Nitric oxide generation, tachyphylaxis and cross-tachyphylaxis from nitrovasodilators in vivo

Blockade of the natriuretic peptide clearance receptor attenuates proteinuria in a mouse model of focal segmental glomerulosclerosis

Glomerular podocytes play a key role in proteinuric diseases. Accumulating evidence suggests that cGMP signaling has podocyte protective effects. The major source of cGMP generation in podocytes is natriuretic peptides. The natriuretic peptide clearance receptor (NPRC) binds and degrades natriuretic peptides. As a result, NPRC inhibits natriuretic peptide-induced cGMP generation. To enhance cGMP generation in podocytes, we blocked natriuretic peptide clearance using the specific NPRC ligand ANP(4-23). We then studied the effects of NPRC blockade in both cultured podocytes and in a mouse transgenic (TG) model of focal segmental glomerulosclerosis (FSGS) created in our laboratory. In this model, a single dose of the podocyte toxin puromycin aminonucleoside (PAN) causes robust albuminuria in TG mice, but only mild disease in non-TG animals. We found that natriuretic peptides protected cultured podocytes from PAN-induced apoptosis, and that ANP(4-23) enhanced natriuretic peptide-induced cGMP generation in vivo. PAN-induced heavy proteinuria in vehicle-treated TG mice, and this increase in albuminuria was reduced by treatment with ANP(4-23). Treatment with ANP(4-23) also reduced the number of mice with glomerular injury and enhanced urinary cGMP excretion, but these differences were not statistically significant. Systolic BP was similar in vehicle and ANP(4-23)-treated mice. These data suggest that: 1. Pharmacologic blockade of NPRC may be useful for treating glomerular diseases such as FSGS, and 2. Treatment outcomes might be improved by optimizing NPRC blockade to inhibit natriuretic peptide clearance more effectively.

Pulmonary Vasodilation by Intravenous Infusion of Organic Mononitrites Of 1,2-Propanediol in Acute Pulmonary Hypertension Induced by Aortic Cross Clamping and Reperfusion: A Comparison With Nitroglycerin in Anesthetized Pigs

INTRODUCTION

Suprarenal aortic cross clamping (SRACC) and reperfusion may cause acute pulmonary hypertension and multiple organ failure.

HYPOTHESIS

The organic mononitrites of 1,2-propanediol (PDNO), an nitric oxide donor with a very short half-life, are a more efficient pulmonary vasodilator and attenuator of end-organ damage and inflammation without significant side effects compared with nitroglycerin and inorganic nitrite in a porcine SRACC model.

METHODS

Anesthetized and instrumented domestic pigs were randomized to either of four IV infusions until the end of the experiment (n = 10 per group): saline (control), PDNO (45 nmol kg min), nitroglycerin (44 nmol kg min), or inorganic nitrite (a dose corresponding to PDNO). Thereafter, all animals were subjected to 90 min of SRACC and 10 h of reperfusion and protocolized resuscitation. Hemodynamic and respiratory variables as well as blood samples were collected and analysed.

RESULTS

During reperfusion, mean pulmonary arterial pressure and pulmonary vascular resistance were significantly lower, and stroke volume was significantly higher in the PDNO group compared with the control, nitroglycerin, and inorganic nitrite groups. In parallel, mean arterial pressure, arterial oxygenation, and fraction of methaemoglobin were similar in all groups. The serum concentration of creatinine and tumor necrosis factor alpha were lower in the PDNO group compared with the control group during reperfusion.

CONCLUSIONS

PDNO was an effective pulmonary vasodilator and appeared superior to nitroglycerin and inorganic nitrite, without causing significant systemic hypotension, impaired arterial oxygenation, or methaemoglobin formation in an animal model of SRACC and reperfusion. Also, PDNO may have kidney-protective effects and anti-inflammatory properties.

Effect of Oral Administration of Losartan, Prazosin, and Verapamil on Peritoneal Solute Transport in Continuous Ambulatory Peritoneal Dialysis Patients

Background

Several intraperitoneally administered drugs have been shown to modify transport of peritoneal solute and fluid. Fewer studies, however, have evaluated the effect of orally administered drugs. The present study was performed to evaluate the effects of oral losartan, prazosin, and verapamil on peritoneal membrane transport during a peritoneal equilibration test (PET), as well as the effects on creatinine clearance (CrCl), Kt/V urea, 24-hour protein in drained dialysate, and drained volume.

