Augmentation of hypoxic pulmonary vasoconstriction in the isolated perfused rat lung by in vitro antagonists of endothelium-dependent relaxation

Persistent Pulmonary Hypertension of the Newborn: Role of Nitric Oxide

Persistent pulmonary hypertension of the newborn (PPHN) is a common cause of respiratory failure in the full-term neonate. Molecular and cellular studies in vascular biology have revealed that endothelial-derived mediators play a critical role in the pathogenesis and treatment of PPHN. Endothelial-derived vasoconstrictors, like endothelin, may increase smooth muscle cell contractility and growth, leading to the physiologic and structural changes observed in the pulmonary arterioles of infants with this disease. On the other hand, decreased production of the endothelial-derived relaxing factor, nitric oxide, may exacerbate pulmonary vasoreactivity and lead to more severe pulmonary hypertension. Exogenous (inhaled) nitric oxide therapy reduces pulmonary vascular resistance and improves oxygenation. The safety and efficacy of this therapy in reducing the need for extracorporeal membrane oxygenation and decreasing long-term morbidity is being tested in several trials nationally and abroad. Understanding the basic mechanisms that regulate the gene expression and production of these vasoactive mediators will lead to improved preventive and therapeutic strategies for PPHN.

NO in the lung

In the lung, nitric oxide (NO) derives from several cellular sources, forming networks of paracrine communication. In pulmonary vessels, NO produced by endothelial cells is a powerful vasodilator. In the airways, NO originates from epithelial cells and from adventitial nerve endings to induce smooth muscle relaxation. Activated macrophages can also produce large quantities of NO during lung immunological reactions. In the normal pulmonary circulation, NO not only mediates vasodilation, but also opposes vasoconstriction, prevents platelet adhesion, controls growth of smooth muscle and influences the composition of the extracellular matrix. During exposure to chronic hypoxia, impaired endothelial NO production contributes to the increased vasomotor tone and vascular remodelling leading to sustained pulmonary hypertension. Exogenous NO gas delivered via the airspaces is a selective pulmonary vasodilator. Inhaled NO is now used as a therapy to treat various forms of pulmonary hypertension and to improve arterial oxygenation during lung injury.

Lung transplantation with cardiopulmonary bypass exaggerates pulmonary vasomotor dysfunction in the transplanted lung

Pulmonary vascular resistance is significantly increased in the transplanted lung. If cardiopulmonary bypass is required, the transplanted lung is reperfused with activated blood elements, which might exacerbate the reperfusion injury. The purpose of this study was to examine the influence of cardiopulmonary bypass on the following mechanisms of pulmonary vasomotor control in a dog model of autologous lung transplantation: (1) endothelium-dependent cyclic guanosine monophosphate-mediated relaxation (response to acetylcholine), (2) endothelium-independent cyclic guanosine monophosphate-mediated relaxation (response to nitroprusside), and (3) beta-adrenergic cyclic adenosine monophosphate-mediated relaxation (response to isoproterenol). Autologous right lung transplants were performed with (n = 4 dogs) and without (n = 5 dogs) bypass. Lungs were stored in cold saline solution (4 degrees C, 3 hours) before reimplantation. Pulmonary vasomotor control mechanisms were studied in isolated pulmonary arterial rings immediately after harvest and 1 hour after reimplantation. Ten rings were studied in each group at each time. Statistical analysis was by analysis of variance. Without bypass, endothelium-dependent cyclic guanosine monophosphate-mediated relaxation and beta-adrenergic cyclic adenosine monophosphate-mediated relaxation were significantly impaired, although endothelium-independent cyclic guanosine monophosphate-mediated relaxation was not. Use of bypass produced significantly greater impairment of both endothelium-dependent cyclic guanosine monophosphate-mediated relaxation and beta-adrenergic cyclic adenosine monophosphate-mediated relaxation. In addition, use of bypass produced significant dysfunction of endothelium-independent cyclic guanosine monophosphate-mediated relaxation as well. We conclude that using cardiopulmonary bypass to perform lung transplantation greatly exaggerates pulmonary vasomotor dysfunction in the transplanted lung. This dysfunction may contribute to significantly higher pulmonary vascular resistance in the transplanted lung if cardiopulmonary bypass is used.

Influence of N omega-nitro-L-arginine methyl ester, LY83583, glybenclamide and L158809 on pulmonary circulation

