Hypothermia attenuates NO production in anesthetized rats with endotoxemia.

Sepsis is often associated with upregulation of nitric oxide production and fever, and it is common to control an excessive febrile response with antipyretic therapy and external cooling. Our aim was to evaluate the effect of hypothermia on NO production in a model of septic shock. Rats were anesthetized, ventilated, and instrumented for hemodynamic monitoring and divided into four groups. Normothermic controls (NC) received saline intravenously and were maintained at 37 °C. Hypothermic controls (HC) received saline but were allowed to become hypothermic. Normothermic endotoxic (NE) received Escherichia Coli lipopolysaccharides (LPS) intravenously to induce endotoxic shock and was maintained at 37 °C. Hypothermic endotoxic (HE) received LPS intravenously and was allowed to become hypothermic. Exhaled NO (NOe) was measured from mixed expired gas at time zero and every 30 min, for 5 h. After injection of LPS, NOe increased substantially in the NE group (700 ± 24 ppb), but increased only to 25 ± 2 ppb in the HE group. NOe increased to 90 ± 3 ppb in the NC group, and to 17.6 ± 3.1 ppb in the HC group after 5 h (P < 0.05), whilst blood pressure remained stable. In the HE group, blood pressure fell immediately after injection of the LPS, but thereafter remained stable despite the rise in NOe. In the NE group, the blood pressure fell gradually, and the animals became hypotensive. During the natural course of endotoxemia in anesthetized rats, allowing severe hypothermia to ensue by not actively managing temperature and hemodynamics resulted in significantly reduced expired NO concentrations, lung injury, and prolonged survival. The clinical benefits of such a finding currently remain unclear and merit further investigation.

Nitric oxide plays a minimal role in hypoxic pulmonary vasoconstriction in isolated rat lungs.

The goal of this study was to elucidate the importance of nitric oxide production during hypoxic pulmonary vasoconstriction (HPV). One group of Sprague Dawley rats received an ip injection of saline (controls), while a second group received an ip injection of Escherichia coli lipopolysacharides (LPS-treated) to render them septic. Three hours later, the animals were anesthetized and prepared for the isolated lung experiment. The lungs were ventilated and perfused with diluted autologous blood (Hct 23%) at constant flow rate while monitoring pulmonary arterial pressure (Pa). Nitric oxide production from the lungs was monitored by measuring its concentration in the mixed exhaled gas (NOe) offline. NOe in the isolated lungs was 2 ppb in controls and 90 ppb in the LPS treated lungs. Hypoxia caused Pa to rise from 10 to 17 mmHg in control lungs, and from 10 to 27 mmHg in the LPS treated lungs. NO production was then manipulated to determine if it affects HPV. NOe was increased by adding L-arginine to the blood, and was blocked by adding nitro-L-arginine (LNA). L-Arginine had minimal effect on NOe in control lungs, but increased NOe in LPS treated lungs, and yet HPV was similar in the 2 groups. Despite inhibition of NO synthesis with nitro-L-arginine (LNA), HPV was potentiated equally in control and in LPS treated lungs (Pa rose by 23 mmHg). Thus NO production did not affect the difference in HPV between control and LPS treated lungs. The results suggest that NO does not plays a primary role in HPV.

Diagnosing sleep apnea in dental patients.

Airway dilator or oral appliance therapy has been recommended with behavioral methods as treatment for patients with primary snoring or mild sleep apnea, unless there are patient-specific contraindications. Airway dilator therapy therefore may be a preferred treatment for patients with mild sleepiness and with fewer than 15 episodes of sleep disordered breathing per hour. The dentist's main diagnostic concern is to separate those patients with moderate or severe sleep apnea, who should be referred to a medical specialist for treatment, from those patients with primary snoring or mild sleep apnea, whom the dentist may treat primarily. Diagnostic tests including clinical predictions formulas, polysomnography, cardiorespiratory sleep studies, and oximetry will be discussed in this review.

Role of nitric oxide in acidosis-induced intestinal injury in anesthetized rats.