Methods

This was an open, controlled, crossover clinical trial performed in 20 patients on continuous ambulatory peritoneal dialysis. All subjects used four 2-L 1.5% glucose dialysis exchanges per day. After a 7-day washout period (without antihypertensives), they had a baseline standard PET and dialysis adequacy assessment performed. Subsequently, they were randomly allocated to receive the first of three study drugs (losartan, prazosin, and verapamil), which were administered orally for a 7-day period. Immediately after each drug period, patients had a new 3-day washout and subsequently started the next drug, until they had received each of the three drugs. On the last day of administration of each drug, patients were subjected to a new PET and adequacy of dialysis evaluation.

Results

None of the studied drugs significantly modified the peritoneal transport of creatinine, glucose, urea, sodium, potassium, or total protein as evaluated by PET. Verapamil significantly increased peritoneal CrCl [51.3 (44.3 – 53.3) vs baseline 45.8 (41.4 – 50.5) L/week/1.73 m2, p < 0.05], weekly Kt/V urea [1.75 (1.60 – 1.78) vs baseline 1.59 (1.54 – 1.73), p < 0.05], and drained dialysate volume [8.80 (8.30 – 8.96) vs baseline 8.44 (8.20 – 8.50) L/day, p < 0.05].

Conclusions

Oral administration of losartan, prazosin, and verapamil did not modify the peritoneal transport of solutes during a 4-hour PET. Oral verapamil significantly increased CrCl, Kt/V urea, and 24-hour drained dialysate volume. It is most likely that verapamil increases peritoneal (hydraulic) conductivity, and then net ultrafiltration volume and convective transport of urea, creatinine, and protein. Verapamil could be considered as an alternative in patients requiring increased dialysis dose and/or ultrafiltration.

A Comparative Study of Inhaled Nitric Oxide and an Intravenously Administered Nitric Oxide Donor in Acute Pulmonary Hypertension

Purpose

Inhaled nitric oxide (iNO) selectively vasodilates the pulmonary circulation but the effects are sometimes insufficient. Available intravenous (iv) substances are non-selective and cause systemic side effects. The pulmonary and systemic effects of iNO and an iv mono-organic nitrite (PDNO) were compared in porcine models of acute pulmonary hypertension.

Methods

In anesthetized piglets, dose–response experiments of iv PDNO at normal pulmonary arterial pressure (n=10) were executed. Dose–response experiments of iv PDNO (n=6) and iNO (n=7) were performed during pharmacologically induced pulmonary hypertension (U46619 iv). The effects of iv PDNO and iNO were also explored in 5 mins of hypoxia-induced increase in pulmonary pressure (n=2-4).

Results

PDNO (15, 30, 45 and 60 nmol NO kg⁻¹ min⁻¹ iv) and iNO (5, 10, 20 and 40 ppm which corresponded to 56, 112, 227, 449 nmol NO kg⁻¹ min⁻¹, respectively) significantly decreased the U46619-increased mean pulmonary arterial pressure (MPAP) and pulmonary vascular resistance (PVR) to a similar degree without significant decreases in mean arterial pressure (MAP) or systemic vascular resistance (SVR). iNO caused increased levels of methemoglobin. At an equivalent delivered NO quantity (iNO 5 ppm and PDNO 45 nmol kg⁻¹ min⁻¹ iv), PDNO decreased PVR and SVR significantly more than iNO. Both drugs counteracted hypoxia-induced pulmonary vasoconstriction and they decreased the ratio of PVR and SVR in both settings.

Conclusion

Intravenous PDNO was a more potent pulmonary vasodilator than iNO in pulmonary hypertension, with no severe side effects. Hence, this study supports the potential of iv PDNO in the treatment of acute pulmonary hypertension.

Treatment with new organic nitrites in pulmonary hypertension of acute experimental pulmonary embolism

Acute pulmonary embolism may cause right heart failure due to increased pulmonary vascular resistance and arterial hypoxemia. Effective vasodilator therapy of the pulmonary hypertension is highly needed. Therefore, we investigated the effects of a newly developed effective pulmonary vasodilator, the organic mononitrites of 1,2-propanediol (PDNO), in a rabbit model of acute pulmonary embolism. In anesthetized and ventilated rabbits, systemic and pulmonary hemodynamics, exhaled nitric oxide (NO), plasma nitrite concentration, and blood gases were monitored. First, dose-response experiments with intravenous and left heart ventricle infusions of PDNO and inorganic nitrite were done in naive animals and in pulmonary hypertension induced by a thromboxane A2 analogue. Second, acute pulmonary embolism was induced and either PDNO or placebo were administered intravenously within 20 minutes and evaluated within 1 hour after pulmonary embolization. PDNO intravenously, in contrast to inorganic nitrite intravenously, increased exhaled NO and counteracted pulmonary hypertension and vasodilated the systemic circulation, dose-dependently, thereby showing efficient NO donation. Pulmonary embolization induced pulmonary hypertension and gas exchange disturbances. PDNO significantly decreased and normalized pulmonary vascular resistance and the right ventricle rate-pressure product, without causing tolerance, with no significant side effects on the systemic circulation, nor on blood-gas values or on methemoglobin formation. In conclusion, PDNO is a NO donor and an efficient vasodilator in the pulmonary circulation. Treatment with this or similar organic nitrites intravenously may be a future option to avoid right heart failure in life-threatening acute pulmonary embolism.

