Breathing pattern, CO2 elimination and the absence of exhaled NO in freely diving Weddell seals

Weddell seals undergo lung collapse during dives below 50 m depth. In order to explore the physiological mechanisms contributing to restoring lung volume and gas exchange after surfacing, we studied ventilatory parameters in three Weddell seals between dives from an isolated ice hole on McMurdo Sound, Antarctica.

METHODS

Lung volumes and CO(2) elimination were investigated using a pneumotachograph, infrared gas analysis, and nitrogen washout. Thoracic circumference was determined with a strain gauge. Exhaled nitric oxide was measured using chemiluminescence.

RESULTS

Breathing of Weddell seals was characterized by an apneustic pattern with end-inspiratory pauses with functional residual capacity at the end of inspiration. Respiratory flow rate and tidal volume peaked within the first 3 min after surfacing. Lung volume reductions before and increases after diving were approximately 20% of the lung volume at rest. Thoracic circumference changed by less than 2% during diving. The excess CO(2) eliminated after dives correlated closely with the duration of the preceding dive. Nitric oxide was not present in the expired gas.

CONCLUSION

Our data suggest that most of the changes in lung volume during diving result from compression and decompression of the gas remaining in the respiratory tract. Cranial shifts of the diaphragm and translocation of blood into the thorax rather than a reduction of thoracic circumference appear to compensate for lung collapse. The time to normalise gas exchange after surfacing was mainly determined by the accumulation of CO(2) during the dive. These findings underline the remarkable adaptations of the Weddell seal for restoring lung volume and gas exchange after diving.

Endothelial nitric oxide synthase limits left ventricular remodeling after myocardial infarction in mice

Background- To investigate the role of endothelial nitric oxide synthase (NOS3) in left ventricular (LV) remodeling after myocardial infarction (MI), the impact of left anterior descending coronary artery ligation on LV size and function was compared in 2- to 4-month-old wild-type (WT) and NOS3-deficient mice (NOS3(-/-)). Methods and Results- Two days after MI, both strains of mice had a similar LV size, fractional shortening, and ejection fraction by echocardiography. Twenty-eight days after MI, both strains had dilated LVs with decreased fractional shortening and lower ejection fractions. Although the infarcted fraction of the LV was similar in both strains, LV end-diastolic internal diameter, end-diastolic volume, and mass were greater, but fractional shortening, ejection fraction, and the maximum rate of developed LV pressure (dP/dt(max)) were lower in NOS3(-/-) than in WT mice. Impairment of diastolic function, as measured by the time constant of isovolumic relaxation (tau) and the maximum rate of LV pressure decay (dP/dt(min)), was more marked in NOS3(-/-) than in WT mice. Mortality after MI was greater in NOS3(-/-) than in WT mice. Long-term administration of hydralazine normalized blood pressure in NOS3(-/-) mice, but it did not prevent the LV dilatation, impaired systolic and diastolic function, and increased LV mass that followed MI. In WT mice, capillary density and myocyte width in the nonischemic portion of the LV did not differ before and 28 days after MI, whereas in NOS3(-/-) mice, capillary density decreased and myocyte width increased after MI, whether or not hydralazine was administered. Conclusions- These results suggest that the presence of NOS3 limits LV dysfunction and remodeling in a murine model of MI by an afterload-independent mechanism, in part by decreasing myocyte hypertrophy in the remote myocardium.

Cessation of platelet-mediated cyclic canine coronary occlusion after thrombolysis by combining nitric oxide inhalation with phosphodiesterase-5 inhibition

OBJECTIVES

We sought to evaluate the ability of type 5 phosphodiesterase (PDE5) inhibitors to augment the antithrombotic effects of inhaled nitric oxide (NO) in a canine model of platelet-mediated coronary thrombosis after thrombolysis.

BACKGROUND

Type 5 phosphodiesterase inhibitors potentiate the ability of NO to inhibit platelet aggregation in vitro by preventing platelet cyclic guanosine monophosphate catabolism. We previously reported that breathing low concentrations of NO gas attenuated, but did not prevent, cyclic flow reductions (CFRs) in a canine model of coronary thrombosis after thrombolysis.

METHODS

Cyclic flow reductions were induced after creation of a left anterior descending coronary artery stenosis, endothelial injury, thrombus formation and thrombolysis. Dogs were either untreated or treated with inhaled NO (20 ppm by volume), intravenous zaprinast, intravenous dipyridamole or the combination of inhaled NO with either PDE5 inhibitor (n = 4 per group).

RESULTS

Cyclic flow reductions ceased, and complete coronary patency was achieved in all dogs after they breathed NO combined with zaprinast (by 12.0+/-4.7 min [mean +/- SEM]) or dipyridamole (by 9.8+/-4.7 min). The frequency of CFRs was unaffected by NO, dipyridamole or zaprinast alone. Systemic arterial blood pressure and bleeding time were unchanged with any treatment. Ex vivo thrombin-induced platelet aggregation in dogs breathing NO and receiving dipyridamole was reduced by 75+/-7% (p < 0.05).

CONCLUSIONS

The PDE5 inhibitors potentiated the antithrombotic properties of inhaled NO in a canine model of platelet-mediated coronary artery thrombosis after thrombolysis, without prolonging the bleeding time or causing systemic hypotension.

Nebulized sildenafil is a selective pulmonary vasodilator in lambs with acute pulmonary hypertension

OBJECTIVE

To determine whether inhalation of aerosolized sildenafil with and without inhaled nitric oxide (NO) causes selective pulmonary vasodilation in a sheep model of pulmonary hypertension.

DESIGN

A controlled laboratory study in instrumented, awake, spontaneously breathing lambs.

SETTING

Animal research laboratory affiliated with a university hospital.

SUBJECT

Twenty Suffolk lambs.

INTERVENTIONS

Lambs were instrumented with a carotid artery catheter, a pulmonary artery catheter, and a tracheostomy tube and studied awake. After baseline measurements, pulmonary hypertension was induced by the continuous infusion of U46619, a thromboxane A2 analog. After breathing three concentrations of inhaled NO (2, 5, and 20 ppm), lambs were divided into two groups. Group 1 (n = 7) breathed aerosols containing 1, 10, and 30 mg of sildenafil alone, and group 2 (n = 4) simultaneously breathed NO (2 and 5 ppm) and aerosols containing 10 mg of sildenafil. Hemodynamic measurements were obtained before and at the end of each drug administration. Venous admixture was calculated, and plasma cyclic guanosine monophosphate and sildenafil concentrations were measured.

MEASUREMENTS AND MAIN RESULTS

Aerosols containing 10 mg and 30 mg of sildenafil selectively decreased the pulmonary artery pressure by 21% +/- 3% and 26% +/- 3%, respectively (p < .05 vs. baseline pulmonary hypertension). When 10 mg of sildenafil was inhaled while simultaneously breathing 2 ppm and 5 ppm NO, the pulmonary artery pressure decreased by 35% +/- 3% and 43% +/- 2% (p < .05 vs. baseline pulmonary hypertension). Inhaled sildenafil did not impair systemic oxygenation, increase right-to-left intrapulmonary shunting, or impair the ability of inhaled NO to reduce right-to-left shunting.

CONCLUSIONS

Nebulized sildenafil is a selective pulmonary vasodilator that can potentiate the pulmonary vasodilating effects of inhaled NO.

Attenuation of hypoxic pulmonary vasoconstriction by endotoxemia requires 5-lipoxygenase in mice

Sepsis and endotoxemia impair hypoxic pulmonary vasoconstriction (HPV), thereby reducing systemic oxygenation. To assess the role of leukotrienes (LTs) in the attenuation of HPV during endotoxemia, the increase in left lung pulmonary vascular resistance (LPVR) before and during left mainstem bronchus occlusion (LMBO) was measured in mice with and without a deletion of the gene encoding 5-lipoxygenase (5-LO). LMBO increased the LPVR equally in saline-challenged wild-type and 5-LO-deficient mice (96+/-20% and 94+/-19%, respectively). Twenty-two hours after challenge with Escherichia coli endotoxin, the ability of LMBO to increase LPVR was markedly impaired in wild-type mice (27+/-7%; P<0.05) but not in 5-LO-deficient mice (72+/-9%) or in wild-type mice pretreated with MK886, an inhibitor of 5-LO activity (76+/-10%). Compared with wild-type mice, endotoxin-induced disruption of lung structures and inflammatory cell influx in the lung were markedly attenuated in 5-LO-deficient mice. Administration of MK571, a selective cysteinyl LT(1) receptor antagonist, 1 hour before endotoxin challenge preserved HPV and attenuated pulmonary injury in wild-type mice but did not prevent the endotoxin-induced increase in pulmonary myeloperoxidase activity. Taken together, these findings demonstrate that a 5-LO product, most likely a cysteinyl LT, contributes to the attenuation of HPV and to pulmonary injury after challenge with endotoxin.

