptoms but also contribute beneficially to neuronal metabolism by minimising side effects at lower doses.

A novel use of methylene blue as a pharmacological tool

INTRODUCTION

A new use of methylene blue as an ulcerogenic agent and the mechanisms involved were identified with an objective to exploit methylene blue as a pharmacological tool to study investigational antiulcer agents.

METHODS

Ulcerogenic potential was assessed using electron microscopy and measurement of an ulcer index after administering methylene blue (5-125 mg kg(-1), p.o.) or absolute ethanol (99%v/v, 2 ml, p.o.) to fasted rats. Estimation of thiobarbituric acid reactive substances, superoxide dismutase, reduced glutathione and catalase was used to assess oxidative stress. H(+)/K(+) ATPase activity, gastric mucosal blood flow and gastric acid secretion were measured to study the mechanism of methylene induced gastric ulcer.

RESULTS

Methylene blue (100 mg kg(-1), p.o.) produced marked ulceration of the gastric mucosa due to increased levels of thiobarbituric acid reactive substances, activities of the H(+)/K(+) ATPase and superoxide dismutase, and decreased blood flow to the gastric mucosa, activity of catalase combined with reduced glutathione levels.

DISCUSSION

It may be concluded that methylene blue activates the H(+)/K(+) ATPase to increase gastric acid secretion and reduces blood supply to gastric mucosa to produce oxidative stress that subsequently causes ulceration of gastric mucosa. Methylene blue can be used as an ulcerogenic agent to study mechanisms of investigational antiulcer agents.

Hyperpyrexia and prolonged postoperative disorientation following methylene blue infusion during parathyroidectomy

A 65-year-old man underwent parathyroidectomy for hyperparathyroidism secondary to renal failure. Intra-operatively he received methylene blue infusion (7.5 mg.kg(-1), a total of 650 mg in 500 ml 0.9% sodium chloride) for visualisation of parathyroid glands. At the end of surgery, following extubation he developed agitation, intense shivering and hyperpyrexia, and his level of consciousness decreased to a Glasgow Coma score of 7. The differential diagnoses included methylene blue toxicity or malignant hyperpyrexia. His lungs were ventilated, and intravenous dantrolene was administered to control hyperpyrexia. Haemodialysis was started to remove the methylene blue dye. We review the literature on the pharmacological actions of methylene blue, and discuss the differential diagnosis and management of this patient.

Relaxation of canine corporal smooth muscle relaxation by ginsenoside saponin Rg3 is independent from eNOS activation

It has been reported that nitric oxide (NO) is involved in the relaxation mechanism of ginsenoside saponin in various smooth muscle in experimental animals. Although ginsenoside Rg(3) showed both endothelium-dependent and -independent component relaxation in vascular smooth muscle, the action mechanism of the relaxation of corporal muscle is not clear. We, thus, investigated the relaxation mechanism of ginsenoside Rg(3) using isolated canine corpus cavernosum. Ginsenoside Rg(3) concentration-dependently relaxed the canine corpus cavernosum that had been contracted by phenylephrine (PE), in which IC(50) was 1.68 x 10(-5) g/ml. Ginsenoside Rg(3) significantly (P < 0.05) potentiated acetylcholine (ACh)-induced relaxation in endothelium intact corpus cavernosum. Methylene blue (MB) but not N(omega)-nitro-L-arginine methylester (L-NAME) or ODQ (1H-[1,2,4]oxadiazol-[4,3-]quinoxsalin-1-one) modified the dose-response curve of ginsenoside Rg(3). Ginsenoside Rg(3) also significantly potentiated relaxation response to UV light in the presence of streptozotocin (STZ), which was almost completely (P < 0.01) blocked by ODQ. Ginsenoside Rg(3) concentration-dependently inhibited corporal phosphodiesterases (PDE), which resulted in increase of cyclic adenosine monophosphate (cAMP) as well as cyclic guanosine monophosphate (cGMP) contents in corporal smooth muscles. MB inhibited the accumulation of cGMP but not cAMP by ginsenoside Rg(3). These results indicate that mechanism responsible for the relaxation by ginsenoside Rg(3) is not by stimulating endothelial nitric oxide synthase (eNOS) of the canine corporal smooth muscle but by increasing cyclic nucleotide levels through PDE inhibition.