The effects of N omega-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase; 6-anilino-5,8-quinolinedione (LY83583), an inhibitor of soluble guanylate cyclase; glybenclamide, a ATP-sensitive K+ channel blocking agent; and 5,7-dimethyl-2-ethyl-3-[[2'-(1H-tetrazol-5-yl)-[1,1']-biphenyl-4- yl]methyl]-3H-imidazo[4,5-b]pyridine (L158809), an angiotensin II type I receptor antagonist, on the response to ventilatory hypoxia were investigated in the isolated blood-perfused rat lung. Under conditions of controlled pulmonary blood flow, and constant left atrial pressure, injections of glybenclamide into the pulmonary arterial perfusion circuit significantly increased baseline pulmonary arterial perfusion pressure, whereas administration of N omega-nitro-L-arginine methyl ester produced smaller increases in baseline tone. Ventilatory hypoxia (3% O2-5% CO2-92% N2) significantly increased pulmonary arterial perfusion pressure and the response was reproducible with respect to time. Following administration of N omega-nitro-L-arginine methyl ester or LY83583, the response to hypoxia was significantly increased, whereas the response to hypoxia was not changed by glybenclamide or atropine. N omega-Nitro-L-arginine methyl ester also significantly enhanced pressor responses to angiotensin II, but had no effect on the pressor response to serotonin. When pulmonary vascular tone was increased with hypoxia, vasodilator responses to acetylcholine were inhibited by N omega-nitro-L-arginine methyl ester and vasodilator responses to levcromakalim were reduced by glybenclamide. In addition, L158809 did not alter the pressor response to hypoxia, whereas responses to angiotensin II were reduced in a selective manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of two K+ channel openers, aprikalim and pinacidil, on hypoxic pulmonary vasoconstriction

This study investigated the effects of two K+ channel openers, aprikalim and pinacidil, on hypoxic pulmonary vasoconstriction induced in isolated rat lung perfused at constant flow. In order to evaluate the mechanism of the hypoxic vasoconstriction we also studied the effects of an inhibitor of the endothelium-derived relaxing factor (EDRF), NG-nitro-L-arginine methyl ester (100 microM), an inhibitor of the guanylate cyclase, methylene blue (30 microM), two K+ channel blockers, glibenclamide (1 microM) and tetraethylammonium (20 mM). In normoxia, NG-nitro-L-arginine methyl ester, methylene blue, glibenclamide or tetraethylammonium did not enhance significantly the baseline perfusion pressure, suggesting that neither EDRF nor K+ channels are involved in the modulation of the low basal pulmonary vascular tone. In hypoxia, aprikalim and pinacidil (0.03-3 microM) induced a concentration-dependent decrease of pulmonary pressure, exhibiting their spasmolytic effects in acute hypoxia. The hypoxic pressure response was significantly increased by NG-nitro-L-arginine methyl ester, methylene blue and tetraethylammonium, but not by glibenclamide suggesting that EDRF and K+ channels other than ATP-sensitive K+ channels are involved in the modulation of the hypoxic pressure response. The spasmolytic effects of aprikalim and pinacidil (1 microM) were not modified by NG-nitro-L-arginine methyl ester, but were partially reduced by tetraethylammonium and completely abolished by glibenclamide, suggesting that these effects are mainly but not exclusively mediated through ATP-sensitive K+ channel opening.

Continuous inhalation of nitric oxide protects against development of pulmonary hypertension in chronically hypoxic rats

Exposure to hypoxia and subsequent development of pulmonary hypertension is associated with an impairment of the nitric oxide (NO) mediated response to endothelium-dependent vasodilators. Inhaled NO may reach resistive pulmonary vessels through an abluminal route. The aim of this study was to investigate if continuous inhalation of NO would attenuate the development of pulmonary hypertension in rats exposed to chronic hypoxia. In conscious rats previously exposed to 10% O2 for 3 wk, short-term inhalation of NO caused a dose-dependent decrease in pulmonary artery pressure (PAP) from 44 +/- 1 to 32 +/- 1 mmHg at 40 ppm with no changes in systemic arterial pressure, cardiac output, or heart rate. In normoxic rats, acute NO inhalation did not cause changes in PAP. In rats simultaneously exposed to 10% O2 and 10 ppm NO during 2 wk, right ventricular hypertrophy was less severe (P < 0.01), and the degree of muscularization of pulmonary vessels at both alveolar duct and alveolar wall levels was lower (P < 0.01) than in rats exposed to hypoxia alone. Tolerance to the pulmonary vasodilator effect of NO did not develop after prolonged inhalation. Brief discontinuation of NO after 2 wk of hypoxia plus NO caused a rapid increase in PAP. These data demonstrate that prolonged inhalation of low concentrations of NO induces sustained pulmonary vasodilation and reduces pulmonary vascular remodeling in response to chronic hypoxia.

Inhaled nitric oxide reverses hypoxic pulmonary vasoconstriction in dogs. A practical nitric oxide delivery and monitoring system

Nitric oxide (NO) is a potent dilator of vascular smooth muscle that likely represents an important endothelium-dependent relaxing factor. Recent interest has focused on inhaled NO as a pulmonary vasodilator. The purpose of this study was to design a reliable NO delivery system with on-line monitoring of NO and nitrogen dioxide (NO2) concentrations, and to test the effects of inhaled NO in a dog model of acute hypoxic pulmonary vasoconstriction (HPV). Six canines were studied. Marked HPV was induced using a hypoxic gas mixture. Using a standard blender. NO was delivered through a volume-cycled ventilator. We were able to rapidly adjust the delivered NO concentration using this system. An on-line chemoluminescence analyzer was used to continuously measure NO and NO2 concentrations. Inhaled NO at 40 and 80 ppm for 30 min rapidly reversed HPV in all animals (PVR 502 +/- 154 dynes.s.cm-5 with hypoxia, 244 +/- 52 with 40 ppm NO, 227 +/- 47 with 80 ppm NO). No significant NO2 or methemoglobin production was noted during the study. We conclude that inhaled NO can be easily delivered through a ventilator and the dose rapidly adjusted, NO and NO2 concentrations can be monitored continuously on-line, inhaled NO rapidly reverses HPV in dogs, and with short-term NO inhalation, there is no significant NO2 or methemoglobin formation. Inhaled NO may, therefore, have a future clinical role as a new agent in the diagnosis and treatment of other forms of pulmonary hypertension.