We investigated the pathogenic mechanism(s) of small intestinal injury during acidosis in relation to circulating nitric oxide (NO) in an experimental rat model. Rats were anesthetized, paralyzed, and mechanically ventilated with room air. Hydrochloric acid (0.16 mmol bolus followed by 0.132 mmol/kg/h) was infused through the jugular vein for 5 hours. Control rats received a saline infusion. Arterial blood gases, blood pressure, and blood pH were measured every 30 minutes. The involvement of NO in this acidosis model was assessed by measuring plasma concentration of nitrite/nitrate (NOx) and by evaluating inducible NO synthase (iNOS) expression in small intestinal mucosa. Intestinal injury was assessed by measuring myeloperoxidase (MPO) activity, thiobarbituric acid reactants (TBARS), and histologic scores. HCl infusion was associated with hypotension, decreased blood pH, increased plasma concentration of NOx, augmented intestinal mucosal iNOS expression, MPO activity, TBARS, and histopathologic injury scores. Pretreatment with an iNOS inhibitor, aminoguanidine (AG, 50 mg/kg), reversed HCl-induced hypotension without a change in blood pH. HCl-induced lesions, MPO activity, TBARS, and plasma NOx production were decreased by AG. Our data show that the pathogenic mechanisms of acidosis-induced small intestinal lesions involve up-regulation of NO production by increased expression of iNOS and augmentation of superoxide radicals and MPO activity.

Inducible nitric oxide synthase in the lung and exhaled nitric oxide after hyperoxia.

The effect of hyperoxia on nitric oxide (NO) production in intact animals is unknown. We described the effects of hyperoxia on inducible nitric oxide synthase (iNOS) expression and NO production in the lungs of rats exposed to high concentrations of oxygen. Animals were placed in sealed Plexiglas chambers and were exposed to either 85% oxygen (hyperoxic group) or 21% oxygen (negative control group). Animals were anesthetized after 24 and 72 h of exposure and were ventilated via a tracheotomy. We measured NO production in exhaled air (E(NO)) by chemiluminescence. The lungs were then harvested and processed for detection of iNOS by immunohistochemistry and Western blotting analysis. The same experiments were repeated in animals exposed to hyperoxia for 72 h after they were infused with L-arginine. We used rats that were injected intraperitoneally with Escherichia coli lipopolysaccharide to induce septic shock as a positive control group. Hyperoxia and septic shock induced expression of iNOS in the lung. However, E(NO) was elevated only in septic shock rats but was normal in the hyperoxic group. Exogenous infusion of L-arginine after hyperoxia did not increase E(NO). To exclude the possibility that in the hyperoxic group NO was scavenged by oxygen radicals to form peroxynitrite, lungs were studied by immunohistochemistry for the detection of nitrotyrosine. Nitrotyrosine was found in septic shock animals but not in the hyperoxic group, further suggesting that NO is not synthesized in rats exposed to hyperoxia. We conclude that hyperoxia induces iNOS expression in the lung without an increase in NO concentration in the exhaled air.

Segmental pulmonary vascular responses to changes in pH in rat lungs: role of nitric oxide.

BACKGROUND: Respiratory or renal failure is associated with changes in blood pH. Changes in pH may have profound effects on vascular tone and reactivity. Site of action of acidosis in the pulmonary vasculature and the role of nitric oxide production remain unclear. METHODS: We utilized isolated rat lung preparation perfused with autologous blood (Hct = 20%, flow rate = 33 ml/min), and investigated the effect of acidosis and alkalosis (induced by ventilation with high and low inspired CO2) on vascular resistance and the role of nitric oxide during resting and elevated tone conditions. Changes in resistance were described in terms of small and large arteries and veins, using the vascular occlusion technique. RESULTS: Acidosis (Pco2 = 66.7 +/- 0.7 mmHg, pH = 7.17 +/- 0.01, Po2 = 255 +/- 3 mmHg) caused vasoconstriction under resting and increased vascular tone conditions (U46619-induced). The changes in resistance occurred primarily in the small arteries. In contrast, alkalosis (Pco2 = 20.1 +/- 0.3 mmHg, pH = 7.61 +/- 0.01, Po2 = 244 +/- 3 mmHg) caused vasodilation only at elevated tone conditions. Nitro-L-arginine (LNA), an inhibitor of nitric oxide synthase, increased vascular resistance slightly but did not modulate the responses to pH, suggesting that such responses are not nitric oxide dependent. During KCl-induced contraction, the effects of pH were abolished. CONCLUSIONS: We conclude that in rat lung, acidosis causes an increase in pulmonary vascular resistance at normal and elevated tone conditions. Furthermore, the response is limited primarily to the small arteries, and is not mediated by nitric oxide. Alkalosis tends to cause the opposite effects. The effects of acidosis and alkalosis were abolished when vascular tone was elevated with a low dose of KCl, suggesting that vascular response to pH may involve changes in membrane potential.

Acidosis stimulates nitric oxide production and lung damage in rats.