Glaucoma - Next Generation Therapeutics: Impossible to Possible

The future of next generation therapeutics for glaucoma is strong. The recent approval of two novel intraocular pressure (IOP)-lowering drugs with distinct mechanisms of action is the first in over 20 years. However, these are still being administered as topical drops. Efforts are underway to increase patient compliance and greater therapeutic benefits with the development of sustained delivery technologies. Furthermore, innovations from biologics- and gene therapy-based therapeutics are being developed in the context of disease modification, which are expected to lead to more permanent therapies for patients. Neuroprotection, including the preservation of retinal ganglion cells (RGCs) and optic nerve is another area that is actively being explored for therapeutic options. With improvements in imaging technologies and determination of new surrogate clinical endpoints, the therapeutic potential for translation of neuroprotectants is coming close to clinical realization. This review summarizes the aforementioned topics and other related aspects.

Organic mononitrites of 1,2-propanediol act as an effective NO-releasing vasodilator in pulmonary hypertension and exhibit no cross-tolerance with nitroglycerin in anesthetized pigs

Purpose

Clinically available intravenous (IV) nitric oxide (NO) donor drugs such as nitroglycerin (GTN) cause systemic hypotension and/or tolerance development. In a porcine model, novel NO donor compounds – the organic mononitrites of 1,2-propanediol (PDNO) – were compared to GTN with regard to pulmonary selectivity and tolerance development. The vasodilatory effects of inorganic nitrite were investigated.

Materials and methods

In anesthetized piglets, central hemodynamics were monitored. At normal pulmonary vascular resistance (PVR), IV infusions of PDNO (15–60 nmol kg⁻¹ min⁻¹), GTN (13–132 nmol kg⁻¹ min⁻¹), and inorganic nitrite (dosed as PDNO) were administered. At increased PVR (by U46619 IV), IV infusions of PDNO (60–240 nmol kg⁻¹ min⁻¹) and GTN (75–300 nmol kg⁻¹ min⁻¹) before and after a 5 h infusion of GTN (45 nmol kg⁻¹ min⁻¹) were given.

Results

At normal PVR, PDNO (n=12) and GTN (n=7) caused significant dose-dependent decreases in mean systemic and pulmonary arterial pressures, whereas inorganic nitrite (n=13) had no significant effect. At increased PVR, PDNO (n=6) and GTN (n=6) significantly decreased mean systemic and pulmonary pressures and resistances, but only PDNO reduced the ratio between pulmonary and systemic vascular resistances significantly. After the 5 h GTN infusion, the hemodynamic response to GTN infusions (n=6) was significantly suppressed, whereas PDNO (n=6) produced similar hemodynamic effects to those observed before the GTN infusion.

Conclusion

PDNO is a vasodilator with selectivity for pulmonary circulation exhibiting no cross-tolerance to GTN, but GTN causes non selective vasodilatation with substantial tolerance development in the pulmonary and systemic circulations. Inorganic nitrite has no vasodilatory properties at relevant doses.

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.

Biodegradable poly(lactic-co-glycolic acid) microspheres loaded with S-nitroso-N-acetyl-D-penicillamine for controlled nitric oxide delivery