Nitric oxide regulates basal systemic and pulmonary vascular resistance in healthy humans

BACKGROUND

The endothelium synthesizes and releases a relaxing factor with the physiochemical properties of nitric oxide (NO). However, the role of endothelium-derived NO in the basal regulation of systemic and pulmonary vascular resistance in humans is not known. Our primary objectives were to determine the effects of inhibiting NO synthesis on blood pressure and systemic vascular resistance and to establish the role of endothelium-derived NO in the regulation of normoxic pulmonary vascular tone.

METHODS AND RESULTS

We studied the systemic and pulmonary hemodynamic effects of NG-monomethyl-L-arginine (L-NMMA, 0.03 to 1.0 mg.kg-1.min-1 IV), an NO synthase inhibitor, in 11 healthy volunteers, aged 33 +/- 2 years. An arterial cannula and a pulmonary artery catheter were placed in each subject to measure blood pressure, pulmonary artery pressure, and pulmonary capillary wedge pressure. Cardiac output was determined by the Fick technique, and systemic and pulmonary vascular resistances were calculated. Serum NO levels (free and protein bound) were measured by chemiluminescence in 5 subjects. Six of the subjects also received phenylephrine (25 to 100 micrograms/min IV) to compare the cardiac hemodynamic effects of L-NMMA with those of a direct-acting vasoconstrictor. L-NMMA caused dose-dependent increases in both blood pressure and systemic vascular resistance. At the highest dose of L-NMMA, there was a 15.5 +/- 1.3% increase in mean blood pressure and a 63.4 +/- 8.2% increase in systemic vascular resistance (each P < .01). Pulmonary vascular resistance increased 39.8 +/- 9.4% (P < .01), whereas mean pulmonary artery pressure did not change. Administration of L-NMMA also reduced cardiac output by 27.8 +/- 2.9% and stroke volume by 15.4 +/- 3.5% (each P < .01). Serum NO levels decreased 65 +/- 10% from basal values (P < .05), confirming inhibition of endogenous NO production. Phenylephrine increased blood pressure to a level comparable to that observed with L-NMMA. The decline in stroke volume was greater with L-NMMA than with phenylephrine (P < .01).

CONCLUSIONS

This study demonstrates that basal release of endothelium-derived NO is directly involved in the determination of systemic vascular resistance and, therefore, blood pressure in healthy humans. In addition, NO regulates basal normoxic pulmonary vascular tone. The complex hemodynamic effects of NO are composite properties of its actions on systemic and pulmonary vascular resistance and cardiac function.

Exercise-induced pulmonary vasoconstriction during combined blockade of nitric oxide synthase and beta adrenergic receptors

We studied the effects of inhibition of nitric oxide (NO) (endothelium-derived relaxation factor) synthase in combination with alpha and beta adrenergic receptor blockade on pulmonary vascular tone during exercise. In paired studies, we exercised sheep on a treadmill at a speed of 4 mph, and measured blood flow and pressures across the pulmonary circulation with and without inhibition of NO synthase (N omega-nitro-L-arginine 20 mg/kg intravenous [i.v.]), alpha receptor blockade (phentolamine 5 mg i.v.), beta receptor blockade (propranolol 1 mg i.v.), and combined alpha and beta receptor blockade. Activation of both types of adrenergic receptors occurs with exercise, and because increased release in NO is hypothesized to occur during exercise, these studies were designed to determine the magnitude of effect and interactions of these competing dilator and constrictor influences. We found that inhibition of NO synthase raised pulmonary vascular resistance (PVR) at rest and that, although a reduction in PVR occurred with exercise from this new baseline, vasoconstriction persisted. Combined beta blockade and NO synthase inhibition unmasked unopposed alpha vasoconstriction; PVR rose at rest and continued to rise with exercise; and mean pulmonary arterial pressures approached very high levels, 43.8 +/- 4.4 cmH2O. Using a distal wedged pulmonary artery catheter technique, most of the vasoconstriction was found to be in vessels upstream from small pulmonary veins. During exercise in sheep there appears to be a high degree of alpha and beta adrenergic-mediated tone in the pulmonary circulation. Endogenous production of NO actively dilates pulmonary vessels at rest and opposes potent alpha-mediated pulmonary vasoconstriction during exercise.