Systemic hypotension during sepsis is thought to be due to nitric oxide (NO) overproduction, but it may also be due to acidosis. We evaluated in healthy rats the consequences of acid infusion on NO and blood pressure. Sprague-Dawley rats were anesthetized, and ventilated with room air. The animals were randomized into four groups. Group 1 (C, n = 10) received only normal saline at rates comparable to the other groups. Group 2 (A1, n = 10) received hydrochloric acid at 0.162 mmol in the first 15 to 30 min, followed by a continuous infusion of 0.058 mmol/h for 5 h. Group 3 (AG+A1, n = 6) was pretreated with aminoguanidine (AG, 50 mg/kg), and HCl was infused as above. Group 4 (A2, n = 7) received HCl at twice the rate used in A1. Nitric oxide concentration in the exhaled gas (ENO), blood gases, and mean arterial pressure were measured every 30 min. Acid infusion in A1 caused the pH to fall gradually from 7.43 +/- 0. 01 to 7.13 +/- 0.05. This moderate decrease in pH was associated with a marked increase in ENO (1.6 +/- 0.3 to 114.2 +/- 22.3 ppb), an increase in plasma nitrite/nitrate (17.3 +/- 3.7 to 35.2 +/- 4.3 microM), and a significant decrease in blood pressure (110.5 +/- 6.3 to 63.3 +/- 15.0 mm Hg). Furthermore, acidosis caused lung inflammation, as suggested by the increase in lung myeloperoxidase activity (282.2 +/- 24.7 to 679.3 +/- 57.3 U/min/g) and lung injury score (1.7 +/- 0.2 to 3.5 +/- 0.6). Acidosis after AG pretreatment was associated with a similar change in pH, but the increase in ENO, nitrite/nitrate, and systemic hypotension were prevented. Furthermore, lung injury was attenuated by AG, as suggested by a lower myeloperoxidase activity, though lung injury score was not altered. In this model, moderate acidosis causes increases in NO, hypotension, and lung inflammation. Lung inflammation and injury are due in part to acidosis and NO production. This is the first report to show a direct effect of chronic acidosis on NO production and lung injury. These results have profound implications on the role of acidosis on NO production and lung injury during sepsis.

The synergistic effect of sympathectomy and hyperbaric oxygen exposure on transcutaneous PO2 in healthy volunteers.

UNLABELLED: The benefit of hyperbaric oxygen (HBO2) exposure is dependent on the oxygen delivery. Such benefit may be limited by the fact that hyperoxia causes vasoconstriction and decreases blood flow. The aim of this study was to determine whether regional sympathectomy attenuates this vasoconstriction response and thus improves oxygen delivery. In a double-blinded manner, healthy volunteers were subjected to HBO2 in a monoplace chamber on two occasions separated by at least 1 wk. Transcutaneous oxygen (tcPO2) and carbon dioxide (tcPCO2) on the forearm were monitored continuously, and blood flow in the axillary artery was measured using angiodynography before and after exposure to HBO2. During one visit, each volunteer received a sympathetic block to the upper extremity by an injection of lidocaine into the brachial plexus at the axilla. During a second visit, the volunteer received a placebo injection of isotonic sodium chloride solution into the brachial plexus of the same side. Skin temperature was recorded on the back of the hand. All subjects exhibited a small but significant increase in skin temperature (2.5%) and in upper limb blood flow (23%) (P < 0.05%) after sympathectomy, but not after isotonic sodium chloride solution injection. Sympathectomy increased tcPO2 marginally while in room air. However, during HBO2, tcPO2 was substantially and significantly higher (409.8+/-98.8 mm Hg) after sympathectomy compared with that after isotonic sodium chloride solution injection (171.3+/-38.1 mm Hg). tcPCO2 did not change significantly after sympathectomy or during HBO2. Thus, sympathectomy presumably improved oxygen delivery by preventing vasoconstriction during hyperoxia. The results suggest that sympathectomy may be a useful adjunct to HBO2 therapy in patients in whom vascular resistance is increased because of sympathetic tone or hyperoxia. IMPLICATIONS: Sympathetic nerve block of the extremities markedly enhances tissue oxygen delivery during hyperbaric oxygen treatment. Sympathectomy may be a beneficial adjunct treatment to hyperbaric oxygen in peripheral vascular insufficiency.

Effects of frusemide on pulmonary capillary pressure in horses exercising on a treadmill.