Nitric oxide (NO) is a fascinating and important endogenous free-radical gas with potent antimicrobial, vasodilating, smooth muscle relaxant, and growth factor stimulating effects. However, its wider biomedical applicability is hindered by its cumbersome administration, since NO is unstable especially in biological environments. In this work, to ultimately develop site-specific controlled release vehicles for NO, the NO donor S-nitroso-N-acetyl-D-penicillamine (SNAP) was encapsulated within poly(lactic-co-glycolic acid) 50:50 (PLGA) microspheres by using a solid-in-oil-in-water emulsion solvent evaporation method. The highest payload was 0.56(±0.01) μmol SNAP/mg microspheres. The in vitro release kinetics of the donor were controlled by the bioerosion of the PLGA microspheres. By using an uncapped PLGA (Mw=24,000-38,000) SNAP was slowly released for over 10days, whereas by using the ester capped PLGA (Mw=38,000-54,000) the release lasted for over 4weeks. The presence of copper ions and/or ascorbate in solution was necessary to efficiently decompose the released NO donor and obtain sustained NO release. It was also demonstrated that light can be used to induce rapid NO release from the microspheres over several hours. SNAP exhibited excellent storage stability when encapsulated in the PLGA microspheres. These new microsphere formulations may be useful for site-specific administration and treatment of pathologies associated with dysfunction in endogenous NO production, e.g. treatment of diabetic wounds, or in diseases involving other biological functions of NO including vasodilation, antimicrobial, anticancer, and neurotransmission.

Vascular and metabolic effects of the haem oxygenase-1 inducer haem arginate in subjects with the metabolic syndrome: A translational cross-over study

This translational randomized and vehicle-controlled cross-over study was performed to assess the impact of haem arginate treatment on haem oxygenase-1 induction, endothelial function and insulin sensitivity in subjects with the metabolic syndrome (n = 14). Both treatment periods consisted of 5 days. Haem arginate or vehicle (l-arginine) was administered intravenously on Days 1 and 3. Forearm blood flow in response to acetylcholine and nitroglycerine was measured by venous occlusion plethysmography (Day 3), insulin sensitivity by a hyperinsulinaemic clamp procedure (Day 5). Haem arginate did not improve endothelial function or insulin sensitivity but significantly reduced the vasodilator response to nitroglycerine (p < 0.01). These negative findings are in contrast to the preclinical data, which may be due to short duration of therapy and limited haem oxygenase-1 induction as well as interference by markedly elevated plasma haem levels observed after haem arginate treatment (p < 0.01). Future studies should pay attention to the delicate balance between sufficient dosing and timely normalization of plasma haem levels.

The nitric oxide-donor molsidomine modulates the innate inflammatory response in a mouse model of muscular dystrophy

Inflammation plays a crucial role in muscle remodeling and repair after acute and chronic damage, in particular in muscular dystrophies, a heterogeneous group of genetic diseases leading to muscular degeneration. Defect of nitric oxide (NO) generation is a key pathogenic event in muscular dystrophies, thus NO donors have been explored as new therapeutics for this disease. We have investigated the immune-modulating effect of one of such drugs, molsidomine, able to slow the progression of muscular dystrophy in the α-Sarcoglican-null mice, a model for the limb girdle muscular dystrophy 2D, sharing several hallmarks of muscle degeneration with other muscular dystrophies. α-Sarcoglican-null mice were treated with molsidomine and drug effects on the inflammatory infiltrates and on muscle repair were assessed at selected time points. We found that molsidomine treatment modulates effectively the characteristics of the inflammatory infiltrate within dystrophic muscles, enhancing its healing function. Initially molsidomine amplified macrophage recruitment, promoting a more efficient clearance of cell debris and effective tissue regeneration. At a later stage molsidomine decreased significantly the extent of the inflammatory infiltrate, whose persistence exacerbates muscle damage: most of the remaining macrophages displayed characteristics of the transitional population, associated with reduced fibrosis and increased preservation of the muscle tissue. The dual action of molsidomine, the already known NO donation and the immunomodulatory function we now identified, suggests that it has a unique potential in tissue healing during chronic muscle damage. This, alongside its already approved use in human, makes molsidomine a drug with a significant therapeutic potential in muscular dystrophies.

Local delivery of nitric oxide: targeted delivery of therapeutics to bone and connective tissues

Non-invasive treatment of injuries and disorders affecting bone and connective tissue remains a significant challenge facing the medical community. A treatment route that has recently been proposed is nitric oxide (NO) therapy. Nitric oxide plays several important roles in physiology with many conditions lacking adequate levels of NO. As NO is a radical, localized delivery via NO donors is essential to promoting biological activity. Herein, we review current literature related to therapeutic NO delivery in the treatment of bone, skin and tendon repair.