Hemodynamic and gas exchange responses to infusion of acetylcholine and inhalation of nitric oxide in patients with chronic obstructive lung disease and pulmonary hypertension

To investigate endothelium-dependent and endothelium-independent nitric oxide (NO) mediated pulmonary vasodilation in patients with chronic obstructive lung disease (COLD), we examined the responses to incremental infusion rates of acetylcholine (ACh) or inhaled NO on hemodynamic and gas exchange. In 13 patients, ACh (15 mg/min) decreased pulmonary artery pressure (Ppa) from 31 +/- 1 to 28 +/- 1 mm Hg (p < 0.01) and systemic arterial pressure while increasing cardiac index from 3.7 +/- 0.4 to 4.7 +/- 0.4 L/min/m2 (p < 0.01). Inhaling 40 parts per million (ppm) NO decreased Ppa from 32 +/- 1 to 26 +/- 1 mm Hg (p < 0.001) with no associated hemodynamic change. ACh reduced PaO2 from 57 +/- 3 to 48 +/- 2 mm Hg (p < 0.01) and increased venous admixture (QVA/QT) from 35 +/- 3 to 45 +/- 3% (p < 0.01). Inhaling 40 ppm NO increased PaO2 from 57 +/- 3 to 60 +/- 3 mm Hg (p < 0.01) and decreased QVA/QT from 36 +/- 3 to 32 +/- 3% (p < 0.01). Pulmonary vascular resistance changes were similar in response to 40 ppm NO or 15 mg/min ACh. In COLD patients, ACh produces both pulmonary and systemic vasodilation but impairs arterial oxygenation whereas inhaled NO induces selective pulmonary vasodilation while improving gas exchange. The resistance to ACh in some patients could be related to pulmonary endothelial dysfunction.

Enhanced nitric oxide synthesis in uremia: implications for platelet dysfunction and dialysis hypotension

Nitric oxide (NO), a potent vasodilator which also inhibits platelet adhesion and aggregation, is generated by endothelial cells and platelets from its precursor L-arginine. Since N-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthesis, normalizes the prolonged bleeding time of uremic rats, it has been suggested that bleeding associated with uremia was due to an excessive NO formation. With the present study we sought to evaluate whether in patients with chronic renal failure--like in uremic rats--defective platelet aggregation were associated with excessive formation of NO and whether uremic plasma promotes NO synthesis by cultured vascular endothelium. Data indicated that plasma L-arginine was higher in uremics than in controls, uremic platelets generated more NO than control platelets, and intraplatelet levels of cGMP (the NO second messenger) were also higher in uremic than in control platelets. Moreover, uremic plasma potently induced NO synthesis by cultured endothelial cells, a phenomenon which was further amplified by adding to uremic plasma endotoxin and interferon gamma. Increased NO biosynthesis may contribute to platelet dysfunction and possibly other manifestations of uremic syndrome, including hemodialysis hypotension.

Effects of pyridoxalated hemoglobin polyoxyethylene conjugate and stroma free hemoglobin on pulmonary vascular responsiveness to vasoactive substances in isolated perfused rat lungs

We examined the effects of pyridoxalated hemoglobin polyoxyethylene conjugate (PHP) and stroma free hemoglobin (SFH) on vascular responsiveness to various vasoactive substances in isolated perfused rat lungs. The lungs isolated from rats were perfused with 6% PHP, 6% SFH, or 6% hydroxyethylstarch (HES) solution, and the effects of intrapulmonary arterial injection of norepinephrine (NE), angiotensin II (ANG-II), acetylcholine (ACh), and nitroglycerin (NG) were examined by measuring perfusion pressure. NE and ANG-II produced a dose-dependent increase in perfusion pressure in all groups. The NE response in the PHP- or SFH-perfused group was significantly larger than that in the HES-perfused one. ACh decreased perfusion pressure in both PHP- and HES-perfused groups but increased perfusion pressure in the SFH-perfused group. NG decreased perfusion pressure in all groups. Present results indicate that pulmonary arterial responses to endothelium-derived relaxing factor (EDRF) induced by ACh would not be affected in the presence of PHP.

Endogenous endothelium-derived relaxing factor opposes hypoxic pulmonary vasoconstriction and supports blood flow to hypoxic alveoli in anesthetized rabbits

Agents that inhibit nitric oxide synthesis augment hypoxic pulmonary vasoconstriction. In an animal model of unilateral alveolar hypoxia, we investigated the hypothesis that endogenous endothelium-derived relaxing factor/nitric oxide opposes hypoxic pulmonary vasoconstriction and supports blood flow to hypoxic alveoli, resulting in a reduction in arterial oxygen tension (PO2). In pentobarbital-anesthetized rabbits, unilateral alveolar hypoxia was produced by ventilation of one lung with 100% oxygen and the other with 100% nitrogen (O2/N2). NG-Nitro-L-arginine methyl ester (0.03 followed by 1.0 mg/kg i.v.) resulted in dose-dependent decreases in the percent of pulmonary blood flow to the N2-ventilated lung and increases in arterial PO2. L-Arginine (1 mg.kg-1.min-1 i.v.) prevented the NG-nitro-L-arginine methyl ester-induced redistribution of blood flow away from hypoxic alveoli and improvement in arterial PO2. Indomethacin (5 mg/kg i.v.) administered during O2/N2 ventilation resulted in a reduction in the percentage of total blood flow to the hypoxic lung and an increase in arterial PO2. However, NG-nitro-L-arginine methyl ester administered in the presence of indomethacin caused additional diversion of blood flow away from the hypoxic lung. The magnitude of the changes suggests that the endothelium-derived relaxing factor/nitric oxide system has the capacity to make a greater contribution than products of cyclooxygenase-mediated arachidonic acid metabolism in supporting blood flow to hypoxic alveoli in the rabbit.