We hypothesised that frusemide would decrease pulmonary capillary pressure in horses during strenuous exercise. Seven horses were tested after receiving saline or frusemide (2 mg/kg bwt) in random order with an interval of at least one week. Measurements were made with the horses standing, exercising at 75, 90 and 100% HRmax (maximal heart rate), and then walking 2 min after cessation of 100% HRmax. The exercise tests lasted for approximately 3 min with an interval of walking between them. Pulmonary artery and oesophageal pressures were recorded continuously and subsequent analysis of the pulmonary artery pressure signal was carried out after subtraction of the oesophageal pressure signal. Pulmonary arterial pressure, pulmonary capillary pressure, pulmonary artery wedge pressure, breathing rate, heart rate and arterial blood gas tensions were recorded at each level of exercise. Pulmonary arterial wedge and pulmonary capillary pressures were determined from the pulmonary arterial waveform after dynamic occlusion of a branch of the pulmonary artery. The resulting decay in pressure was submitted to exponential curve fitting and the amplitude on this curve at the moment of occlusion was recorded as pulmonary capillary pressure. When adjusted for horse and exercise intensity, horses receiving frusemide had lower pulmonary capillary and wedge pressures (adjusted least-squares means = 36 mmHg and 28 mmHg, respectively) when compared with control values (adjusted least-squares means = 41 mmHg (P = 0.042) and 35 mmHg (P = 0.002), respectively). Pulmonary arterial pressure, breathing rate, heart rate and arterial blood gas tensions did not differ between treatments at any exercise intensity. We conclude that frusemide reduces pulmonary capillary and wedge pressures. This is compatible with reduced transcapillary filtration and, therefore, reduced accumulation of lung water at exercise. It may also account for the putative protective effect of frusemide against exercise-induced pulmonary haemorrhage.

Neutrophil sequestration in the lung following acute aortic occlusion starts during ischaemia and can be attenuated by tumour necrosis factor and nitric oxide blockade.

OBJECTIVES: To investigate the role of lower extremity ischaemia in acute lung injury with special emphasis on the role of tumour necrosis factor (TNF) and nitric oxide (NO) as mediators of neutrophil (PMN) chemotaxis in the lung. DESIGN: Prospective randomised study. MATERIALS AND METHODS: Sprague-Dawley rats were randomized into four groups: group 1 (x-clmap): aorta clamped just above the bifurcation for 3 h; group 2 (AG): 50 mg/kg aminoguanidine, a specific inducible NO synthase (iNOS) inhibitor, was administered prior to aortic occlusion; group 3 (Steroids): 1 mg/kg dexamethasone was administered prior to aortic occlusion; and group 4 (TNFbp): 2 mg/kg TNFbp, a PEGylated dimeric form of the high affinity TNF receptor I (R1) was administered prior to aortic occlusion to block TNF action. Groups 2, 3 and 4 were subjected to the same ischaemia time as group 1. NO concentration in the exhaled gas (ENO) was measured in 30 min intervals. At the end of the 3 h ischaemia, one lung was excised and fixed for routine histological evaluation, and the other underwent bronchoalveolar lavage (BAL). PMN chemotaxis towards the BAL fluid was then measured using the blindwell technique. RESULTS: ENO in group 1 increased from 0.9 +/- 0.3 ppb at baseline, to 41.3 +/- 9.2 ppb at the end of ischaemia. Animals in this group exhibited significant lung inflammation. Aminoguanidine, dexamethasone and TNFbp blocked NO production (peak ENO values of 7.2 +/- 1.9, 12.6 +/- 1.3 and 8.9 +/- 1.7 ppb for groups 2, 3 and 4 respectively), decreased PMN chemotaxis and sequestration in the lung, and attenuated lung inflammation. CONCLUSIONS: Acute lung injury resulting from distal aortic occlusion starts during ischaemia. TNF and NO blockade decrease PMN chemotaxis and sequestration and attenuate the lung injury process.

Treatment of septic shock in rats with nitric oxide synthase inhibitors and inhaled nitric oxide.