Antiinflammatory properties of inducible nitric oxide synthase in acute hyperoxic lung injury

The objective of this study was to determine whether endogenous nitric oxide (NO), specifically the inducible NO synthase isoform (iNOS: NOS II), reduces or amplifies lung injury in mice breathing at a high oxygen tension. Previous studies have shown that exogenous (inhaled) NO protects against hyperoxia-induced lung injury, and that endogenous NO derived from iNOS inhibits leukocyte recruitment and protects against lung injury induced by lipopolysaccharide. In the present study, hyperoxia (> 98% O(2) for 72 h) induced acute lung injury in both wild-type and iNOS-deficient mice as determined by elevated albumin and lactate dehydrogenase levels in bronchoalveolar lavage fluid (BALF) and by increased extravascular lung water. Lung injury was greater in iNOS-deficient mice than in wild-type mice and was associated with an increased number of polymorphonuclear leukocytes in BALF. iNOS messenger RNA expression levels increased in the lungs of wild-type hyperoxic mice. Nitrotyrosine, a marker of reactive NO species, was expressed in both wild-type and iNOS-deficient mice in hyperoxia, indicating an iNOS-independent pathway for protein nitration. We conclude that iNOS is capable of reducing pulmonary leukocyte accumulation and lung injury. The data indicate that iNOS induction serves as a protective mechanism to minimize the effects of acute exposure to hyperoxia.

Inhibition of lung phosphodiesterase improves responsiveness to inhaled nitric oxide in isolated-perfused lungs from rats challenged with endotoxin

OBJECTIVES

To investigate the ability of phosphodiesterase (PDE) selective inhibitors to improve responsiveness to inhaled nitric oxide (NO) in isolated-perfused lungs of rats pretreated with endotoxin/lipopolysaccharide (LPS).

DESIGN AND SETTING

Prospective, controlled animal study in the animal research facility of a university hospital.

INTERVENTIONS

Sixteen hours after adult Sprague-Dawley rats were injected intraperitoneally with 0.4 mg/ kg E. coli 0111:B4 LPS administration, lungs were isolated and perfused, and the thromboxane mimetic U46619 was employed to increase the mean pulmonary artery pressure by 5-7 mmHg. The lungs were then ventilated with or without 0.4 ppm NO, and erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA; PDE type 2 inhibitor), milrinone (PDE type 3 inhibitor), or zaprinast (inhibitor of PDE types 5 and 9) were added to the perfusate.

MEASUREMENTS AND RESULTS

In the presence of EHNA (12.5, 25, 50 microM) the vasodilator response to inhaled NO was not greater than in its absence (0.25 +/- 0.25, 0.5 +/- 0.25, 0.75 +/- 0.25 mmHg vs. 0.25 +/- 0.25, 0.5 +/- 0.25, 0.75 +/- 0.25 mmHg, respectively). In the presence of milrinone (125, 250, 500 nM), the vasodilator response to inhaled NO was also not improved. In contrast, zaprinast (3.7, 7.4, 14.8 microM) augmented the pulmonary vasodilatory effect of inhaled NO in lungs from LPS-pretreated rats from 0.25 +/- 0.25, 0.5 +/- 0.25, 0.75 +/- 0.25 mmHg to 0.75 +/- 0.25, 1.5 +/- 0.5, 1.75 +/- 0.75 mmHg, respectively (p < 0.05).

CONCLUSIONS

Our results demonstrate that inhibition of pulmonary PDE enzyme activity with zaprinast increases vasodilator responsiveness to inhaled NO in lungs obtained from rats 16 h after LPS challenge.

Comparison of the effects of nitric oxide, nitroprusside, and nifedipine on hemodynamics and right ventricular contractility in patients with chronic pulmonary hypertension

STUDY OBJECTIVES

The effects of inhaled nitric oxide (NO) on hemodynamics and right ventricular (RV) contractility were compared with those of nitroprusside and nifedipine in 14 patients with severe chronic pulmonary hypertension.

STUDY DESIGN

Micromanometer and balloon-tipped right heart catheterization were performed. Inhaled NO, IV nitroprusside, and sublingual nifedipine were administered sequentially while patients breathed > 90% oxygen.

SETTING

Cardiac catheterization laboratory in a tertiary care teaching hospital.

PATIENTS

Fourteen patients with severe pulmonary hypertension unrelated to left ventricular dysfunction.

MEASUREMENTS AND RESULTS

During NO inhalation, mean systemic arterial pressure (MAP) was unchanged, but pulmonary artery (PA) pressure ([mean +/- SEM] 49 +/- 2 mm Hg vs 44 +/- 2 mm Hg; p < 0.01), pulmonary vascular resistance (PVR; 829 +/- 68 vs 669 +/- 64 dyne x s x cm(-5); p < 0.01) and RV end-diastolic pressure (RVEDP; 12 +/- 1 vs 10 +/- 1 mm Hg; p < 0.01) decreased. Stroke volume index (SVI; 31 +/- 2 vs 35 +/- 3 mL/m(2); p < 0.05) increased, and the first derivative of RV pressure at 15 mm Hg developed pressure (RV +dP/dt at DP15) was unchanged. During nitroprusside administration, MAP decreased (105 +/- 5 vs 76 +/- 5 mm Hg; p < 0.01), PA was unchanged (48 +/- 2 vs 45 +/- 3 mm Hg; p = not significant), and PVR decreased (791 +/- 53 vs 665 +/- 53 dyne x s x cm(-5); p < 0.01). RV +dP/dt at DP15 increased (425 +/- 22 vs 465 +/- 29 mm Hg/s; p < 0.05), but SVI was unchanged. Nifedipine decreased MAP (103 +/- 5 vs 94 +/- 5 mm Hg; p < 0.01), PA and PVR were unchanged, RVEDP increased (12 +/- 1 vs 14 +/- 2 mm Hg; p < 0.01), and RV +dP/dt at DP15 decreased (432 +/- 90 vs 389 +/- 21 mm Hg/s; p < 0.05).

CONCLUSIONS

Inhaled NO is a selective pulmonary vasodilator in patients with chronic pulmonary hypertension that improves cardiac performance without altering RV contractility. Nitroprusside caused a similar degree of pulmonary vasodilation. In contrast to inhaled NO, nitroprusside caused systemic hypotension associated with an increase in RV contractility. Acute administration of nifedipine did not cause pulmonary vasodilation, but RVEDP increased and RV contractility decreased.

Exhaled nitric oxide production by nitric oxide synthase-deficient mice

Nitric oxide (NO) is produced in the nasal cavities, airways, and lungs and is exhaled by normal animals and humans. Although increased exhaled NO concentrations in airway inflammation have been associated with increased airway expression of nitric oxide synthase 2 (NOS 2), it is uncertain which NOS isoform is responsible for baseline levels of exhaled NO. We therefore studied wild-type mice and mice with a congenital deficiency of NOS 1, NOS 2, or NOS 3. By studying a closed chamber in which the exhaled gas of a group of mice was collected, gaseous NO production rates were measured. Wild-type mice exhaled 362 +/- 35 x 10(-15) mol g(-1) min(-1) NO (mean +/- SE, n = 16 groups of five mice), NOS 1-deficient mice exhaled 592 +/- 74 x 10(-15) mol g(-1) min(-1) NO (n = 15 groups, p < 0.05 versus wild-type and NOS 2-deficient mice), NOS 2-deficient mice 330 +/- 74 x 10(-15) mol g(-1) min(-1) NO (n = 14 groups) and NOS 3-deficient mice 766 +/- 101 x 10(-15) mol g(-1) min(-1) NO (n = 16 groups, p < 0.001 versus wild-type and NOS 2-deficient mice). Pharmacological NOS inhibition with L-NAME decreased (p < 0.05) the exhaled NO production rate of wild-type and NOS 3-deficient but not of NOS 2-deficient mice. L-Arginine administration increased exhaled NO production rate in all but NOS 2-deficient mice. Absence of NOS 1 or 3 is associated with increased murine exhaled NO production rates. Since NOS 2-deficient mice were the only genotype to lack substrate- and inhibitor-regulated changes of NO exhalation, we suggest that NOS 2 is an important isoform contributing to exhaled NO exhalation in healthy mice.

Congenital deficiency of nitric oxide synthase 2 protects against endotoxin-induced myocardial dysfunction in mice

BACKGROUND

Sepsis can be complicated by severe myocardial dysfunction and is associated with increased nitric oxide (NO) production by inducible NO synthase (NOS2). To investigate the role of NOS2 in endotoxin-induced myocardial dysfunction in vivo, we studied wild-type and NOS2-deficient mice.

METHODS AND RESULTS

Serial echocardiographic parameters of myocardial function were measured before and at 4, 7, 16, and 24 hours after an endotoxin challenge. Seven hours after challenge with either endotoxin or saline, systemic and left ventricular pressures were measured, and the first derivative of left ventricular developed pressure (dP/dt), slope of the end-systolic pressure-dimension relationship (Slope(LVESPD)), and time constant of isovolumic relaxation (tau) were calculated. Endotoxin challenge in wild-type mice decreased left ventricular fractional shortening, velocity of circumferential shortening, dP/dt(max), Slope(LVESPD), and dP/dt(min) and increased time constant tau. Endotoxin-induced myocardial dysfunction was associated with increased ventricular NOS2 gene expression and cGMP concentrations. Seven hours after endotoxin challenge, NOS2-deficient mice had greater fractional shortening, dP/dt(max), and Slope(LVESPD) than did endotoxin-challenged wild-type mice. Measures of diastolic function, dP/dt(min) and time constant tau, were preserved in endotoxin-challenged NOS2-deficient mice. After endotoxin challenge in wild-type mice, early (3-hour) inhibition of NOS2 with L-N:(6)-(1-iminoethyl)lysine hydrochloride prevented, whereas later (7-hour) inhibition could not reverse, endotoxin-induced myocardial dysfunction.

CONCLUSIONS

These results suggest that NOS2 is required for the development of systolic and diastolic dysfunction in murine sepsis.