Is Nitric Oxide (NO) the Last Word in Radiosensitization? A Review

As a short-lived radical that diffuses across membranes, rather than interacting with membrane-bound receptors, nitric oxide (NO) represents a significant departure from synthetically derived radiosensitizers. An endogenous compound, NO may equal or surpass its molecular cousin, oxygen, as a hypoxic radiosensitizer, through pleiotropic phenotypic effects on tumor perfusion, cell signaling, mitochondrial respiration, the fixation of radiation-induced damage, and the radioprotection of normal tissue. However, unlike oxygen, in the context of radiosensitization, the clinical role and utility of NO are poorly understood, with often contradictory and controversial reported effects: whether NO functions as a radiosensitizer may ultimately be contextual to the tumor microenvironment. This may make NO manipulation an ideal candidate for a personalized radiosensitization approach tailored to specific patient and tumor types/microenvironmental characteristics. Effective delivery of NO both systemically and directly to the tumor may be critical to the success of this approach. Compounds that release NO or NO precursors have the potential to drive innovation and result in a new fertile branch of the radiosensitizer tree.

Formation of new bioactive organic nitrites and their identification with gas chromatography-mass spectrometry and liquid chromatography coupled to nitrite reduction

Nitric oxide (NO) donors, notably organic nitrates and nitrites are used therapeutically but tolerance develops rapidly, making the use of e.g. nitroglycerin difficult. NO donation in the pulmonary vascular bed might be useful in critically ill patients. Organic nitrites are not associated with tachyphylaxis but may induce methaemoglobinemia and systemic hypotension which might hamper their use. We hypothesised that new lung-selective NO donors can be identified by utilizing exhaled NO as measure for pulmonary NO donation and systemic arterial pressure to monitor hypotension and tolerance development. Solutions of alcohols and carbohydrates were reacted with NO gas and administered to ventilated rabbits for evaluation of in vivo NO donation. Chemical characterization was made by liquid chromatography with on-line nitrite reduction (LC-NO) and by gas chromatography-mass spectrometry (GC-MS). In vivo experiments showed that the hydroxyl-containing compounds treated with NO gas yielded potent NO donors, via nitrosylation to organic nitrites. Analyses by LC-NO showed that the reaction products were able to release NO in vitro. In GC-MS the reaction products were determined to be the organic nitrites, where some are new chemical entities. Non-polar donors preferentially increased exhaled NO with less effect on systemic blood pressure whereas more polar molecules had larger effects on systemic blood pressure and less on exhaled NO. We conclude that new organic nitrites suitable for intravenous administration are produced by reacting NO gas and certain hydroxyl-containing compounds in aqueous solutions. Selectivity of different organic nitrites towards the pulmonary and systemic circulation, respectively, may be determined by molecular polarity.

Chronic nitric oxide synthase blockade desensitizes the heart to the negative metabolic effects of nitric oxide

The consequences of chronic nitric oxide synthase (NOS) blockade on the myocardial metabolic and guanylyl cyclase stimulatory effects of exogenous nitric oxide (NO) were determined. Thirty-three anesthetized open-chest rabbits were randomized into four groups: control, NO donor S-nitroso-N-acetyl-penicillamine (SNAP, 10(-4 )M), NOS blocking agent N(G)-nitro-L-arginine methyl ester (L-NAME, 20 mg/kg/day) for 10 days followed by a 24 hour washout and L-NAME for 10 days followed by a 24 hour washout plus SNAP. Myocardial O(2) consumption was determined from coronary flow (microspheres) and O(2) extraction (microspectrophotometry). Cyclic GMP and guanylyl cyclase activity were determined by radioimmunoassay. There were no baseline metabolic, functional or hemodynamic differences between control and L-NAME treated rabbits. SNAP in controls caused a reduction in O(2) consumption (SNAP 5.9+/-0.6 vs. control 8.4+/-0.8 ml O(2)/min/100 g) and a rise in cyclic GMP (SNAP 18.3+/-3.8 vs. control 10.4+/-0.9 pmol/g). After chronic L-NAME treatment, SNAP caused no significant changes in O(2) consumption (SNAP 7.1+/-0.8 vs. control 6.4+/-0.7) or cyclic GMP (SNAP 14.2+/-1.8 vs. control 12.1+/-1.3). In controls, guanylyl cyclase activity was significantly stimulated by SNAP (216.7+/-20.0 SNAP vs. 34.4+/-2.5 pmol/mg/min base), while this increase was blunted after L-NAME (115.9+/-24.5 SNAP vs. 24.9+/-4.7 base). These results demonstrated that chronic NOS blockade followed by washout blunts the response to exogenous NO, with little effect on cyclic GMP or myocardial O(2) consumption. This was related to reduced guanylyl cyclase activity after chronic L-NAME. These results suggest that, unlike many receptor systems, the NO-cyclic GMP signal transduction system becomes downregulated upon chronic inhibition.