OBJECTIVE: To evaluate the effect of treatment with a combination of nitric oxide synthase inhibitors and inhaled nitric oxide on systemic hypotension during sepsis. DESIGN: Prospective, randomized, controlled study on anesthetized animals. SETTING: A cardiopulmonary research laboratory. SUBJECTS: Forty-seven male adult Sprague-Dawley rats. INTERVENTIONS: Animals were anesthetized, mechanically ventilated with room air, and randomized into six groups: a) the control group (C, n=6) received normal saline infusion; b) the endotoxin-treated group received 100 mg/kg i.v. of Escherichia coli lipopolysaccharide (LPS, n=9); c) the third group received LPS, and 1 hr later the animals were treated with 100 mg/kg i.v. Nw-nitro-L-arginine (LNA, n=9); d) the fourth group received LPS, and after 1 hr, the animals were treated with 100 mg/kg i.v. aminoguanidine (AG, n=9); e) the fifth group received LPS and 1 hr later was treated with LNA plus 1 ppm inhaled nitric oxide (LNA+NO, n=7); f) the sixth group received LPS and 1 hr later was treated with aminoguanidine plus inhaled NO (AG+NO, n=7). Inhaled NO was administered continuously until the end of the experiment. MEASUREMENTS AND MAIN RESULTS: Systemic mean blood pressure (MAP) was monitored through a catheter in the carotid artery. Mean exhaled NO (ENO) was measured before LPS (T0) and every 30 mins thereafter for 5 hrs. Arterial blood gases and pH were measured every 30 mins for the first 2 hrs and then every hour. No attempt was made to regulate the animal body temperature. All the rats became equally hypothermic (28.9+/-1.2 degrees C [SEM]) at the end of the experiment. In the control group, blood pressure and pH remained stable for the duration of the experiment, however, ENO increased gradually from 1.3+/-0.7 to 17.6+/-3.1 ppb after 5 hrs (p< .05). In the LPS treated rats, MAP decreased in the first 30 mins and then remained stable for 5 hrs. The decrease in MAP was associated with a gradual increase in ENO, which was significant after 180 mins (58.9+/-16.6 ppb) and reached 95.3+/-27.5 ppb after 5 hrs (p< .05). LNA and AG prevented the increase in ENO after LPS to the level in the control group. AG caused a partial reversal of systemic hypotension, which lasted for the duration of the experiment. LNA reversed systemic hypotension almost completely but only transiently for 1 hr, and caused severe metabolic acidosis in all animals. The co-administration of NO with AG had no added benefits on MAP and pH. In contrast, NO inhalation increased the duration of the reversal in MAP after LNA, alleviated the degree of acidosis, and decreased the mortality rate (from 55% to 29%). CONCLUSIONS: In this animal model, LPS-induced hypotension was alleviated slightly and durably after AG, but only transiently after LNA. Furthermore, co-administration of NO with AG had no added benefits but alleviated the severity of metabolic acidosis and mortality after LNA. We conclude that nitric oxide synthase (NOS) inhibitors, given as a single large bolus in the early phase of sepsis, can exhibit some beneficial effects. Administration of inhaled NO with NOS inhibitors provided more benefits in some conditions and therefore may be a useful therapeutic combination in sepsis. NO production in sepsis does not seem to be a primary cause of systemic hypotension. Other factors are likely to have a major role.

Role of nitric oxide and tumor necrosis factor on lung injury caused by ischemia/reperfusion of the lower extremities.

PURPOSE: Acute aortic occlusion with subsequent ischemia/reperfusion (I/R) of the lower extremities is known to predispose to lung injury. The pathophysiologic mechanisms of this injury are not clear. In the present study, we studied the role of tumor necrosis factor (TNF) and nitric oxide (NO) in lung injury caused by lower extremity I/R. METHODS: A rat model in which the infrarenal aorta was cross-clamped for 3 hours followed by 1 hour of reperfusion was used. The rats were randomized into five groups: group 1, aorta exposed but not clamped; group 2, aorta clamped for 3 hours, followed by 1 hour of reperfusion; group 3, 1 mg/kg dexamethasone administered before the aorta was clamped; group 4, 25 mg aminoguanidine, a specific inducible NO synthase (iNOS) inhibitor, administered before the aorta was clamped; and group 5, 2 mg/kg TNFbp, a PEG-ylated dimeric form of the high-affinity p55 TNF receptor I (RI), administered before the aorta was clamped. NO concentration in the exhaled gas (ENO) was measured, as an index of NO production by the lung, in 30 minute intervals during I/R. Serial arterial blood samples for TNF assay were obtained during the course of the experiment. At the end of the experiment, the lungs were removed and histologically examined for evidence of injury. RESULTS: ENO in group 2 increased from 0.7 +/- 0.3 ppb at baseline to 54.3 +/- 7.5 ppb at the end of ischemia and remained stable during reperfusion (54.6 +/- 8.5 ppb at the end of reperfusion). ENO production was blocked by aminoguanidine, by dexamethasone, and by TNFbp given before aortic occlusion. Serum TNF in groups 2, 3 and 4 increased rapidly during early ischemia, reaching its peak value 60 minutes after occlusion of the aorta, then gradually declined to baseline levels at the end of ischemia, and remained low during reperfusion. TNFbp decreased serum TNF concentration significantly when it was given before aortic occlusion. Histologic examination of the lungs at the end of the experiment revealed that aminoguanidine, dexamethasone, and TNFbp had a protective effect on the lungs. CONCLUSIONS: Serum TNF increases rapidly during lower extremity ischemia and causes increased production of NO from the lung by upregulating iNOS. Increased NO is associated with more severe lung injury, and iNOS blockade has beneficial effects on the lung. TNF blockade before ischemia decreases NO production by the lung and attenuates lung injury. ENO can be used as an early marker of lung injury caused by lower extremity I/R.