Effects of intravenous Zaprinast and inhaled nitric oxide on pulmonary hemodynamics and gas exchange in an ovine model of acute respiratory distress syndrome

BACKGROUND

Inhaled nitric oxide (No) selectively dilates the pulmonary vasculature and improves gas exchange in acute respiratory distress syndrome. Because of the very short half-life of NO, inhaled NO is administered continuously. Intravenous Zaprinast (2-o-propoxyphenyl-8-azapurin-6-one), a cyclic guanosine monophosphate phosphodiesterase inhibitor, increases the efficacy and prolongs the duration of action of inhaled NO in models of acute pulmonary hypertension. Its efficacy in lung injury models is uncertain. The authors hypothesized that the use of intravenous Zaprinast would have similar beneficial effects when used in combination with inhaled NO to improve oxygenation and dilate the pulmonary vasculature in a diffuse model of acute lung injury.

METHODS

The authors studied two groups of sheep with lung injury produced by saline lavage. In the first group, 0, 5, 10, and 20 ppm of inhaled NO were administered in a random order before and after an intravenous Zaprinast infusion (2 mg/kg bolus followed by 0.1 mg. kg-1. min-1). In the second group, inhaled NO was administered at the same concentrations before and after an intravenous infusion of Zaprinast solvent (0.05 m NaOH).

RESULTS

After lavage, inhaled NO decreased pulmonary arterial pressure and resistance with no systemic hemodynamic effects, increased arterial oxygen partial pressure, and decreased venous admixture (all P < 0.05). The intravenous administration of Zaprinast alone decreased pulmonary artery pressure but worsened gas exchange (P < 0.05). Zaprinast infusion abolished the beneficial ability of inhaled NO to improve pulmonary gas exchange and reduce pulmonary artery pressure (P < 0. 05 vs. control).

CONCLUSIONS

This study suggests that nonselective vasodilation induced by intravenously administered Zaprinast at the dose used in our study not only worsens gas exchange, but also abolishes the beneficial effects of inhaled NO.

Nitric oxide inhalation decreases pulmonary artery remodeling in the injured lungs of rat pups

Vascular injury causes the muscularization of peripheral pulmonary arteries, which is more pronounced in the infant than in the adult lung. Although inhaled NO gas attenuates pulmonary artery remodeling in hypoxic rats, whether or not it protects the lung by mitigating vasoconstriction is unknown. This investigation tested whether inhaled NO decreases the muscularization of injured pulmonary arteries in rat pups by modulating vascular tone. One week after monocrotaline administration, the percentage of muscularized rat pup lung arteries was increased by >3-fold. Nevertheless, monocrotaline exposure did not cause right ventricular hypertrophy, pulmonary hypertension, or vasoconstriction. In addition, it did not increase the expression of markers of inflammation (interleukin-1beta, intercellular adhesion molecule-1, and E-selectin) or of platelet-mediated thrombosis (GPIbalpha). Continuous inhalation of 20 ppm NO gas prevented the neomuscularization of the pulmonary arteries in pups with lung injury. Moreover, a 3-fold increase in cell proliferation and 30% decrease in cell numbers in pulmonary arteries caused by monocrotaline exposure was prevented by NO inhalation. These data indicate that inhaled NO protects infants against pulmonary remodeling induced by lung injury by mechanisms that are independent of pulmonary tone, inflammation, or thrombosis.

Sildenafil is a pulmonary vasodilator in awake lambs with acute pulmonary hypertension

BACKGROUND

Phosphodiesterase type 5 (PDE5) hydrolyzes cyclic guanosine monophosphate in the lung, thereby modulating nitric oxide (NO)/cyclic guanosine monophosphate-mediated pulmonary vasodilation. Inhibitors of PDE5 have been proposed for the treatment of pulmonary hypertension. In this study, we examined the pulmonary and systemic vasodilator properties of sildenafil, a novel selective PDE5 inhibitor, which has been approved for the treatment of erectile dysfunction.

METHODS

In an awake lamb model of acute pulmonary hypertension induced by an intravenous infusion of the thromboxane analog U46619, we measured the effects of 12.5, 25, and 50 mg sildenafil administered via a nasogastric tube on pulmonary and systemic hemodynamics (n = 5). We also compared the effects of sildenafil (n = 7) and zaprinast (n = 5), a second PDE5 inhibitor, on the pulmonary vasodilator effects of 2.5, 10, and 40 parts per million inhaled NO. Finally, we examined the effect of infusing intravenous l-NAME (an inhibitor of endogenous NO production) on pulmonary vasodilation induced by 50 mg sildenafil (n = 6).

RESULTS

Cumulative doses of sildenafil (12.5, 25, and 50 mg) decreased the pulmonary artery pressure 21%, 28%, and 42%, respectively, and the pulmonary vascular resistance 19%, 23%, and 45%, respectively. Systemic arterial pressure decreased 12% only after the maximum cumulative sildenafil dose. Neither sildenafil nor zaprinast augmented the ability of inhaled NO to dilate the pulmonary vasculature. Zaprinast, but not sildenafil, markedly prolonged the duration of pulmonary vasodilation after NO inhalation was discontinued. Infusion of l-NAME abolished sildenafil-induced pulmonary vasodilation.

CONCLUSIONS

Sildenafil is a selective pulmonary vasodilator in an ovine model of acute pulmonary hypertension. Sildenafil induces pulmonary vasodilation via a NO-dependent mechanism. In contrast to zaprinast, sildenafil did not prolong the pulmonary vasodilator action of inhaled NO.

Inhaled nitric oxide

Nitric oxide (NO) is a free-radical gas that is an important signaling molecule in pulmonary vessels. Endogenous NO produced in endothelial cells from oxygen and L-arginine diffuses into smooth muscle cells in the vascular wall and causes vasodilatation. NO that diffuses into the blood vessel lumen is avidly bound by hemoglobin and does not cause important systemic vasodilatation. Inhaling low levels of NO rapidly and safely decreases pulmonary artery hypertension in many patients without causing systemic hypotension. In hypoxemic newborns with pulmonary hypertension, clinical studies indicate that inhaled NO increases systemic oxygen levels and decreases the requirement for extracorporeal membrane oxygenation. NO also has been observed to regulate cell proliferation. Recent studies suggest that inhaled NO selectively modulates the pulmonary artery proliferative response that is associated with lung injury. These later studies may indicate that inhaled NO can be applied to attenuate or prevent pulmonary artery disease in patients with injured lungs.

Congenital NOS2 deficiency protects mice from LPS-induced hyporesponsiveness to inhaled nitric oxide

BACKGROUND

In animal models, endotoxin (lipopolysaccharide) challenge impairs the pulmonary vasodilator response to inhaled nitric oxide (NO). This impairment is prevented by treatment with inhibitors of NO synthase 2 (NOS2), including glucocorticoids and L-arginine analogs. However, because these inhibitors are not specific for NOS2, the role of this enzyme in the impairment of NO responsiveness by lipopolysaccharide remains incompletely defined.

METHODS

To investigate the role of NOS2 in the development of lipopolysaccharide-induced impairment of NO responsiveness, the authors measured the vasodilator response to inhalation of 0.4, 4, and 40 ppm NO in isolated, perfused, and ventilated lungs obtained from lipopolysaccharide-pretreated (50 mg/kg intraperitoneally 16 h before lung perfusion) and untreated wild-type and NOS2-deficient mice. The authors also evaluated the effects of breathing NO for 16 h on pulmonary vascular responsiveness during subsequent ventilation with NO.

RESULTS

In wild-type mice, lipopolysaccharide challenge impaired the pulmonary vasodilator response to 0.4 and 4 ppm NO (reduced 79% and 45%, respectively, P < 0.001), but not to 40 ppm. In contrast, lipopolysaccharide administration did not impair the vasodilator response to inhaled NO in NOS2-deficient mice. Breathing 20 ppm NO for 16 h decreased the vasodilator response to subsequent ventilation with NO in lipopolysaccharide-pretreated NOS2-deficient mice, but not in lipopolysaccharide-pretreated wild-type, untreated NOS2-deficient or untreated wild-type mice.

CONCLUSIONS

In response to endotoxin challenge, NO, either endogenously produced by NOS2 in wild-type mice or added to the air inhaled by NOS2-deficient mice, is necessary to impair vascular responsiveness to inhaled NO. Prolonged NO breathing, without endotoxin, does not impair vasodilation in response to subsequent NO inhalation. These results suggest that NO, plus other lipopolysaccharide-induced products, are necessary to impair responsiveness to inhaled NO in a murine sepsis model.

Hypoxic pulmonary blood flow redistribution and arterial oxygenation in endotoxin-challenged NOS2-deficient mice

Sepsis and endotoxemia impair hypoxic pulmonary vasoconstriction (HPV), thereby reducing arterial oxygenation and enhancing hypoxemia. Endotoxin induces nitric oxide (NO) production by NO synthase 2 (NOS2). To assess the role of NO and NOS2 in the impairment of HPV during endotoxemia, we measured in vivo the distribution of total pulmonary blood flow (QPA) between the right (QRPA) and left (QLPA) pulmonary arteries before and after left mainstem bronchus occlusion (LMBO) in mice with and without a congenital deficiency of NOS2. LMBO reduced QLPA/QPA equally in saline-treated wild-type and NOS2-deficient mice. However, prior challenge with Escherichia coli endotoxin markedly impaired the ability of LMBO to reduce QLPA/QPA in wild-type, but not in NOS2-deficient, mice. After endotoxin challenge and LMBO, systemic oxygenation was impaired to a greater extent in wild-type than in NOS2-deficient mice. When administered shortly after endotoxin treatment, the selective NOS2 inhibitor L-NIL preserved HPV in wild-type mice. High concentrations of inhaled NO attenuated HPV in NOS2-deficient mice challenged with endotoxin. These findings demonstrate that increased pulmonary NO levels (produced by NOS2 or inhaled at high levels from exogenous sources) are necessary during the septic process to impair HPV, ventilation/perfusion matching and arterial oxygenation in a murine sepsis model.