Intermedin/adrenomedullin-2 dilates the rat pulmonary vascular bed: dependence on CGRP receptors and nitric oxide release

Intermedin/adrenomedullin-2 (IMD/AM2) is a 47 amino acid peptide formed by enzymatic degradation of preprointermedin. The present study was undertaken to investigate the effects of rat IMD (rIMD) in the isolated buffer perfused rat lung (IBPR) under resting conditions and under conditions of elevated pulmonary vasoconstrictor tone (PVT). Under resting conditions in the IBPR, rIMD had little or no activity. When PVT was actively increased by infusion of U46619, bolus injection of IMD decreased pulmonary arterial pressure (PAP) in a dose-dependent manner. Since the pulmonary perfusion rate and left atrial pressure were constant, these reductions in PAP directly reflect reductions in pulmonary vascular resistance (PVR). The pulmonary vasodilator response to rIMD, when compared to calcitonin gene-related peptide (CGRP) on a molar basis, was greater at the lowest and midrange doses. The degree of inhibition by CGRP8-37 on pulmonary vasodilator response to rIMD was significantly less when compared to CGRP. Pretreatment with L-nitro-arginine-methyl ester (L-NAME), unlike meclofenamate and glybenclamide, significantly reduced the pulmonary vasodilator responses to rIMD. rIMD administration induced cross-tachyphylaxis to the pulmonary vasodilator response to CGRP whereas CGRP administration did not alter the ability of rIMD to dilate the IBPR. Pulmonary vasodilator responses to repeated injections of rIMD did not undergo tachyphylaxis. The present data demonstrate rIMD possesses direct vasodilator activity in the rat pulmonary vascular bed. The present data suggest activation of CGRP1 receptors and release of nitric oxide (NO*) mediate the pulmonary vasodilator response to rIMD whereas cyclooxygenase products and KATP channels do not contribute to the pulmonary vasodilator response to rIMD. The ability of rIMD to induce heterologous desensitization of CGRP1 receptor activation, to retain much of its pulmonary vasodilator activity after inhibition of CGRP1 receptors, and to lack homologous desensitization together suggests the pulmonary, unlike the systemic, vasodilator response to rIMD may depend on other vasodilator mechanisms including receptors in the calcitonin-receptor-like-receptor (CRLR) family.

Correlation between pulmonary gas exchange and basal and nitroglycerin (GTN)-induced exhaled nitric oxide (eNO) in patients undergoing cardiac surgery

The relationship between eNO and events in the alveolar-capillary unit in acute lung injury remains to be established. Since endogenous eNO largely originates from the airway epithelium, but nitroglycerin (GTN)-induced eNO is due to microvascular/alveolar metabolism, we have proposed to use basal and GTN-induced eNO as metabolic markers of the airway--and microvascular/alveolar function, respectively. The current work investigates the relationship between basal and GTN-induced eNO and oxygenation parameters (PaO(2)/FiO(2) ratio) in patients undergoing cardiac surgery utilising cardiopulmonary bypass (CPB). Breath by breath eNO measurements were made in 10 patients before, and 1 and 3 h after CPB either under basal conditions or following intravenous administration of GTN (1, 2 and 3 microg/kg). Basal eNO remained unchanged, whereas GTN-induced eNO was reduced following CPB. Also, there was a transient reduction in PaO(2)/FiO(2) ratio 1 h after CPB (32+/-4 vs. 44+/-3 kPa). A negative correlation was found between oxygenation and basal eNO by Pearson's correlation test and linear regression analysis suggesting that decreased oxygenation was associated with increased basal eNO. In contrast, a decrease in GTN-induced eNO positively correlated with reduced oxygenation index (R=0.533, p=0.002). These data suggest that differential relationships exist between basal and nitrovasodilator-induced eNO and oxygenation indices during subclinical lung injury in patients following CPB and that GTN-induced eNO evolution may reflect better microvascular events and injury.