Hypoxic contraction of pre-stretched human pulmonary artery

To clarify the mechanism of hypoxic pulmonary vasoconstriction in man, human pulmonary artery segments (2 mm O.D.) were suspended and changes in isometric force were measured. The arteries were contracted by hypoxia (PO2 43 +/- 2 Torr) developing a tension of 127 +/- 36 mg over the course of 15 min. This contraction was completely blocked by 10(-6) M L-isoproterenol, 10(-6) M nitroglycerin, partially blocked by 10(-8)-10(-6) M verapamil, unchanged by 10(-6) M phentolamine, 10(-6) M L-propranolol, 10(-6) M diphenhydramine, 10(-6) M guanethidine, 10(-7) M FPL 55712 and enhanced by 10(-6) M BAY K 8644, 10(-3) M procaine, 3 x 10(-6) M quinacrine, 10(-6) M indomethacin or 10(-6) M methylene blue. Removal of the endothelium significantly enhanced the magnitude of hypoxia-induced contraction. These results suggest that the human pulmonary artery constricts in response to hypoxia, at least in part, through activation of the voltage-dependent Ca2+ channels and that neither alpha, beta, H1 receptors, the lipoxygenase pathway nor neural reflexes are involved. They also show that the endothelium is not required for hypoxic contraction and that its presence reduces sensitivity to hypoxia.

Direct role for potassium channel inhibition in hypoxic pulmonary vasoconstriction

Cellular mechanisms responsible for hypoxic pulmonary vasoconstriction were investigated in pulmonary arterial cells, isolated perfused lung, and pulmonary artery rings. Three K+ channel antagonists, Leiurus quinquestriatus venom, tetraethylammonium, and 4-aminopyridine, mimicked the effects of hypoxia in isolated lung and arterial rings by increasing pulmonary artery pressure and tension and also inhibited whole cell K+ currents in isolated pulmonary arterial cells. Reduction of oxygen tension from normoxic to hypoxic levels directly inhibited K+ currents and caused membrane depolarization in isolated canine pulmonary arterial smooth muscle cells but not in canine renal arterial smooth muscle cells. Nisoldipine or high buffering of intracellular Ca2+ concentration with [1,2-bis(2)aminophenoxy] ethane-N,N,N',N'-tetraacetic acid prevented hypoxic inhibition of K+ current, suggesting that a Ca(2+)-sensitive K+ channel may be responsible for the hypoxic response. These results indicate that K+ channel inhibition may be a key event that links hypoxia to pulmonary vasoconstriction by causing membrane depolarization and subsequent Ca2+ entry.

Relationship between chronic hypoxia and in vitro pulmonary relaxation mediated by endothelium-derived relaxing factors in human chronic obstructive lung disease

Endothelium-derived relaxing factors (EDRF) are paracrine vasodilator substances released by endothelial cells. There is compelling evidence to suggest that EDRF may play an important role in the modulation of vascular tone in the systemic circulation. However, the role of EDRF-mediated pulmonary relaxation in chronic lung disease is unknown. The authors have, therefore, investigated endothelium-dependent relaxation of isolated pulmonary arteries (PAs) obtained from 18 patients undergoing heart-lung transplantation for end-stage chronic hypoxic cor pulmonale (HCP). Control PAs were obtained from 10 patients, none of whom had evidence of HCP, and who underwent lobectomy for lung carcinoma. All vascular rings were studied immediately after lung excision. PA rings from control patients dose-dependently relaxed to cumulative doses of acetylcholine (ACh, 10(-10) to 10(-5) M), achieving a maximal relaxation of 73.2 +/- 4.4% from precontraction to phenylephrine. By contrast, PA rings from HCP patients achieved only 42.1 +/- 6.7% of maximal relaxation (p less than 0.01). Sodium nitroprusside (10(-4) M) relaxed all PA rings, with and without endothelium (carefully removed before study), obtained from both control and HCP patients. The endothelium-dependent maximal relaxation to ACh was positively related to pretransplant values of PaO2 (r = 0.59; p less than 0.01), but no relationship was found with either PaCO2 (r = -0.41) or FEV1 (r = -0.14). The authors conclude that pulmonary relaxation mediated by EDRF is impaired in human HCP and suggest that such impairment may be related to severity of the preexisting chronic hypoxemia.

Responses to vasodilator drugs on pulmonary artery preparations from pulmonary hypertensive rats