Evidence for a possible role for nitric oxide in the modulation of heart activity in Achatina fulica and Helix aspersa

The effects of nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine, S-nitroso-l-glutathione, sodium nitroprusside and sodium nitrite were investigated on the activity of the isolated hearts of Achatina fulica and Helix aspersa. NO donors inhibited heart activity in a concentration-dependent manner. The only exception was sodium nitroprusside, which excited H. aspersa heart. The inhibitory effects of these NO donors were reduced by the NO scavenger, methylene blue, the guanylyl cyclase inhibitor, 1H-(1,2,4) Oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), and potentiated by 8-Br-cGMP and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). Acetylcholine also inhibited the heart activity, and this inhibition was reduced by methylene blue and ODQ. Positive NADPH-diaphorase staining was located in the outer pericardial layer of the heart of A. fulica. The present results provide evidence that NO may modulate the activity of gastropod hearts, and this modulation may modify the inhibitory action of acetylcholine on heart activity.

Use of methylene blue and N,O-carboxymethylchitosan to prevent postoperative adhesions in a rat uterine horn model

OBJECTIVES

To compare the anti-adhesion potential of 1% methylene blue (MB) solution and 2% N,O-carboxymethylchitosan (NOCC) gel in a rat uterine horn model.

DESIGN

Experimental animal study.

SETTING

University medical center.

ANIMAL(S)

Forty female Wistar albino rats randomized into four groups.

INTERVENTION(S)

We examined the effects of 1% MB solution and 2% NOCC gel to reduce the extent and severity of postoperative adhesions in a rat uterine horn model: no adjuvant therapy in control group, 2 mL of normal saline (NS) solution in NS group, 2 mL of 1% MB solution in MB group, and 2 mL of 2% NOCC gel in NOCC group was instilled onto uterine horns of the rats.

MAIN OUTCOME MEASURE(S)

Adhesions were scored according to their extent and severity.

RESULT(S)

The extent and severity scores of adhesions in MB and NOCC groups were significantly lower than those of control and NS groups. There was no statistically significant difference between the extent and severity scores of adhesions between MB and NOCC groups.

CONCLUSION(S)

These findings suggest that MB and 2% NOCC gel should be considered as an adjuvant in the prevention of postoperative intra-abdominal adhesions. Future experimental and clinical studies are required to find their optimal formulation and usage.

Muscarinic regulation of cardiac ion channels

The parasympathetic component of the autonomic nervous system plays an important role in the physiological regulation of cardiac function by exerting significant influence over the initiation as well as propagation of electrical impulses, in addition to being able to regulate contractile force. These effects are mediated in whole or in part through changes in ion channel activity that occur in response to activation of M(2) muscarinic cholinergic receptors following release of the neurotransmitter acetylcholine. The coupling of M(2) receptor activation to most changes in cardiac ion channel function can be explained by one of two general paradigms. The first involves direct G protein-dependent regulation of ion channel activity. The second involves indirect regulation of ion channel activity through modulation of cAMP-dependent responses. This review focuses on recent advances in our understanding of the mechanisms by which M(2) muscarinic receptor activation both inhibits and facilitates cAMP-dependent ion channel responses in the heart.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Heparin attenuates norepinephrine-induced venoconstriction

Reversal by heparin of norepinephrine-induced constriction of normal hand veins was studied. Venous size was measured using a linear variable differential transformer (LVDT) during infusions of saline, norepinephrine, insulin and norepinephrine, and graded doses of heparin with norepinephrine. Heparin reduced the venoconstrictive effects of norepinephrine (p < 0.01), with the effects beginning at 18.5 nmol/min (0.05 U/min) and reaching a maximum between 185 nmol/min and 1.85 mumol/min (0.5 and 5 U/min). Maximal heparin-induced venorelaxation correlated with the maximal insulin effect within individuals (r = 0.8, p < 0.01) and was unchanged by the addition of insulin. Methylene blue, a non-specific inhibitor of the nitric oxide cGMP cascade, reduced heparin-induced venorelaxation. In conclusion, heparin in either physiologic or pharmacologic concentration attenuated norepinephrine-induced venoconstriction. A common mechanism of venorelaxation by heparin and insulin is not excluded given the correlation and lack of additivity of maximum effects and their inhibition by methylene blue.