Increased nitric oxide in exhaled gas is an early marker of hypovolemic states.

Acute hemorrhage is associated with a variety of physiologic and metabolic alterations, including vascular hyporeactivity and endothelial cell dysfunction. The lung is a major target organ during hemorrhagic shock. The effect of acute hemorrhage on NO production in the lung is not well described. In the present study we examined the effect of acute hemorrhage on exhaled NO (NOe), and studied how changes in blood volume and flow affect NOe. Anesthetized and mechanically ventilated rabbits were used. The effect of acute hemorrhage by slow exsanguination on NOe was examined using chemiluminescence. Because hemorrhagic shock is associated with decreased pulmonary blood flow, we established an isolated lung preparation perfused with autologous blood (Hct = 17.4%) and studied the effect of pulmonary flow rate on NOe independent of metabolic changes. In order to separate the effect of flow from the effect of changes in blood volume, we examined the effect of flow in isolated lungs perfused with a blood-free albumin solution (PAS). In the isolated lung, ventilation was similar to that used in the intact animal, and temperature, pH, pCO2, and PO2 were kept normal. Prior to exsanguination, baseline NOe in the intact animal was 24 +/- 3 ppb. At 5, 10, 15, and 20 min after initiating the hemorrhage, NOe rose to 31 +/- 3, 51 +/- 7, 94 +/- 10, and 154 +/- 16 ppb, respectively (P < 0.05). During baseline conditions in the blood-perfused isolated lungs (200 ml/min), NOe was 35 +/- 3 ppb. When flow was decreased to 70 and 0 ml/min, NOe increased to 37 +/- 3 and 56 +/- 6 ppb, respectively (P < 0.001). During baseline conditions in the PAS-perfused lungs (70 ml/min), NOe was 94 +/- 13 ppb and was unaffected by changes in flow. The perfusion pressure in the isolated lungs was in the normal range. Reduction in blood flow rate in the isolated lung was associated with less than twofold increase in NOe. This was attributed to reduction in red blood cell volume and not due to changes in blood flow rate. Reduction in flow in the intact animal during hemorrhage generated more than threefold increase in NOe, suggesting that neurohumoral mediators, in addition to changes in flow, play an important role in determining. NOe in the intact condition. NOe began to rise immediately after exsanguination began, and therefore may be a useful early marker of acute hemorrhagic shock and hypovolemia. This information may be useful in the intensive care setting.

Segmental pulmonary vascular responses to ATP in rat lungs: role of nitric oxide.

ATP exhibits vascular pressor and depressor responses in a dose- and tone-dependent manner. The vascular site of ATP-induced contraction or dilation has not previously been characterized. Using the vascular occlusion technique, we investigated the effects of ATP in isolated rat lungs perfused with autologous blood (hematocrit = 20%) and described its action during resting and elevated tone in terms of changes in resistances of the small and large arteries and veins. During resting tone, ATP (10(-5) M) caused contraction primarily in the small arteries and, to some extent, in the small veins, suggesting that P2x purinoceptors are present in these small vessels. During hypoxia, ATP caused dilation primarily in the small arteries, suggesting that P2y purinoceptors are predominant in small arteries. During U-46619-induced contraction, which occurred evenly throughout the four segments, ATP caused dilation in the large arteries and veins but not in the small arteries and veins. After treatment with N omega-nitro-L-arginine to inhibit nitric oxide synthesis, ATP-induced contraction was potentiated, and its dilatory effects during hypoxia were attenuated. The action of ATP was independent of prostanoids, because its constrictor and dilatory responses were not affected significantly by indomethacin. In conclusion, the results indicate that the effects of ATP on the pulmonary vasculature are primarily due to P2x and P2y purinoceptors in the small arteries. Contribution of these purinoceptors in other vessels to changes in total vascular resistance in rat lung was minor.

Determinants of nitric oxide in exhaled gas in the isolated rabbit lung.