1. Relaxant responses to six vasodilator drugs, with different mechanisms of action, were examined on noradrenaline (0.1 microM)-contracted ring preparations of pulmonary artery and aorta taken from rats with pulmonary hypertension induced by monocrotaline or chronic hypoxia. 2. On pulmonary artery preparations from monocrotaline-treated rats, compared with controls, (a) the maximum relaxation to pinacidil and cromakalim was significantly increased, but their potency (negative log EC50) was unchanged, (b) the potencies of nitroprusside and sodium nitrite were significantly reduced (10 fold and 3 fold respectively), but there was no change in the maxima, (c) for nicorandil there was an increase in maximum relaxation and a decrease in potency (3 fold), and (d) for atriopeptin II there was no change in potency or maximum. 3. The increase in maximum relaxation for pinacidil and the decrease in potency for nitroprusside were also demonstrated in pulmonary artery preparations from rats with chronic hypoxic pulmonary hypertension. The other four drugs were not examined in preparations from hypoxic rats. 4. In both models of pulmonary hypertension, no change in maximum response or potency was seen on aortic preparations for any of the vasodilator drugs. 5. In control preparations, none of the drugs was more potent on pulmonary artery than on aorta (i.e. they were not pulmonary-selective). In preparations from pulmonary hypertensive rats, pinacidil was selective for pulmonary artery, in contrast to nitroprusside which was selective for aorta.6. It is concluded that the development of pulmonary hypertension in rats is accompanied by changes in the responsiveness of the pulmonary arteries to some vasodilator drugs; whether or not these changes occur depends on the mechanism of action of the vasodilator drug, but they are independent of the method of inducing pulmonary hypertension.7. It is postulated that the reduction in potency seen for nitroprusside, sodium nitrite and nicorandil may be due to desensitization of soluble guanylate cyclase in pulmonary vascular smooth muscle in pulmonary hypertension.

Endothelium-derived relaxing factor and the pulmonary circulation

Endothelium-derived relaxing factor (EDRF) is probably identical to nitric oxide (NO) and is released by the vascular endothelium both in the basal unstimulated state and in response to a wide range of physical and chemical stimuli. Since it was first described 10 years ago, evidence is accumulating that it is an important modulator of vascular smooth muscle tone. EDRF acts on the pulmonary vascular bed as on the systemic circulation. EDRF release to pharmacologic stimuli is impaired in pulmonary arteries from patients with chronic hypoxemia. This impairment is associated with severity of respiratory failure and of structural change of vessel walls. Disturbance of EDRF activity may be important in the pathophysiology of pulmonary vascular disease. This brief review describes the current status of experimental studies concerning the possible role of EDRF on the pulmonary circulation in normal conditions and in the pathogenesis of pulmonary hypertension.

Effects of water deprivation and morphine administration on atrial natriuretic peptide mRNA levels in rat auricles

We investigated the influences of two potent stimuli, water deprivation (5 days) and morphine administration (100 mg/kg), on the level of atrial natriuretic peptide (ANP) mRNA in the rat auricles. The ANP mRNA level was measured by Northern blot hybridization analysis. The plasma concentration of ANP decreased in water deprived rats, and the ANP mRNA levels in both auricles of these rats were lower than those of the control, particularly in the left auricle. Thirty minutes after the injection of morphine, the plasma concentration of ANP markedly increased, while morphine increased the right auricular ANP mRNA level 4 hr after the administration. These data suggest that these stimuli can change ANP gene expression in the auricles and that the changes are induced differentially in both auricles.

Measurement of endothelial cytosolic calcium concentration and nitric oxide production reveals discrete mechanisms of endothelium-dependent pulmonary vasodilatation

Nitric oxide is an endothelium-derived relaxing factor. Conversion of L-arginine to nitric oxide follows mediator-induced elevation of endothelial cytosolic calcium concentration. However, not all endothelium-dependent vasodilatation is caused by endothelium-derived relaxing factor, and few studies have correlated changes in vascular tone with measurement of free cytosolic calcium concentration or nitric oxide. The effects of three endothelium-dependent vasodilators (acetylcholine, bradykinin, and A23187) on vascular tone and nitric oxide production were studied in proximal rat pulmonary artery rings. Changes in free cytosolic calcium concentration and nitric oxide production were also studied in bovine pulmonary artery endothelial cells. A23187 and bradykinin caused pulmonary vasodilatation, nitric oxide production, and elevation of endothelial calcium concentrations. Although acetylcholine caused endothelium-dependent vasodilatation, it reduced free cytosolic calcium concentration and failed to increase nitric oxide levels. Acetylcholine-induced dilatation was partially inhibited by meclofenamate but was unaffected by ouabain. Acetylcholine, unlike bradykinin and A23187, does not act through a nitric oxide-dependent mechanism in the rat pulmonary vasculature.

Role of endothelium-derived relaxing factor in regulation of vascular tone and remodeling. Update on humoral regulation of vascular tone

In addition to preserving the permselectivity of the vascular wall and providing an antithrombogenic surface, the vascular endothelium contributes importantly to the regulation of vasomotor tone. Indeed, the endothelium participates in the conversion of angiotensin I to angiotensin II; the enzymatic inactivation of several plasma constituents such as bradykinin, norepinephrine, serotonin, and ADP; and the synthesis and release of vasodilator substances such as prostacyclin and the recently discovered endothelium-derived relaxing factor (EDRF). The diffusible EDRF released from the endothelium is nitric oxide or a substance closely related to it such as nitrosothiol. The endothelium also synthesizes and releases vasoconstrictive factors, including products derived from arachidonic acid metabolism and the recently discovered peptide endothelin. An increasing body of evidence from experimental and clinical studies indicates that EDRF and endothelium-derived contracting factors play an important role in vascular physiology and pathology. It has become apparent that the balance of these factors may be a major determinant of systemic and regional hemodynamics. Moreover, through generally opposite effects on growth-related vascular changes, contracting factors such as endothelin and relaxing factors such as EDRF also may be important determinants of the vascular response to injury in various disease states such as atherosclerosis and hypertension. It is clear that the vascular endothelium is a complex and dynamic organ. Understanding endothelium function in normal physiology and disease states is of potential clinical importance and should be the focus of future investigation.