Methylene blue based protein solder for vascular anastomoses: an in vitro burst pressure study

BACKGROUND AND OBJECTIVE

Attempts at sutureless anastomoses have used protein-based solders containing chromophores [Oz et al., J Vasc Surg 1990;11:718; Poppas et al., J Urol 1998150:1052] to enhance the strength of laser anastomoses. Reports have described the use of indocyanine green [Oz et al., Surg Forum 1989;316.], fuschin, and fluorescein isothiocyanate as chromophores [Chuck et al. , Lasers Surg Med 1989;9:471; Vance et al., Lasers Med Sci 1988;3:219]. Methylene blue (MB) is a chromophore with absorption peaks in the 600-700 nm region whose use has not been reported in laser-assisted vascular anastomoses. Therefore, we set out to produce and characterise a MB-containing protein solder. The absorption and burst pressure characteristics have been investigated and described as well as a brief review of the chemical and biological properties of MB.

STUDY DESIGN/MATERIALS AND METHODS

The MB and porcine serum albumin (PSA)-based solder was produced and used to form end-to-end anastomoses in porcine splenic arteries. The solder was activated using a laser diode emitting at 670 nm. The burst pressures of the anastomoses were tested, and the results analysed as a function of MB concentration and absorption. In addition, the relationship between MB concentration and absorption was examined.

RESULTS

A dose-response relationship was found between the measured absorption of the solder and the burst pressure of the anastomoses formed. Burst pressures exceeding physiological levels were found. Changes in MB concentration revealed a marked negative deviation from Beer's law at 670 nm, owing to the monomer-dimer-trimer equilibria.

CONCLUSION

PSA with MB solder is able to form high-quality end-to-end anastomoses, with immediate burst pressure profiles similar to those previously described for sutured [Quigley et al., Microsurgery 1985;6:229], lasered [Quigley et al., Microsurgery 1985;6:229], and soldered anastomoses [Small et al., J Clin Laser Med Surg 1997;15:205]. The relationship between burst pressure strength and chromophore absorption is discussed.

Not acute but chronic hypertriglyceridemia is associated with impaired endothelium-dependent vasodilation: reversal after lipid-lowering therapy by atorvastatin

There is controversy regarding the relation between hypertriglyceridemia (HTG) and endothelial function. This study was designed to investigate endothelial function in a patient group with chronic HTG, before and during lipid-lowering therapy by atorvastatin. In addition, the effects of acute HTG on endothelial function were studied in normolipidemic individuals. Eight male patients with chronic HTG were studied before and after 6 weeks of lipid-lowering treatment with 80 mg atorvastatin once daily. Ten age-matched control subjects were studied at baseline and immediately after a high-dose infusion of artificial triglycerides. The endothelium-dependent response to serotonin was attenuated in the HTG group, whereas the response to acetylcholine was comparable to the response in the control group. The response to the endothelium-independent vasodilator nitroprusside was comparable in both groups. In response to atorvastatin therapy, serum triglyceride and cholesterol levels decreased significantly by 43% (paired t test, P=0.017) and 38% (paired t test, P=0.012), respectively. After 6 weeks of treatment, the forearm blood flow response to serotonin improved from 63% to 106% (ANOVA, P<0.001). Induction of acute HTG in the control subjects did not affect the forearm blood flow responses to serotonin and nitroprusside; however, the response to acetylcholine was paradoxically increased. In conclusion, patients with chronic HTG have an impaired endothelium-dependent vasodilation to serotonin that is normalized after 6 weeks of lipid-lowering therapy by atorvastatin.

Effects of a novel guanylyl cyclase inhibitor on the vascular actions of nitric oxide and peroxynitrite in immunostimulated smooth muscle cells and in endotoxic shock

OBJECTIVE

Nitric oxide (NO), produced by the inducible isoform of NO synthase (NOS) in circulatory shock exerts cytotoxic and vasodilator effects. Part of these effects are mediated by formation of peroxynitrite, a toxic oxidant produced by the rapid reaction of NO and superoxide. Other parts of the vascular actions of NO in shock are thought to be mediated by the action of NO on the soluble guanylyl cyclase (GC) in the smooth muscle and subsequent decrease in the intracellular calcium levels. Using 1H-(1,2,4)oxadiazolo(4,3-alpha)quinoxalin-1 -one (ODQ), a potent inhibitor of GC, we studied the role of GC activation in the NO- and peroxynitrite-related vascular alterations.

DESIGN

In vitro: Controlled experiment using cultured rat aortic smooth muscle cells. In vivo: Prospective, randomized, controlled animal study.

SETTING

Experimental laboratory.

SUBJECTS

Male Wistar rats and male Swiss mice.