Nitric oxide concentrations in the exhaled gas (NOe) increases during various inflammatory conditions in humans and animals. Little is known about the sources and factors that influence NOe. NOe at end expiration was measured by chemiluminescence in an isolated, blood-perfused rabbit lung. The average end-expiratory concentration over 10 breaths was used. The effect of positive end-expiratory pressure (PEEP), flow rate, pH, hypoxia, venous pressure, and flow pulsatility on NOe were determined. At constant blood flow, increasing PEEP from 1 to 5 cm H2O elicited a reproducible increase in NOe from 49 +/- 7 to 53 +/- 8 parts per billion (ppb) (p < 0.05). When blood pH was increased from 7.40 to 7.74 by breathing low CO2 gas, NOe rose from 45 +/- 7 to 55 +/- 7 ppb (p < 0.001). Hypoxia caused a dose-dependent decrease in NOe from 37 +/- 3 during baseline to 23 +/- 2 during ventilation with 0% O2 (p < 0.01). Venous pressure elevation from 0 to 5 and 10 mm Hg decreased NOe from 32 +/- 5, to 26 +/- 5 and 24 +/- 5 ppb, respectively (p < 0.05). Switching from steady to pulsatile flow (same man flow) resulted in a small, albeit significant reduction in NOe; 30 +/- 4 to 28 +/- 4 ppb (p < 0.05). Changes in flow rate between 200 and 20 ml/min were associated with small changes in NOe; however, when flow was stopped, NOe rose substantially to 56 +/- 6 ppb (p < 0.05). The changes in NOe were rapid (1 to 2 min) and reversible. The results suggest that NOe is influenced by ventilatory and hemodynamic variables, pH, and hypoxia. We suggest that caution must be taken when interpreting changes in exhaled NO in humans or experimental animals. Changes in total and regional blood flow, capillary blood volume, ventilation, hypoxia, and pH should not be overlooked.

Analysis of the double occlusion which provides four pressure gradients.

The arterial, double and venous occlusions are used to partition pulmonary vascular resistance into four segments. In this study, we tested whether the same can be accomplished from one double occlusion. In an isolated canine lung left lower lobe perfused with blood (flow rate = 500 mL.min-1), the pulmonary arterial and venous pressures (Pa and Pv, respectively) were measured directly. Arterial, double, and venous occlusions were performed and analysed as usual (Method 1) to measure pressures in small arteries and small veins (Pa' and Pv', respectively) and capillary pressure (Pc). Alternatively, one double occlusion was analysed (Method 2), not only for Pa, Pv and Pc, but also as independent arterial and venous occlusions to measure Pa' and Pv'. Method 1 yielded Pa, Pa', Pc, Pv', and Pv (Baseline) of 14.2 +/- 1.7, 10.8 +/- 1.6, 8.9 +/- 1.9, 7.3 +/- 1.5 and 1.3 +/- 0.6 mmHg, respectively (1 mmHg = 0.133 kPa). Method 2 yielded values for the same five pressures equal to 14.7 +/- 2.1, 11.0 +/- 2.2, 8.9 +/- 1.9, 7.3 +/- 1.3 and 1.3 +/- 0.6 mmHg, respectively. There was no significant difference in the pressure profile obtained using the two methods, nor were there differences during hypoxia and angiotensin infusion. These results suggest that a more thorough analysis of the double occlusion can provide the same information about distribution of vascular resistance as provided by a combination of the three occlusions. The advantage of the new approach is that fewer occlusions are needed and resistance distribution can be assessed during a transient response. Because all pressures are derived from one occlusion, the pressures would be more accurate relative to each other.

Half-life of nitric oxide in aqueous solutions with and without haemoglobin.

Nitric oxide (NO) has been linked to many regulatory functions in mammalian cells. Studies of NO release are hampered by the short half-life of the molecule. In the blood, NO disappears within seconds because it binds avidly with haemoglobin (Hb). The relationship between Hb concentration and NO disappearance, however, has not been described. In this study we utilized an amperometric NO sensor (WPI, Sarasota, FL) to monitor continuously the disappearance of NO from an aqueous solution when Hb (free or as red blood cells) was added. The calibration and linearity of the NO sensor was checked frequently using a chemical reaction to generate a known concentration of NO. An aliquot of NO solution (prepared from authentic gas) was added to a glass beaker containing 20 ml saline to generate NO concentration of approximately 1200 nM. Under our experimental conditions (PO2 = 40 mmHg), NO concentration fell slowly over 20 min with a half-life of 445 s. However, when haemoglobin was added, NO disappeared rapidly in proportion to Hb concentration. The results suggest that rapid binding of NO to Hb occurs in a 4:1 ratio. The maximum rate constant of NO disappearance due to binding with Hb was 2 x 10(5) M-1 s-1. The 4:1 binding ratio between NO:Hb may be used as a tool to quantitate NO release in some biological assays. The study supports the notion that NO acts as an autocoid because it disappears rapidly in the presence of Hb and is not likely to act as a circulating humoral substance. The NO sensor was useful for monitoring of NO concentration in Hb free solutions, but its response time limits its use in blood.