Low-dose inhaled nitric oxide in acutely burned children with profound respiratory failure

BACKGROUND

Inhaled nitric oxide (NO) is a rapidly acting selective pulmonary vasodilator that partially reverses the pathophysiology of acute respiratory distress syndrome (ARDS).

METHODS

After human studies approval, we studied 11 burned children with severe ARDS in a trial of inhaled NO therapy, assessing its effect on intrapulmonary shunt as measured by the PaO2/FiO2 ratio (PFR). There were 12 episodes of administration; 1 child was treated twice.

RESULTS

The children had an average age of 8.3 +/- 4.8 years (mean +/- SEM, range 11 months to 14 years) and average burn size of 64% +/- 22%. At the time of enrollment, the PFR averaged 95 +/- 50 and Murray lung score 3.1 +/- 0.5. Inhaled NO was begun an average of 6.3 +/- 5.5 days after injury and was administered for an average of 7.8 +/- 7.2 days at an average dose of 6.7 +/- 2.4 parts per million. PFR improved an average of 162% +/- 214%. Eight of the 11 children (73%) survived. The 3 nonsurvivors had similar admission PFR values (100 +/- 75 versus 93 +/- 44, P = .089) but a significantly less favorable initial response to inhaled NO, with a percentage of improvement in PFR at 1 hour after enrollment of 7.3% +/- 6.4% versus 213% +/- 226% (P = .026). There were no complications related to NO administration.

CONCLUSIONS

Inhaled NO can be safely administered to treat ARDS in children with acute burns and appears to improve their ventilatory management. An immediate improvement in PFR with inhaled NO may correlate with survival.

Three-dimensional echocardiographic assessment of left ventricular wall motion abnormalities in mouse myocardial infarction

We applied 3-dimensional echocardiographic reconstruction to assess left ventricular (LV) volumes, function, and the extent of wall motion abnormalities in a murine model of myocardial infarction (MI). Consecutive parasternal short-axis planes were obtained at 1-mm intervals with a 13-MHz linear array probe. End-diastolic and end-systolic LV volumes were calculated by Simpson's rule, and the ejection fraction and cardiac output were derived. Echocardiography-derived cardiac output was validated by an aortic flow probe in 6 mice. Echocardiography was then performed in 9 mice before and after the left anterior descending coronary artery was ligated. Wall motion was assessed, and the ratio of the abnormally to normally contracting myocardium was calculated. After MI occurred, LV end-diastolic volume and LV end-systolic volume increased (33 +/- 10 vs 24 +/- 6 microL, P <.05 and 24 +/- 9 vs 10 +/- 4 microL, P <.001), whereas cardiac output decreased (4.2 +/- 1.5 mL/min vs 6.6 +/- 2.3 mL/min, P <.01). Forty percent of the myocardium was normokinetic, 24% was hypokinetic, and 36% was akinetic. Echocardiography can measure LV volumes and regional and global function in a murine model of myocardial infarction, thereby providing the potential to quantitate and compare the responses of various transgenic mice to MI and its therapies.

Echocardiographic determination of risk area size in a murine model of myocardial ischemia

Genetically altered mice are useful to understand cardiac physiology. Myocardial contrast echocardiography (MCE) assesses myocardial perfusion in humans. We hypothesized it could evaluate murine myocardial perfusion before and after acute coronary ligation. MCE was performed before and after this experimental myocardial infarction (MI) in anesthetized mice by intravenous injection of contrast microbubbles and transthoracic echo imaging. Time-video intensity curves were obtained for the anterior, lateral, and septal myocardial walls. After MI, MCE defects were compared with the area of no perfusion measured by Evans blue staining. In healthy animals, intramyocardial contrast was visualized in all the cardiac walls. The anterior wall had a higher baseline video intensity (53 +/- 17 arbitrary units) than the lateral (34 +/- 13) and septal (27 +/- 13) walls (P < 0.001) and a lower increase in video intensity after contrast injection [50 +/- 17 vs. 60 +/- 24 (lateral) and 65 +/- 29 (septum), P < 0.01]. After MI, left ventricular (LV) dimensions were enlarged, and the shortening fraction was decreased. A perfusion defect was imaged with MCE in every mouse, with a correlation between MCE perfusion defect size (35 +/- 13%) and the nonperfused area by Evans blue (37 +/- 16%, y = 0.77x + 6.1, r = 0.93, P < 0. 001). Transthoracic MCE is feasible in the mouse and can accurately detect coronary occlusions and quantitate nonperfused myocardium.

Inhibition of nitric oxide synthase prevents hyporesponsiveness to inhaled nitric oxide in lungs from endotoxin-challenged rats

BACKGROUND

Inhalation of nitric oxide (NO) selectively dilates the pulmonary circulation and improves arterial oxygenation in patients with adult respiratory distress syndrome (ARDS). In approximately 60% of patients with septic ARDS, minimal or no response to inhaled NO is observed. Because sepsis is associated with increased NO production by inducible NO synthase (NOS2), the authors investigated whether NOS inhibition alters NO responsiveness in rats exposed to gram-negative lipopolysaccharide (LPS).

METHODS

Sprague-Dawley rats were treated with 0.4 mg/kg Escherichia coli O111:B4 LPS with or without dexamethasone (inhibits NOS2 gene expression; 5 mg/kg), L-NAME (a nonselective NOS inhibitor; 7 mg/kg), or aminoguanidine (selective NOS2 inhibitor; 30 mg/kg). Sixteen hours after LPS treatment, lungs were isolated-perfused; a thromboxane-analog U46619 was added to increase pulmonary artery pressure (PAP) by 5 mmHg, and the pulmonary vasodilator response to inhaled NO was measured.

RESULTS

Ventilation with 0.4, 4, and 40 ppm NO decreased the PAP less than in lungs of LPS-treated rats (0.75+/-0.25, 1.25+/-0.25, 1.75+/-0.25 mmHg) than in lungs of control rats (3+/-0.5, 4.25+/-0.25, 4.5+/-0.25 mmHg; P < 0.01). Dexamethasone treatment preserved pulmonary vascular responsiveness to NO in LPS-treated rats (3.75+/-0.25, 4.5+/-0.25, 4.5+/-0.5 mmHg, respectively; P < 0.01 vs. LPS, alone). Responsiveness to NO in LPS-challenged rats was also preserved by treatment with L-NAME (3.0+/-1.0, 4.0+/-1.0, 4.0+/-0.75 mmHg, respectively; P < 0.05 vs. LPS, alone) or aminoguanidine (1.75+/-0.25, 2.25+/-0.5, 2.75+/-0.5 mmHg, respectively; P < 0.05 vs. LPS, alone). In control rats, treatment with dexamethasone, L-NAME, and aminoguanidine had no effect on inhaled NO responsiveness.

CONCLUSION

These observations demonstrate that LPS-mediated increases in pulmonary NOS2 are involved in decreasing responsiveness to inhaled NO.

Determination of right ventricular structure and function in normoxic and hypoxic mice: a transesophageal echocardiographic study

BACKGROUND

Noninvasive cardiac evaluation is of great importance in transgenic mice. Transthoracic echocardiography can visualize the left ventricle well but has not been as successful for the right ventricle (RV). We developed a method of transesophageal echocardiography (TEE) to evaluate murine RV size and function.

METHODS AND RESULTS

Normoxic and chronically hypoxic mice (F(IO2)=0.11, 3 weeks) and agarose RV casts were scanned with a rotating 3.5F/30-MHz intravascular ultrasound probe. In vivo, the probe was inserted in the mouse esophagus and withdrawn to obtain contiguous horizontal planes at 1-mm intervals. In vitro, the probe was withdrawn along the left ventricular posterior wall of excised hearts. The borders of the RV were traced on each plane, allowing calculation of diastolic and systolic volumes, RV mass, RV ejection fraction, stroke volume, and cardiac output. RV wall thickness was measured. Echo volumes obtained in vitro were compared with cast volumes. Echo-derived cardiac output was compared with measurements of an ascending aortic Doppler flow probe. Echo-derived RV free wall mass was compared with true RV free wall weight. There was excellent agreement between cast and TEE volumes (y=0.82x+6.03, r=0.88, P<0.01) and flow-probe and echo cardiac output (y=1.00x+0.45, r=0.99, P<0.0001). Although echo-derived RV mass and wall thickness were well correlated with true RV weight, echo-derived RV mass underestimated true weight (y=0.53x+2.29, r=0.81, P<0.0001). RV mass and wall thickness were greater in hypoxic mice than in normoxic mice (0.78+/-0.19 versus 0.51+/-0.14 mg/g, P<0.03, 0.50+/-0.03 versus 0.38+/-0.03 mm, P<0.04).

CONCLUSIONS

TEE with an intravascular ultrasound catheter is a simple, accurate, and reproducible method to study RV size and function in mice.