Nitroglycerin-patch induced tolerance is associated with reduced ability of nitroglycerin to increase exhaled nitric oxide

Nitroglycerin (GTN), used in the treatment of ischemic heart disease, acts through the liberation of nitric oxide (NO). However, its clinical use is limited due to tolerance development. Expired NO was used as an indicator of GTN-bioactivation and was measured together with plasma nitrite and mean arterial pressure (MAP) during GTN indicator infusions. The model was applied in rabbits subjected to various time periods of low-dose GTN pretreatment by patch application for 1, 24 and 72 h. Pretreatment with GTN-patch resulted in significant attenuation of expired NO from the GTN indicator infusion in the 24 h and 72 h pretreatment groups compared to placebo (72 h). Dose-response curves with increasing GTN infusions after 24 h GTN-patch pretreatment revealed a significant attenuation of the MAP decrease compared to placebo. GTN-induced changes in plasma nitrite correlated to increases in expired NO and decreases in MAP. This indicates that expired NO could serve as an indicator of NO generation from GTN in the vascular system. We conclude that GTN tolerance is associated with reduced capacity to generate NO from GTN. Care should be taken in using MAP-reduction to evaluate tolerance since high indicator doses could liberate sufficient amounts of NO to elicit maximal MAP decrease even in tolerant animals.

Endothelin ETA receptor antagonist did not affect development of tolerance to glyceryl trinitrate in rat

Glyceryl trinitrate (GTN), extensively used for the treatment of cardiovascular disorders, is associated with rapid development of tolerance. The exact mechanism responsible for tolerance development to GTN is still not known. Recently, it has been demonstrated that GTN tolerance is associated with increased expression of endothelin (ET). This study was carried out to determine the effect of ET(A) receptor antagonist, BMS182874, on the development of tolerance to GTN in urethane-anaesthetized rats. Diastolic blood pressure (DBP), systolic blood pressure (SBP) and heart rate (HR) were continuously recorded in vehicle- and BMS182874 (3 mg kg(-1), i.v.)-treated rats. GTN was infused at the rate of 10 microg min(-1), intravenously for 4 h. Tolerance to GTN was determined using challenge doses of GTN (10, 30 and 90 microg, i.v.). GTN produced a fall in DBP, SBP and an increase in HR. In vehicle-treated rats, the fall in SBP before induction of GTN tolerance was 28 +/- 2, 43 +/- 4 and 52 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. However, following GTN infusion (10 microg min(-1), i.v. for 4 h) a rapid development of tolerance was observed and the fall in SBP was 1 +/- 1, 9 +/- 4 and 15 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. Similarly, in BMS182874-treated rats the fall in SBP in non-tolerant rats was 28 +/- 4, 42 +/- 4 and 48 +/- 5 mmHg with 10, 30 and 90 microg GTN, respectively. In BMS182874-treated rats following GTN infusion (10 microg min(-1), i.v. for 4 h) a rapid development of tolerance was observed and the fall in SBP was 4 +/- 3, 10 +/- 2 and 13 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. The decrease in DBP and SBP in vehicle- and BMS182874-treated GTN-tolerant rats was statistically similar. These results suggest that ET(A) receptor antagonist BMS182874 did not affect development of tolerance to GTN in rats.

Endogenous nitric oxide release by vasoactive drugs monitored in exhaled air

Direct measurements of endogenous nitric oxide (NO) release is of great interest but difficult to perform in vivo. We hypothesized that endogenous NO release from vasoactive substances would be detectable in exhaled air. Exhaled NO was measured after intravenous injections of various endothelium-dependent and endothelium-independent vasoactive drugs, in anesthetized pigs and humans. In pigs, a dose-dependent release of exhaled NO was observed for acetylcholine (ACh), bradykinin, substance P, endothelin (ET)-1, and nitroglycerine. Each compound had an individual and highly reproducible release pattern. Bradykinin-induced NO release was enhanced by angiotensin converting enzyme inhibition. ET receptor antagonism markedly reduced the response in exhaled NO to ET-1, whereas atropin abolished the NO response to ACh. NO synthase inhibition abolished basal levels of exhaled NO as well as the responses in exhaled NO to all compounds except nitroglycerine. In humans, ACh evoked a dose-dependent increase of NO levels in exhaled air. NO release by endogenous vasoactive agonists can be measured online in the exhaled air of pigs and humans. These novel findings may be useful when characterizing NO release from compounds that interfere with NO synthesis or drugs that act as donors of NO. Moreover, the possibility of using exhaled NO as an indicator of pulmonary endothelial dysfunction merits further studies.