Inhaled nitric oxide. A selective pulmonary vasodilator reversing hypoxic pulmonary vasoconstriction

Background. The gas nitric oxide (NO) is an important endothelium-derived relaxing factor, inactivated by rapid combination with heme in hemoglobin. Methods and Results. Awake spontaneously breathing lambs inhaled 5-80 ppm NO with an acutely constricted pulmonary circulation due to either infusion of the stable thromboxane endoperoxide analogue U46619 or breathing a hypoxic gas mixture. Within 3 minutes after adding 40 ppm NO or more to inspired gas, pulmonary hypertension was reversed. Systemic vasodilation did not occur. Pulmonary hypertension resumed within 3-6 minutes of ceasing NO inhalation. During U46619 infusion pulmonary vasodilation was maintained up to 1 hour without tolerance. In the normal lamb, NO inhalation produced no hemodynamic changes. Breathing 80 ppm NO for 3 hours did not increase either methemoglobin or extravascular lung water levels nor modify lung histology compared with control lambs. Conclusions. Low dose inhaled NO (5-80 ppm) is a selective pulmonary vasodilator reversing both hypoxia- and thromboxane-induced pulmonary hypertension in the awake lamb [corrected].

Impairment of endothelium-dependent pulmonary-artery relaxation in chronic obstructive lung disease

BACKGROUND

Endothelial cells release endothelium-derived relaxing factor (EDRF) in a variety of vascular beds, including the pulmonary circulation. However, the role of EDRF-mediated pulmonary-artery relaxation in chronic hypoxic lung disease is unknown.

METHODS

We studied endothelium-dependent relaxation mediated by EDRF in vitro in pulmonary arteries that had been obtained from 22 patients undergoing heart-lung transplantation for end-stage chronic obstructive lung disease. Control pulmonary arteries were obtained from 15 patients undergoing lobectomy for lung carcinoma who did not have evidence of other chronic lung disease. The responses of all vascular rings (external diameter, 1.2 to 3.4 mm) to the endothelium-dependent vasodilators acetylcholine and adenosine diphosphate were studied immediately after lung excision.

RESULTS

Pulmonary arterial rings from the patients with chronic lung disease developed a greater tension (2.19 +/- 0.16 g) in response to phenylephrine (10(-6) M) than the rings from control patients (1.28 +/- 0.18 g, P less than 0.05). Inhibition of EDRF synthesis by treatment with NG-monomethyl-L-arginine (10(-4) M) eliminated this difference, increasing the tension in the rings from the controls (P less than 0.01) but not in those from the patients with chronic lung disease. Rings from control patients relaxed in response to cumulative doses (10(-10) to 10(-5) M) of acetylcholine (maximal relaxation, 81.3 +/- 3.9 percent) and adenosine diphosphate (maximal relaxation, 85.3 +/- 2.6 percent). By contrast, rings from patients with chronic obstructive lung disease achieved only 41.3 +/- 4.8 percent of maximal relaxation in response to acetylcholine (n = 32) and 49.4 +/- 5.5 percent in response to adenosine diphosphate (n = 24) (P less than 0.001, as compared with control rings). Rings from both the controls and the patients with chronic lung disease relaxed similarly in response to the endothelium-independent vasodilator sodium nitroprusside (10(-4) M). There was an inverse correlation between the degree of intimal thickening and the level of maximal relaxation of the rings from the patients with chronic lung disease (r = -0.60, P less than 0.001). Maximal relaxation was also related directly to the partial pressure of arterial oxygen before transplantation (r = 0.68, P less than 0.01) and inversely to the partial pressure of arterial carbon dioxide before transplantation (r = -0.55, P less than 0.01), but not to the forced expiratory volume in one second (r = 0.19, P not significant).

CONCLUSIONS

Endothelium-dependent pulmonary-artery relaxation in vitro is impaired in arteries from patients with end-stage chronic obstructive lung disease. Such impairment may contribute to the development of pulmonary hypertension in chronic hypoxic lung disease.

Difference in effect of inhibitors of energy metabolism on endothelium-dependent relaxation of rat pulmonary artery and aorta

Previous studies have suggested that systemic artery endothelial cell production of the nitrovasodilator endothelium-derived relaxing factor (EDRF) is dependent upon oxidative energy production. This study was undertaken to test if pulmonary artery (PA) EDRF has a similar requirement for oxidative phosphorylation. The effects of inhibitors of oxidative phosphorylation and glycolysis on endothelium-dependent relaxation were studied in rat aortic and PA rings. In aortic rings, 0.1 microM rotenone and 0.1 microM antimycin A, and, to a lesser extent, 50 mM 2-deoxyglucose, inhibited endothelium-dependent relaxation to acetylcholine and adenosine diphosphate. Relaxation to the receptor-independent calcium ionophore A23187 was less severely affected, and relaxation to the direct smooth muscle dilator sodium nitroprusside was unaffected. The inhibitors had much less effect on PA relaxation, decreasing the potency but not the efficacy of the endothelium-dependent dilators. These results suggest that the dependence on oxidative energy production for endothelium-dependent relaxation may differ between the systemic and pulmonary vascular beds, and that in pulmonary arterial endothelium, oxidative energy production may not be required for receptor-mediated production and/or release of EDRF. The resistance of PA endothelium to decreases in oxidative energy production may contribute to the normally low tone maintained in this circuit in vivo.