Inhaled nitric oxide improves exercise capacity in patients with severe heart failure and right ventricular dysfunction

Fourteen cardiac transplant candidates were studied with cardiopulmonary exercise testing at baseline and while breathing nitric oxide (40 ppm). Oxygen consumption at the anaerobic threshold was improved by breathing nitric oxide in patients with pulmonary hypertension and in patients with an elevated left ventricular end-diastolic volume index.

Sustained pulmonary hypertension and right ventricular hypertrophy after chronic hypoxia in mice with congenital deficiency of nitric oxide synthase 3

Chronic hypoxia induces pulmonary hypertension and right ventricular (RV) hypertrophy. Nitric oxide (NO) has been proposed to modulate the pulmonary vascular response to hypoxia. We investigated the effects of congenital deficiency of endothelial NO synthase (NOS3) on the pulmonary vascular responses to breathing 11% oxygen for 3-6 wk. After 3 wk of hypoxia, RV systolic pressure was greater in NOS3-deficient than in wild-type mice (35+/-2 vs 28+/-1 mmHg, x+/-SE, P < 0.001). Pulmonary artery pressure (PPA) and incremental total pulmonary vascular resistance (RPI) were greater in NOS3-deficient than in wild-type mice (PPA 22+/-1 vs 19+/-1 mmHg, P < 0.05 and RPI 92+/-11 vs 55+/-5 mmHg.min.gram.ml-1, P < 0.05). Morphometry revealed that the proportion of muscularized small pulmonary vessels was almost fourfold greater in NOS3-deficient mice than in wild-type mice. After 6 wk of hypoxia, the increase of RV free wall thickness, measured by transesophageal echocardiography, and of RV weight/body weight ratio were more marked in NOS3-deficient mice than in wild-type mice (RV wall thickness 0.67+/-0.05 vs 0.48+/-0.02 mm, P < 0.01 and RV weight/body weight ratio 2.1+/-0.2 vs 1.6+/-0.1 mg. gram-1, P < 0.05). RV hypertrophy produced by chronic hypoxia was prevented by breathing 20 parts per million NO in both genotypes of mice. These results suggest that congenital NOS3 deficiency enhances hypoxic pulmonary vascular remodeling and hypertension, and RV hypertrophy, and that NO production by NOS3 is vital to counterbalance pulmonary vasoconstriction caused by chronic hypoxic stress.

Pulmonary vasodilation by nitric oxide gas and prodrug aerosols in acute pulmonary hypertension

Sodium 1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (DEA/NO; Et2N-[N(O)NO]Na) is a compound that spontaneously generates nitric oxide (NO). Because of its short half-life (2.1 min), we hypothesized that inhaling DEA/NO aerosol would selectively dilate the pulmonary circulation without decreasing systemic arterial pressure. We compared the pulmonary selectivity of this new NO donor with two other reference drugs: inhaled NO and inhaled sodium nitroprusside (SNP). In seven awake sheep with pulmonary hypertension induced by the infusion of U-46619, we compared the hemodynamic effects of DEA/NO with those of incremental doses of inhaled NO gas. In seven additional awake sheep, we examined the hemodynamic effects of incremental doses of inhaled nitroprusside (i.e., SNP). Inhaled NO gas selectively dilated the pulmonary vasculature. Inhaled DEA/NO produced nonselective vasodilation; both systemic vascular resistance (SVR) and pulmonary vascular resistance (PVR) were reduced. Inhaled SNP selectively dilated the pulmonary circulation at low concentrations (< or = 10(-2)M), inducing a decrease of PVR of up to 42% without any significant decrease of SVR(-5%), but nonselectively dilated the systemic circulation at larger doses (> 10(-2)M). In conclusion, despite its short half-life, DEA/NO is not a selective pulmonary vasodilator compared with inhaled NO. Inhaled SNP appears to be selective to the pulmonary circulation at low doses but not at higher levels.

Cyclic-GMP-binding, cyclic-GMP-specific phosphodiesterase (PDE5) gene expression is regulated during rat pulmonary development

Increased nitric oxide (NO) production plays a critical role in the mammalian pulmonary vascular adaptation to extrauterine life. NO activates soluble guanylate cyclase, increasing intracellular cGMP concentrations, thereby inducing relaxation of vascular smooth muscle. cGMP is inactivated by cyclic nucleotide phosphodiesterases (PDEs). One PDE isozyme, PDE5, specifically hydrolyzes cGMP, is abundant in lung tissues, and modifies the pulmonary vasodilatory response to exogenous NO. To investigate the regulation of PDE5 gene expression during pulmonary development, PDE5 mRNA levels, as well as cGMP-metabolizing PDE enzyme activity, were measured in the lungs of perinatal and adult rats. RNA blot hybridization revealed that PDE5 mRNA was detectable in fetal lung tissue as early as 18.5 d of the 22-d term gestation and reached maximal levels in neonatal lungs. mRNA levels in adult rat lungs were 3-4-fold less than the levels measured in lungs of 1- and 8-d-old rats. Pulmonary cGMP hydrolytic activity in 1-d-old animals was 30-fold greater than the cGMP hydrolytic activity of adult rat lungs. Zaprinast, a specific PDE5 antagonist, inhibited 52 and 56% of cGMP hydrolytic activity in lungs of 1- and 8-d-old rats, respectively, but only 18% of the activity in adult lungs. In situ hybridization revealed that PDE5 mRNA transcripts were present in the vascular smooth muscle cells of neonatal and adult lungs. PDE5 mRNA was also detected in the alveolar walls of neonatal rat lungs. These results demonstrate that the gene encoding PDE5 is abundantly expressed in the lungs of perinatal rats, and is available to participate in the mammalian pulmonary vascular transition to extrauterine life. Extravascular PDE5 gene expression in neonatal lungs suggests a potentially important nonvascular role for this enzyme during pulmonary development.

Selective pulmonary vasodilation induced by aerosolized zaprinast

BACKGROUND

Zaprinast, an inhibitor of guanosine-3',5'-cyclic monophosphate (cGMP)-selective phosphodiesterase, augments smooth muscle relaxation induced by endothelium-dependent vasodilators (including inhaled nitric oxide [NO]). The present study was designed to examine the effects of inhaled nebulized zaprinast, alone, and combined with inhaled NO.

METHODS

Eight awake lambs with U46619-induced pulmonary hypertension sequentially breathed two concentrations of NO (5 and 20 ppm), followed by inhalation of aerosols generated from solutions containing four concentrations of zaprinast (10, 20, 30, and 50 mg/ml). The delivered doses of nebulized zaprinast at each concentration (mean +/- SD) were 0.23 +/- 0.06, 0.49 +/- 0.14, 0.71 +/- 0.24, and 1.20 +/- 0.98 mg x kg(-1) x min(-1), respectively. Each lamb also breathed NO (5 and 20 ppm) and zaprinast (0.23 +/- 0.06 mg x kg[-1] x min[-1]) in combination after a 2-h recovery period.

RESULTS

Inhaled NO selectively dilated the pulmonary vasculature. Inhaled zaprinast selectively dilated the pulmonary circulation and potentiated and prolonged the pulmonary vasodilating effects of inhaled NO. The net transpulmonary release of cGMP was increased by inhalation of NO, zaprinast, or both. The duration of the vasodilation induced by zaprinast inhalation was greater than that induced by NO inhalation.

CONCLUSIONS

Aerosolization of a cGMP-selective phosphodiesterase inhibitor alone or combined with NO may be a useful noninvasive therapeutic method to treat acute or chronic pulmonary hypertension.

Selective pulmonary vasodilation by intravenous infusion of an ultrashort half-life nucleophile/nitric oxide adduct

BACKGROUND

PROLI/NO (C5H7N3O4Na2 x CH3OH) is an ultrashort-acting nucleophile/NO adduct that generates NO (half-life 2 s at 37 degrees C and pH 7.4). Because of its short half-life, the authors hypothesized that intravenous administration of this compound would selectively dilate the pulmonary vasculature but cause little or no systemic hypotension.

METHODS

In eight awake healthy sheep with pulmonary hypertension induced by 9,11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F2alpha, the authors compared PROLI/NO with two reference drugs-inhaled NO, a well-studied selective pulmonary vasodilator, and intravenous sodium nitroprusside (SNP), a nonselective vasodilator. Sheep inhaled 10, 20, 40, and 80 parts per million NO or received intravenous infusions of 0.25, 0.5, 1, 2, and 4 microg x kg-1 x min-1 of SNP or 0.75, 1.5, 3, 6, and 12 microg x kg-1 x min-1 of PROLI/NO. The order of administration of the vasoactive drugs (NO, SNP, PROLI/NO) and their doses were randomized.

RESULTS

Inhaled NO selectively dilated the pulmonary vasculature. Intravenous SNP induced nonselective vasodilation of the systemic and pulmonary circulation. Intravenous PROLI/NO selectively vasodilated the pulmonary circulation at doses up to 6 microg x kg-1 x min-1, which decreased pulmonary vascular resistance by 63% (P < 0.01) from pulmonary hypertensive baseline values without changing systemic vascular resistance. At 12 microg x kg-1 x min-1, PROLI/NO decreased systemic and pulmonary vascular resistance and pressure. Exhaled NO concentrations were higher during PROLI/NO infusion than during SNP infusion (P < 0.01 with all data pooled).

CONCLUSIONS

The results suggest that PROLI/NO could be a useful intravenous drug to vasodilate the pulmonary circulation selectively.