Site of action of endogenous nitric oxide on pulmonary vasculature in rats.

The effect of endogenous nitric oxide (NO) on the pulmonary hypoxic vasoconstriction was studied in isolated and blood perfused rat lungs. By applying the occlusion technique we partitioned the total pulmonary vascular resistance (PVR) into four segments: (1) large arteries (Ra), (2) small arteries (Ra'), (3) small veins (Rv'), and (4) large veins (Rv). The resistances were evaluated under baseline (BL) conditions and during; hypoxic vasoconstriction and acetylcholine (Ach) which was injected during hypoxic vasoconstriction. After recovery from hypoxia and Ach, Nomega-nitro-L-arginine (L-NA) was added to the reservoir and the responses to hypoxia and Ach were reevaluated. Before L-NA, hypoxia caused significant increase in the resistances of all segments (P < 0.05), with the largest being in Ra and Ra'. Ach-induced relaxation during hypoxia occurred in Ra, Ra' and Rv' (P < 0.05). L-NA did not change the basal tone of the pulmonary vasculature significantly. However, after L-NA, hypoxic vasoconstriction was markedly enhanced in Ra, Ra', and Rv' (P < 0.01) compared with the hypoxic response before L-NA. Ach-induced relaxation was abolished after L-NA. We conclude that, in rat lungs, inhibition of NO production during hypoxia enhances the response in the small arteries and veins as well as in the large arteries. The results suggest that hypoxic vasoconstriction in the large pulmonary arteries and small vessels is attenuated by NO release.

Pulmonary capillary pressure during exercise in horses.

The object of this study was to relate pulmonary capillary pressure to arterial and wedge pressures during exercise. Pulmonary vascular pressures were measured in six standardbred horses exercising at speeds equivalent to 75, 90, and 100% of maximal heart rate. Vascular pressures were measured with transducer-tip catheters and expressed relative to esophageal pressure. Pulmonary capillary pressure was estimated by the arterial-occlusion technique modified for exercise. Mean pulmonary arterial, capillary and wedge pressures increased from 30.5 +/- 6.3, 17.8 +/- 4.3, and 13.4 +/- 1.6 mmHg, respectively, at rest, to 70.5 +/- 5.2, 42.1 +/- 5.3, and 38.4 +/- 5.6 mmHg, respectively, at maximal exercise. The largest part of the increase occurred during the first level of exertion. With exercise, the pressure across the lung barely doubled at a time when the cardiac output would have increased at least fivefold. Thus the absolute resistance in both pre- and postcapillary segments must have decreased. The capillary and wedge pressures rose similarly, whereas the difference between them did not change with exertion. The fractional resistance of the precapillary segment increased with exercise. The postcapillary resistance, initially 28% of the total pulmonary vascular resistance, fell to 9% at maximal exercise. The rise (to approximately 45 mmHg) in pulmonary capillary pressure with exertion is consistent with an increase in transvascular filtration.

Perioperative distribution of pulmonary vascular resistance in patients undergoing coronary artery surgery.

This study was undertaken to measure distribution of pulmonary vascular resistance (PVR) perioperatively in patients undergoing coronary artery bypass grafting (CABG) and to examine the effects of cardiopulmonary bypass (CPB) on pulmonary capillary pressure (Pc) relative to wedge pressure (Pw). Pulmonary artery catheters were placed before anesthetic induction in 18 patients scheduled for elective CABG and systemic hemodynamic variables were measured. Pulmonary artery pressure was recorded during balloon inflation and stored for off-line determination of Pc. Data were collected prior to induction (baseline), as well as after induction and intubation, skin incision, sternotomy, protamine administration, and chest closure. At each data point, downstream (capillary plus venous segments) resistance (Rds) contributed approximately 60% of total PVR and did not change significantly during the operation. PVR decreased (P < 0.05) after CPB and protamine administration, primarily due to a decrease in the absolute magnitude of the upstream (arterial) resistance. Administration of large-dose opioid anesthesia had no significant effect (P > 0.05) on total PVR or on segmental distribution of vascular resistance. At all data points, Pc was significantly larger than Pw (P < 0.05). This study demonstrates that perioperative measurement of Pc is feasible, that during CABG under these conditions, relative contribution of arterial and venous resistances remain relatively unchanged, that Pc is always larger than Pw, and that the administration of large-dose opioid anesthesia has a minimal effect on pulmonary vascular hemodynamics.