Effect of in vivo nitrate tolerance on hypersensitivity to NO donors after NO-synthase blockade

Animals treated with nitric oxide synthase (NOS) inhibitors exhibit marked hypersensitivity to the blood pressure lowering effects of exogenous nitric oxide (NO) donors. We used this model as a sensitive index to evaluate the relative importance of reduced biotransformation of glyceryl trinitrate (GTN) to NO in the development of nitrate tolerance. NOS-blockade hypertension using N(G)-nitro-L-arginine methyl ester (L-NAME) caused a marked enhancement of the mean arterial pressure (MAP) decrease mediated by GTN in nontolerant rats. However, even large doses of GTN were unable to change the MAP in GTN-tolerant, NOS-blockade hypertensive animals. In contrast, the MAP responses to the spontaneous NO donor sodium nitroprusside (SNP) were completely unaltered in either tolerant rats or tolerant NOS-blockade hypertensive animals, indicating that NO-dependent vasodilatory mechanisms remain intact despite the development of GTN tolerance. The MAP-lowering effects of GTN in NOS-blockade hypertensive animals were restored 48 h after cessation of chronic GTN exposure. These alterations in the pharmacodynamic response to GTN during tolerance development and reversal were associated with parallel changes in the pattern of GTN metabolite formation, suggesting that the activity of one or more enzymes involved in nitrate metabolism was altered as a consequence of chronic GTN exposure. These findings suggest that the vasodilation resulting from the vascular biotransformation of GTN to NO (or a closely related species) is severely compromised in nitrate-tolerant animals, and that although other mechanisms may contribute to the vascular changes observed following the development of GTN tolerance, decreased GTN bioactivation is likely the most important.

Mechanisms of nitric oxide generation from nitroglycerin and endogenous sources during hypoxia in vivo

Nitroglycerin (GTN), often used in conditions of cardiovascular ischaemia, acts through the liberation of nitric oxide (NO) and the local concentration of NO in the tissue is responsible for any biological effect. However, little is known about the way in which the concentration of NO from GTN and other NO-donors is influenced by low oxygen tension in the target tissues. To evaluate the impact of changes in oxygen tension in the metabolism of NO-donors we measured exhaled NO in anaesthetized rabbits in vivo and expired NO and perfusate nitrite (NO(2)(-)) in buffer-perfused lungs in situ. The impact of acute hypoxia on NO formation from GTN, isosorbide-5-mononitrate (ISMN), dissolved authentic NO, NO(2)(-) and NO generated from endogenous NO-synthase (NOS) was studied in either model. Acute hypoxia drastically increased exhaled NO concentrations from all NO-donors studied, both in vivo and in the perfused lung. During similar conditions endogenous NO generation from NOS was strongly inhibited. The effects were most pronounced at less than 3% inspired oxygen. The mechanisms for the increased NO-formation during hypoxia seems to differ between GTN- and NO(2)(-)-derived NO. The former phenomenon is likely due to diminished breakdown of NO. In conclusion, hypoxic conditions preserve very high local NO concentrations generated from organic nitrates in vivo and we suggest that this might benefit preferential vasodilation in ischaemic tissue regions. Our findings point out the necessity to consider the influence of oxygen tension when studying the action of NO-donors.

Nitroglycerin tolerance in human vessels: evidence for impaired nitroglycerin bioconversion

BACKGROUND

The basis for progressive attenuation of the effects of organic nitrates during long-term therapy (nitrate tolerance) remains controversial; proposed mechanisms include impaired nitrate bioconversion resulting in decreased release of nitric oxide (NO) from nitrates and/or increased NO clearance through a reaction with incrementally generated superoxide (O(2)(-)).

METHODS AND RESULTS

Patients undergoing elective coronary artery bypass were randomized to receive 24 hours of intravenously infused nitroglycerin (NTG; nitrate group) or no nitrate therapy (control group). Discarded segments of the internal mammary artery and saphenous vein were used to examine (1) vascular responsiveness to NTG, sodium nitroprusside, and the calcium ionophore A23187; (2) bioconversion of NTG to 1,2- and 1,3-glyceryl dinitrate; and (3) the generation of O(2)(-). Responses to NTG were reduced 3- to 5-fold in vessels from the nitrate group compared with control vessels (P:<0. 01 for both types of segments), whereas responses to sodium nitroprusside and A23187 were unchanged. Tissue content of 1, 2-glyceryl dinitrate was lower (P:=0.012) in the saphenous veins from the nitrate group than in those from the control group. O(2)(-) generation was greater (P:<0.01) in internal mammary artery samples from the nitrate group than in those from the control group. However, incremental O(2)(-) generation induced by an inhibitor of superoxide dismutase did not affect NTG responses.

CONCLUSIONS

NTG tolerance in patients with coronary artery disease is nitrate-specific and is associated with evidence of impaired NTG bioconversion. Tolerance was associated with incremental O(2)(-) generation, but short-term elevation of O(2)(-) did not affect NTG responsiveness, suggesting increased NO clearance by O(2)(-) has a minimal contribution to tolerance.