Low concentrations of nitric oxide increase oxygen affinity of sickle erythrocytes in vitro and in vivo

The hallmark of sickle cell disease (SCD) is the polymerization of deoxygenated sickle hemoglobin (HbS). In SCD patients, one strategy to reduce red blood cell (RBC) sickling is to increase HbS oxygen affinity. Our objective was to determine if low concentrations of nitric oxide (NO) gas would augment the oxygen affinity of RBCs containing homozygous HbS (SS). Blood containing normal adult hemoglobin (AA) or SS RBCs was incubated in vitro in the presence of varying concentrations of NO up to 80 ppm, and oxygen dissociation curves (ODCs) were measured. In addition, blood was obtained from three AA and nine SS volunteers, before and after breathing 80 ppm NO in air for 45 min, and the ODCs were measured. Exposure of SS RBCs to 80 ppm NO in vitro for 5 min or longer decreased the partial pressure of oxygen at which hemoglobin is 50% saturated with oxygen (P50), an average of 15% (4.8+/-1.7 mmHg mean+/-SE; P < 0.001). The increase in SS RBC oxygen affinity correlated with the NO concentration. The P50 of AA RBCs was unchanged (P > 0.1) by 80 ppm NO. In SS volunteers breathing 80 ppm NO for 45 min, the P50 decreased (P < 0.001) by 4.6+/-2.0 mmHg. 60 min after NO breathing was discontinued, the RBC P50 remained decreased in five of seven volunteers in whom the ODC was measured. There was no RBC P50 change (P > 0.1) in AA volunteers breathing NO. Methemoglobin (Mhb) remained low in all subjects breathing NO (SS Mhb 1.4+/-0.5%), and there was no correlation (r = 0.02) between the reduction in P50 and the change in Mhb. Thus, low concentrations of NO augment the oxygen affinity of sickle erythrocytes in vitro and in vivo without significant Mhb production. These results suggest that low concentrations of NO gas may offer an attractive new therapeutic model for the treatment of SCD.

Pulmonary vasoconstriction and hypertension in mice with targeted disruption of the endothelial nitric oxide synthase (NOS 3) gene

NO, synthesized in endothelial cells by endothelial NO synthase (NOS 3), is believed to be an important endogenous pulmonary vasodilator substance that contributes to the normal low pulmonary vascular resistance. To selectively investigate the role of NOS 3 in the pulmonary circulation, mice with targeted disruption of the NOS 3 gene were studied. Pulmonary hemodynamics were studied by measuring pulmonary artery pressure, left ventricular end-diastolic pressure, and lower thoracic aortic flow by using a novel open-chest technique. Transient partial occlusion of the inferior vena cava was used to assess the pulmonary artery pressure-flow relationship. Tension developed by isolated pulmonary artery segments after acetylcholine stimulation was measured in vitro. The histological appearance of NOS 3-deficient and wild-type murine lungs was compared. NOS 3-deficient mice (n = 27), when compared with wild-type mice (n = 32), had pulmonary hypertension (pulmonary artery pressure, 19.0 +/- 0.8 versus 16.4 +/- 0.6 mm Hg [mean +/- SE]; P < .05) that was due to an increased total pulmonary resistance (62 +/- 6 versus 33 +/- 2 mm Hg.min.g.mL-1; P < .001). In vitro, acetylcholine induced vasodilation in the main pulmonary arteries of wild-type but not NOS 3-deficient mice. The morphology of the lungs of NOS 3-deficient mice did not differ from that of wild-type mice. We conclude that NOS 3 is a key enzyme responsible for providing basal pulmonary NO release. Congenital NOS 3 deficiency produces mild pulmonary hypertension in mice.

Inhaled nitric oxide and persistent pulmonary hypertension of the newborn. The Inhaled Nitric Oxide Study Group

BACKGROUND

Persistent pulmonary hypertension of the newborn causes systemic arterial hypoxemia because of increased pulmonary vascular resistance and right-to-left shunting of deoxygenated blood. Inhaled nitric oxide decreases pulmonary vascular resistance in newborns. We studied whether inhaled nitric oxide decreases severe hypoxemia in infants with persistent pulmonary hypertension.

METHODS

In a prospective, multicenter study, 58 full-term infants with severe hypoxemia and persistent pulmonary hypertension were randomly assigned to breathe either a control gas (nitrogen) or nitric oxide (80 parts per million), mixed with oxygen from a ventilator. If oxygenation increased after 20 minutes and systemic blood pressure did not decrease, the treatment was considered successful and was continued at lower concentrations. Otherwise, it was discontinued and alternative therapies, including extracorporeal membrane oxygenation, were used.

RESULTS

Inhaled nitric oxide successfully doubled systemic oxygenation in 16 of 30 infants (53 percent), whereas conventional therapy without inhaled nitric oxide increased oxygenation in only 2 of 28 infants (7 percent). Long-term therapy with inhaled nitric oxide sustained systemic oxygenation in 75 percent of the infants who had initial improvement. Extracorporeal membrane oxygenation was required in 71 percent of the control group and 40 percent of the nitric oxide group (P=0.02). The number of deaths was similar in the two groups. Inhaled nitric oxide did not cause systemic hypotension or increase methemoglobin levels.

CONCLUSIONS

Inhaled nitric oxide improves systemic oxygenation in infants with persistent pulmonary hypertension and may reduce the need for more invasive treatments.

Hyporesponsiveness to inhaled nitric oxide in isolated, perfused lungs from endotoxin-challenged rats

Inhaled nitric oxide (iNO) causes selective pulmonary vasodilation and improves oxygenation in patients with the adult respiratory distress syndrome (ARDS). Approximately 30% of ARDS patients fail to respond to iNO. Because sepsis syndrome often accompanies a decreased response to iNO, we investigated NO responsiveness in isolated, perfused lungs from rats exposed to lipopolysaccharide (LPS). Eighteen hours after intraperitoneal injection of 0.5 mg/kg LPS, rat lungs were isolated, perfused, and preconstricted with U-46619. Ventilation with 0.4, 4, and 40 parts per million by volume NO vasodilated LPS-pretreated lungs 75, 47, and 42% less than control lungs (P < 0.01 value differs at each concentration). The diminished vasodilatory response to iNO was associated with decreased NO-stimulated guanosine 3',5'-cyclic monophosphate (cGMP) release into the perfusate. Soluble guanylate cyclase activity did not differ in lung extracts from LPS-pretreated and control rats. LPS increased pulmonary cGMP-phosphodiesterase (PDE) activity by 40%. The PDE-sensitive cGMP analogue 8-bromoguanosine 3',5'-cyclic monophosphate vasodilated lungs from LPS-pretreated rats less than lungs from control rats. In contrast, the PDE-insensitive 8-para-chlorophenylthioguanosine 3',5'-cyclic monophosphate vasodilated lungs equally from both groups. After LPS challenge, the rat pulmonary vasculature becomes hyporesponsive to iNO. Hyporesponsiveness to iNO appears partly attributable to increased pulmonary cGMP-PDE activity.

Localizing antithrombotic and vasodilatory activity with a novel, ultrafast nitric oxide donor

Reaction of nitric oxide (NO) with L-proline in methanolic sodium methoxide yields a diazeniumdiolate product, C5H7N3O4Na2.CH3OH (PROLI/NO), that can be stabilized in basic solution but that dissociates to proline (1 mol) and NO (2 mol) with a half-life of only 1.8 s at pH 7.4 and 37 degrees C. This kinetic behavior has allowed the generation of highly localized antiplatelet and vasodilatory effects. By infusing solutions containing 4 microM PROLI/NO in 0.1 M sodium hydroxide at the rate of 1 nmol.min-1 immediately upstream from a polyester vascular graft in the unheparinized baboon circulatory system, for example, platelet deposition at the normally thrombogenic graft surface was substantially reduced relative to controls receiving only 0.1 M sodium hydroxide. In a second study, infusion of PROLI/NO into the right atrium of sheep with induced pulmonary hypertension selectively dilalated the lung vasculature, dose-dependently reducing the pulmonary arterial pressure by as much as 9 mmHg with no observable effect on the systemic arterial pressure at an infusion rate of up to 24 nmol.kg-1.min-1. PROLI/NO could also be formulated as an insoluble polymer blend that released NO smoothly for prolonged periods. The results suggest that localized delivery of diazeniumdiolates such as PROLI/NO which generate NO with extreme rapidity on entering the blood stream may hold considerable promise for inhibition of thrombus formation, selective dilation of the vasculature, and other research and clinical applications.

Inhaled nitric oxide increases coronary artery patency after thrombolysis

BACKGROUND

Nitric oxide (NO) and nitrosovasodilators that release NO inhibit platelet aggregation. The antithrombotic effect of intravenously infused nitrosovasodilators is usually accompanied by systemic vasodilation. Inhaled NO is a pulmonary vasodilator that does not produce systemic hemodynamic effects. This study examines the antithrombotic effect of inhaled NO in a canine model of platelet-mediated coronary artery reocclusion after thrombolysis.

METHODS AND RESULTS

In 25 anesthetized dogs, a segment of the left anterior descending coronary artery was traumatized and a high-grade stenosis created. Thrombus was injected at this site, and tissue plasminogen activator was administered, producing cyclic flow variations (CFVs) in 24 of 25 dogs. CFV frequency was unchanged in dogs not breathing NO but decreased by 35 +/- 9% (P < .05) and 53 +/- 7% (P < .01) while dogs breathed 20 and 80 parts per million (ppm) NO, respectively. The coronary artery patency ratio (fraction of time during which the coronary artery was patent; CAPR) was unchanged in dogs not treated with NO but increased from 51 +/- 7% to 64 +/- 8% while breathing 20 ppm NO (P < .01) and from 49 +/- 3% to 75 +/- 7% while breathing 80 ppm NO (P < .01). The increased CAPR during 80 ppm NO administration persisted during a 45-minute posttreatment period (70 +/- 7%, P < .05 versus baseline). NO inhalation did not change systemic hemodynamics. In a pharmacological model of coronary vasoconstriction, inhaled NO did not reverse the effect of the thromboxane A2 agonist U-46619. In vitro ADP-induced platelet aggregation was inhibited by NO gas.