INTERVENTIONS

In vitro: a) Stimulation of rat aortic smooth muscle cells with bacterial lipopolysaccharide (LPS) and gamma-interferon, measurement of the production of nitrite and nitrate (breakdown products of NO), and suppression of mitochondrial respiration for 24 to 48 hrs, in the presence or absence of ODQ; and b) in norepinephrine-precontracted endothelium-denuded thoracic aortic rings, exposure to LPS (10 ng/mL) in the presence or absence of ODQ. In vivo: Rats treated in vivo with LPS (10 mg/kg iv for 3 hrs) and mice challenged with 60 mg/kg LPS ip, in the presence or absence of ODQ.

MEASUREMENTS AND MAIN RESULTS

Stimulation of rat aortic smooth muscle cells with bacterial LPS and gamma-interferon induced the production of nitrite and nitrate (breakdown products of NO) and suppression of mitochondrial respiration for 24 to 48 hrs. The amount of NO produced was slightly enhanced with ODQ (10-100 EM), whereas the suppression of mitochondrial respiration was not affected by ODQ (1-100 microM). ODQ did not affect the degree of suppression of mitochondrial respiration in response to NO donor agents or to peroxynitrite. Exposure to LPS (10 ng/mL) for 6 hrs caused a time-dependent relaxation of norepinephrine-precontracted endothelium-denuded thoracic aortic rings. This response was caused by the expression of inducible NOS and could be blocked by pharmacologic inhibitors of NOS such as N(G)-methylL-arginine. ODQ (1 microM) prevented the LPS-induced loss of vascular tone in this experimental system. Similar to the in vitro responses, there was a significant suppression of the norepinephrine-induced contractions in ex vivo experiments, in which rings were taken from animals treated in vivo with LPS (10 mg/kg for 3 hrs). ODQ treatment in vitro (1 microM) caused a complete restoration of the contractile responses. In mice challenged with 60 mg/kg LPS ip, ODQ (20 mg/kg), given either as a pretreatment or as a 4-hr posttreatment, improved survival at 24-144 hrs.

CONCLUSION

These studies indicate that GC activation does not contribute to NO- or peroxynitrite-induced cytotoxicity but does contribute to the vascular hyporeactivity induced by endotoxin in vitro and in vivo. GC inhibition alone is sufficient to influence survival in a murine model of severe sepsis.

Comparison of two soluble guanylyl cyclase inhibitors, methylene blue and ODQ, on sodium nitroprusside-induced relaxation in guinea-pig trachea

To clarify further the role of cyclic GMP in mediating the relaxant response in guinea-pig trachea induced by sodium nitroprusside (SNP), the effects of soluble guanylyl cyclase inhibitors, methylene blue and 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) on SNP-induced muscle relaxation and cyclic GMP accumulation were determined. SNP (0.3-100 microM) evoked a concentration-dependent relaxation of guinea-pig isolated tracheas precontracted with 0.3 microM carbachol. Preincubation of the preparations with methylene blue (10, 30 and 100 microM) resulted in a slight but concentration-dependent prevention of the relaxant response to SNP. In contrast, the relaxation to SNP was extensively prevented by 3 microM ODQ and almost abolished by 10 microM ODQ. SNP (30 microM) induced a significant elevation of cyclic GMP accumulation (from 1.34+/-0.14 to 5.39+/-0.28 pmol mg(-1) protein, n= 5; P<0.001), which was partially attenuated by 100 microM methylene blue (3.11+/-0.51 pmol mg(-1) protein, n=5; P<0.05), whereas completely abolished by 10 microM ODQ (1.31+/-0.28 pmol mg(-1) protein, n = 5; P<0.001). Methylene blue, but not ODQ and Nomega-nitro-L-arginine methyl ester (L-NAME), caused a concentration-dependent contraction in the tracheal preparation. The tension produced by 100 microM methylene blue was 41.8+/-4.3% (0.3 microM carbachol as 100%; n = 12). Moreover, the non-selective muscarinic receptor antagonist atropine and the M3-selective antagonist 4-diphenylacetoxy-N-methylpiperidine methiodine greatly inhibited the contractile effect evoked by methylene blue (100 microM). In conclusion, this study provides substantial evidence that SNP-induced muscle relaxation in guinea-pig trachea is completely via a cyclic GMP-dependent mechanism. Furthermore, ODQ, but not methylene blue, will likely become an important tool in differentiating between cyclic GMP-dependent and -independent effects of nitric oxide.