CONCLUSIONS

Inhaled NO at concentrations of 20 and 80 ppm increases coronary patency and decreases CFV frequency in a canine model of platelet-mediated coronary reocclusion after thrombolysis without producing systemic hemodynamic effects.

Combined effects of NO inhalation and intravenous PGF2 alpha on pulmonary circulation and gas exchange in an ovine ARDS model

OBJECTIVES

Inhalation of nitric oxide (NO) selectively dilates pulmonary vessels in well-ventilated regions. Prostaglandin F2 alpha (PGF2 alpha) is a vasoconstrictor and is reported to enhance hypoxic pulmonary vasoconstriction. The objective of this study was to examine whether the combination of intravenous PGF2 alpha and inhaled NO in ARDS lungs has a beneficial effect on oxygenation.

DESIGN

We investigated the effect of intravenous PGF2 alpha infusion (0.05-10.0 micrograms/kg per min) with and without NO inhalation (60 ppm) on the hemodynamics and gas exchange in an ovine ARDS model, examining the pulmonary artery pressure versus the flow plot by varying cardiac output.

MEASUREMENTS AND RESULTS

After lung lavage, NO inhalation reduced the mean pulmonary arterial pressure (MPAP) by decreasing the zero-flow pressure intercept from 10.6 +/- 3.8 (mean +/- SD) to 8.5 +/- 3.8 mmHg (p < 0.05) with no significant change in slope. NO inhalation improved PaO2 from 56 +/- 12 to 84 +/- 38 mmHg (p < 0.005) and reduced pulmonary shunt from 65 +/- 5 to 53 +/- 8% (Qs/Qt) (p < 0.001). The dose-dependent effects of PGF2 alpha infusion were: (1) increased MPAP attributed to an increased slope in pulmonary artery pressure-flow plot; (2) decreased cardiac index; (3) decreased Qs/Qt with unchanged PaO2. The dose-dependent decrease in Qs/Qt after PGF2 alpha infusion was attributed to the decreased cardiac output.

CONCLUSIONS

It is suggested that inhalation of NO reduced the critical vascular pressure near alveoli without affecting upstream vessels, while infused PGF2 alpha constricted the larger upstream pulmonary artery vessels without appreciably affecting the critical pressure. Inhalation of NO into well-ventilated lung areas shifted perfusion to well-oxygenated areas, and there was no supplemental shift in blood flow by adding an infusion of PGF2 alpha.

Chronic inhalation of nitric oxide inhibits neointimal formation after balloon-induced arterial injury

Systemic and local intravascular NO administration inhibits neointimal formation after vascular injury in animal models. NO appears to attenuate smooth muscle proliferation both directly and indirectly by preventing the release of growth factors. Inhalation of low concentrations of NO dilates pulmonary vascular smooth muscle but does not cause systemic vasodilatation. Recently, NO inhalation was found to inhibit platelet function in vivo. We studied the effects of NO inhalation on neointimal formation after balloon-induced injury of the adult rat carotid artery. Beginning 60 minutes before carotid injury, rats breathed either air with 0 or 80 ppm NO for 14 days. Rats were killed, carotid arteries were fixed and paraffin-embedded, and neointimal formation was measured by analyzing the ratio of intimal to medial areas (I/M ratio) in carotid artery cross sections. Intimal hyperplasia was evident in both groups of animals, but I/M ratios were 43% less in animals breathing 80 ppm NO for 2 weeks than in animals breathing air alone (0.78 +/- 0.12 and 1.37 +/- 0.11 [mean +/- SE], respectively; P < .02). Similarly, 1 week after carotid injury, neointimal formation was less in rats breathing 80 ppm NO than in rats breathing air alone (I/M ratio, 0.39 +/- 0.11 versus 0.76 +/- 0.06; P < .02). Breathing 20 ppm NO for 2 weeks or 80 ppm NO for 1 week followed by air alone for 1 week did not attenuate neointimal formation measured at 14 days. In anesthetized rats breathing 80 ppm NO or air alone for 1 hour, neither systemic blood pressure nor bleeding time differed. These observations demonstrate that inhaling 80 ppm NO inhibits neointimal formation after balloon-induced carotid artery injury in rats. NO inhalation may represent a safe and novel method of preventing restenosis after percutaneous angioplasty.

Inhaled nitric oxide. A bronchodilator in mild asthmatics with methacholine-induced bronchospasm

Nitric oxide (NO) reduces airway tone in the methacholine-treated guinea pig. We examined whether low levels of inhaled NO gas would relax airway smooth muscle tone in patients with mild asthma subjected to methacholine-induced bronchospasm. Thirteen adult volunteers with mild asthma inspired increasing concentrations of methacholine until their baseline forced expiratory volume in one second (FEV1, 3.29 +/- 0.17 L, mean +/- SEM) decreased by > or = 20% (2.33 +/- 0.18 L, p < 0.01). Thereafter, they sequentially inhaled 100 parts per million (ppm) NO, 40% O2; 40% O2; and 100 ppm NO, 40% O2 while spirometry was performed. Subsequent inhalation of isoproterenol returned the FEV1 levels to baseline. Inhaling 100 ppm NO increased FEV1 to 2.66 +/- 0.18 L (p < 0.01), and this increase was maintained after NO was discontinued. FEV1 did not change during the second period of NO inhalation. Similar results were observed for vital capacity, but no significant effect was noted on forced expiratory flow at 25% of vital capacity or peak expiratory flow. Subjects were then divided into a responder subgroup, which showed a mean increase in FEV1 after initial NO inhalation of 560 +/- 150 ml, and a nonresponder subgroup, which showed a mean increase in FEV1 of 129 +/- 29 ml. Our data suggest that inhalation of nitric oxide by patients with mild asthma with methacholine-induced bronchospasm results in a minor but significant relaxation of airway tone.

Splenic contraction, catecholamine release, and blood volume redistribution during diving in the Weddell seal

The spleen of the Weddell seal (Leptonychotes weddelli) may contract and inject red blood cells (RBCs) into the peripheral circulation during diving, but evidence for this hypothesis is indirect. Accordingly, we measured splenic dimensions by ultrasonography, plasma catecholamine concentrations, hemoglobin concentration, and hematocrit in five Weddell seals before and after intravenous epinephrine during halothane anesthesia and while awake at the surface after voluntary dives. Spleen size was reduced immediately after epinephrine injection or after the seal surfaced. Within the first 2 min after the seal surfaced, cephalocaudal splenic length was 71 +/- 2% (mean +/- SD; P < 0.05) and splenic thickness was 71 +/- 4% (P < 0.05) of the maximal resting values. Splenic size increased (half-time = 6-9 min) after the seal surfaced and was inversely correlated with plasma epinephrine and norepinephrine concentrations. Hemoglobin concentration increased from 17.5 +/- 5.3 g/dl (measured during general anesthesia) to 21.9 +/- 3.7 g/dl (measured in the first 2 min after surfacing). At these same times, the hematocrit increased from 44 +/- 12 to 55 +/- 8%. These values decreased (half-time = 12-16 min) after the seal surfaced. We estimate 20.1 liters of RBCs were sequestered at rest, presumably in the spleen, and released either on epinephrine injection or during diving. Catecholamine release and splenic contraction appear to be an integral part of the voluntary diving response of Weddell seals.

In vivo lipopolysaccharide pretreatment inhibits cGMP release from the isolated-perfused rat lung

Administration of bacterial lipopolysaccharide (LPS) to rats stimulates synthesis of nitric oxide (NO), a free radical molecule that activates soluble guanylate cyclase, thereby increasing intracellular guanosine 3',5'-cyclic monophosphate (cGMP) concentration and inducing systemic vasodilatation. To investigate the effect of endotoxemia on the pulmonary NO/cGMP signal transduction system, we measured the release of cGMP by isolated-perfused lungs of rats that received an intraperitoneal injection of LPS (1 mg/kg) or saline 2 days earlier. Over 90 min, 1.4 +/- 0.78 and 0.079 +/- 0.016 nmol cGMP accumulated in pulmonary perfusates of saline- and LPS-treated rats, respectively (P < 0.05). Despite addition to the perfusate of Zaprinast, superoxide dismutase, or A23187, markedly less cGMP was released from the lungs of rats exposed to LPS than from the lungs of control rats. In contrast, after ventilation with 100 parts per million NO gas, cGMP accumulating in the perfusate of the lungs of both groups of rats was markedly increased, and the quantity of cGMP released from the lungs of LPS-treated rats was similar to that released by control rat lungs (2.8 +/- 0.57 vs. 3.3 +/- 0.88 nmol, P = NS). With the use of immunoblot techniques, equal concentrations of constitutive endothelial NO synthase were detected in the lungs of rats treated with saline or LPS. These results demonstrate that the NO/cGMP signal transduction system is abnormal in the lungs of rats exposed to LPS, at least in part, at the level of endothelial NO synthase activation.

Hormonal regulatory adjustments during voluntary diving in Weddell seals

Subadult male Weddell seals were instrumented with microcomputer-based backpacks and were then monitored during voluntary diving and recovery periods in McMurdo Sound, Antarctica. Depth and duration of diving, swim speed, and dive pattern were routinely monitored. An indwelling venous catheter was used to collect plasma samples at various time periods before and following diving episodes, so that changes in plasma concentrations of hormones and of metabolites could be measured. Adrenergic and nitroxidergic regulatory effects were assessed indirectly by measuring concentration changes in catecholamine and cyclic guanosine monophosphate (cGMP), respectively. The studies found that (i), except for dives of less than several minutes, epinephrine and norepinephrine both increased as a function of diving duration, then rapidly decreased during recovery (with a half time of about 10 min), (ii) that the changes in catecholamine concentrations correlated with splenic contraction and an increase in circulating red blood cell mass (hematocrit), (iii) that the changes in catecholamines, especially [epinephrine], were inversely related to insulin/glucagon ratios, which mediated a postdiving hyperglycemia, and (iv) that in long dives (but not short ones) the changes in catecholamines correlated with increasing reliance on anaerobic metabolism, indicated by increased plasma lactate concentrations. These diving-catecholamine relationships during voluntary diving at sea were similar to those observed during enforced submergence (simulated diving) under controlled laboratory conditions. At the end of diving, even while catecholamine concentrations were still high, many of the above effects were rapidly reversed and the reversal appeared to correlate with accelerated nitric oxide production, indirectly indicated by increased plasma cGMP concentrations. Taken together, the data led to the hypothesis of important adrenergic regulation of the diving response in seals, with rapid reversal at the end of diving and during recovery being regulated by nitroxidergic mechanisms.

Inhaled nitric oxide does not alter endotoxin-induced nitric oxide synthase activity during rat lung perfusion

Nitric oxide (NO) has been demonstrated to decrease its own synthesis in tissue preparations. We tested the hypothesis that endogenous NO synthesis induced by lipopolysaccharides (LPS) would be decreased by exogenous NO during isolated lung perfusion. Rats were pretreated with either saline or LPS 48 h before lung harvest. Endogenous NO synthase activity was measured as conversion of L-[14C]-arginine to L-[14C]citrulline during 90 min of perfusion. NO (100 ppm) was added to the ventilating gas during perfusion of lungs from one group of control or LPS-treated rats. A second group of control or LPS-treated rats was exposed chronically to 100 ppm NO for the 48 h before lung harvest, in addition to receiving 100 ppm NO added to the ventilating gas during lung perfusion. We conclude that conversion of L-[14C]arginine to L-[14C]citrulline was minimal in control lungs and increased in response to LPS pretreatment. NO added to the ventilating gas for the 90 min of ex vivo perfusion did not alter the rate of L-[14C]citrulline production. In vivo exposure to 100 ppm NO for 48 h did not alter the induction of inducible NO synthase activity as measured during ex vivo lung perfusion. This indicates that inhaled NO does not exert negative-feedback inhibition on inducible NO synthase in the ex vivo rat lung.

Myoglobin saturation in free-diving Weddell seals

Although the consumption of myoglobin-bound O2 (MbO2) stores in seal muscles has been demonstrated in seal muscles during laboratory simulations of diving, this may not be a feature of normal field diving in which measurements of heart rate and lactate production show marked differences from the profound diving response induced by forced immersion. To evaluate the consumption of muscle MbO2 stores during unrestrained diving, we developed a submersible dual-wavelength laser near-infrared spectrophotometer capable of measuring MbO2 saturation in swimming muscle. The probe was implanted on the surface of the latissimus dorsi of five subadult male Weddell seals (Leptonychotes weddelli) released into a captive breathing hole near Ross Island, Antarctica. Four seals had a monotonic decline of muscle O2 saturation during free diving to depths up to 300 m with median slopes of -5.12 +/- 4.37 and -2.54 +/- 1.95%/min for dives lasting < 17 and > 17 min, respectively. There was no correlation between the power consumed by swimming and the desaturation rate. Two seals had occasional partial muscle resaturations late in dives, indicating transfer of O2 from circulating blood to muscle myoglobin. Weddell seals partially consume their MbO2 stores during unrestrained free diving.

Effects of targeted neuronal nitric oxide synthase gene disruption and nitroG-L-arginine methylester on the threshold for isoflurane anesthesia

BACKGROUND

Considerable evidence suggests that nitric oxide plays a role in synaptic transmission in the central and peripheral nervous system. Nonselective inhibition of nitric oxide synthase by nitroG-L-arginine methylester (L-NAME) reduces the minimum alveolar concentration of halothane anesthesia. The effects of selective neuronal nitric oxide synthase inhibition on the anesthetic requirements in mice congenitally deficient in neuronal nitric oxide synthase (knockout mice) were examined.

METHODS

Isoflurane minimum alveolar concentration and righting reflex ED50 (RRED50) were determined in knockout and wild-type mice. Subsequently, the effects of intraperitoneal L-NAME on minimum alveolar concentration and RRED50 of knockout and wild-type mice were examined. In a separate experiment, the effects of week-long administration of L-NAME were examined in wild-type mice. Isoflurane minimum alveolar concentration and RRED50 were measured on the 8th day and were repeated after an acute intraperitoneal dose of L-NAME.

RESULTS

Targeted disruption of the neuronal nitric oxide synthase gene did not modify isoflurane minimum alveolar concentration and RRED50 of knockout mice. Acute intraperitoneal L-NAME decreased the minimum alveolar concentration and RRED50 of wild-type but did not alter those values in knockout mice. The wild-type mice, when given L-NAME for a week, showed a minimum alveolar concentration and RRED50 identical to that of untreated wild-type mice.

CONCLUSIONS

Although acute nonselective inhibition of nitric oxide synthase reduces the anesthetic requirements of wild-type mice, a chronic congenital deficiency of neuronal nitric oxide synthase or a week of L-NAME treatment of wild-type mice does not produce a state of greater sensitivity to the effects of isoflurane anesthesia.

Constitutive endothelial nitric oxide synthase gene expression is regulated during lung development

Nitric oxide (NO), a potent vasodilator, is a free-radical gas synthesized from L-arginine by nitric oxide synthases (NOS). NO appears to have an important role in perinatal changes in pulmonary vascular resistance. We previously identified mRNA encoding the constitutive endothelial NOS (ceNOS) isoform in human pulmonary tissue. To begin investigating functions of this enzyme in perinatal pulmonary development, we measured ceNOS mRNA and immunoreactivity in the developing rat lung. With the use of RNA blot hybridization, abundant pulmonary ceNOS mRNA was detected during the late fetal and postnatal period. The highest levels were detected within 24 h after birth, and elevated mRNA levels persisted for 16 days. In contrast, much lower levels of ceNOS mRNA were found in adult rat lung. With the use of immunoblot techniques, ceNOS protein levels were found to be correlated with mRNA levels. To identify the pulmonary cell types expressing the ceNOS gene, in situ hybridization with a digoxigenin-labeled cRNA probe was performed on sections from lungs of 1-day-old and adult rats. In lungs from 1-day-old rats, ceNOS mRNA was detected in alveolar and serosal epithelial cells as well as in endothelial cells lining small and medium-sized blood vessels. In contrast, in adult lungs, ceNOS gene transcripts were detected in rare endothelial cells. These observations suggest that ceNOS gene expression is regulated during lung development and that ceNOS is available to participate in the postnatal reduction of pulmonary vascular resistance. ceNOS gene expression in nonendothelial cells in the neonatal rat lung suggests that NO may also contribute to nonvascular functions in the developing lung.

Prolonged pulmonary vasodilator action of inhaled nitric oxide by Zaprinast in awake lambs

Inhaled nitric oxide (NO) has been shown to selectively dilate the pulmonary vasculature. Zaprinast, an inhibitor of guanosine 3',5'-cyclic monophosphate-specific phosphodiesterase, augments smooth muscle relaxation induced by endothelium-dependent vasodilators. The present study was designed to determine whether intravenous administration of Zaprinast potentiates the vasodilating effects or prolongs the duration of action of intermittent NO inhalation. Eight awake lambs with U-46619-induced pulmonary hypertension breathed three concentrations of NO (5, 10, and 20 ppm) in a random order before and during an intravenous Zaprinast infusion (0.1 mg.kg-1.min-1). Inhaled NO decreased pulmonary arterial pressure (PAP) in a dose-dependent fashion, with mean PAP reduction at 5, 10, and 20 ppm NO inhalation of 6 +/- 1, 7 +/- 1, and 9 +/- 1 (SE) mmHg, respectively. Although the Zaprinast infusion did not change the magnitude of mean PAP reduction, it caused a statistically significant reduction of pulmonary vascular resistance and prolonged the duration of action of inhaled NO (half-times of vasodilator response to 5, 10, and 20 ppm NO inhalation: 1.9 +/- 0.1, 2.1 +/- 0.2, and 2.1 +/- 0.2 min, respectively; half-times of NO inhalation with Zaprinast: 9.7 +/- 1.7, 11.5 +/- 2.2, and 12.3 +/- 2.0, respectively). Plasma concentrations as well as the transpulmonary differences of guanosine 3',5'-cyclic monophosphate were increased by the Zaprinast infusion during NO inhalation. A stable level of pulmonary vasodilation was demonstrated in four additional lambs by combining intermittent NO breathing with an intravenous infusion of Zaprinast.