Endothelium-derived relaxing factor: basic review and clinical implications

EDRF is a potent, endogenous vasodilator that is produced and released from endothelial cells and subsequently causes the relaxation of VSM through the activation of soluble guanylate cyclase and an increase in VSM cyclic GMP. Structurally, EDRF is likely to be NO or a related nitrogen oxide-containing compound. It is synthesized in endothelial and other cell types from L-arginine by a calcium-calmodulin and NADPH-dependent enzyme. Its action is very similar to the nitrovasodilators that act directly on VSM. EDRF is present in all vascular beds, large and small vessels, and in a wide range of species. Its role in human vascular physiology and pathophysiology is just beginning to be understood. EDRF is a potent endogenous vasodilator and inhibitor of platelet aggregation and adhesion. Its activity is impaired in hypertension and atherosclerosis, and its absence due to endothelial damage may play a role in cerebral and coronary vasospasm. It is a mediator of flow-dependent vasodilation, and its inhibition by hypoxia may contribute to the hypoxic pulmonary vasoconstrictor response. Endothelial cell damage and impairment of EDRF production may also contribute to acute and chronic pulmonary hypertension. A further understanding of the chemical nature and synthetic pathways of EDRF should lead to the production of analogs and antagonists, which may play an important role in future treatments for atherosclerosis, myocardial infarction, angina, hypertension, and other vascular diseases. The recent realization that EDRF serves as the second messenger for guanylate cyclase activation and cyclic GMP production in a variety of cell types outside of the cardiovascular system, including renal and respiratory epithelium, cerebellar neurons, macrophages, and adrenocytes, suggests even broader implications. The importance of EDRF to the anesthesiologist may go beyond an understanding of its role in cardiovascular physiological and pathophysiological states. Initial studies have shown that the endothelium may play a role in mediating the vascular actions of anesthetics, and that anesthetics can inhibit the production, release, or action of EDRF. How are these interactions mediated? Are there significant differences between anesthetics with regard to their effects on EDRF? Is there a clinically significant effect of anesthetics on basal activity of EDRF, or only in response to exogenous stimulation? Conversely, it is important to determine if alterations in endothelial cell function by various disease states such as hypertension, atherosclerosis, adult respiratory distress syndrome, cerebral vasospasm, and others cause changes in the vascular actions of anesthetics. The potential interactions of anesthetics with EDRF production and action in cell types other than the endothelium have not yet been explored.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelium-derived relaxing factor inhibits hypoxic pulmonary vasoconstriction in rats

The hypothesis that endothelium-derived relaxing factor (EDRF) modulates hypoxic pulmonary vasoconstriction (HPV) was tested in isolated, blood-perfused rat lungs ventilated with gas mixtures of 21% O2-5% CO2-74% N2 (normoxia) or of 3% O2-5% CO2-92% N2 (hypoxia); 30 microM NG-monomethyl-L-arginine (L-NMMA), an inhibitor of EDRF production, caused a reduction in the endothelium-dependent relaxant response to acetylcholine (ACh) from 62 +/- 7, 88 +/- 4, and 100 +/- 4% to 26 +/- 8, 49 +/- 12, and 75 +/- 7% at ACh concentrations of 1, 10, and 100 microM, respectively (p less than 0.05 at all concentrations), indicating that L-NMMA acts via the inhibition of EDRF production. L-NMMA induced a concentration-related augmentation in HPV of 20 +/- 5, 32 +/- 8, and 34 +/- 8% at concentrations of 30, 300, and 1,000 microM (p less than 0.05, compared with a vehicle control group at all concentrations). The pressor response to a dose of angiotensin II (A-II), which produced the same increase in pulmonary artery pressure as that induced by hypoxia, was also significantly augmented (2 +/- 0.6%), but to a lesser extent. The augmentation of HPV by 30 microM L-NMMA was completely reversed by 1 mM L-arginine (a precursor of EDRF), but not by D-arginine (an isomer of L-arginine). One and 6 mM L-arginine, but not 6 mM D-arginine caused a significant inhibition of HPV by 20 +/- 2 and 47 +/- 12% (p less than 0.05, compared with the vehicle control group) and a small but not significant reduction in A-II-mediated contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Loss of endothelium-dependent relaxant activity in the pulmonary circulation of rats exposed to chronic hypoxia

To determine whether exposure to chronic hypoxia and subsequent development of pulmonary hypertension induces alterations of endothelium-dependent relaxation in rat pulmonary vascular bed, we studied isolated lung preparations from rats exposed to either room air (controls) or hypoxia (H) during 1 wk (1W-H), 3 wk (3W-H), or 3W-H followed by 48 h recovery to room air (3WH + R). In lungs pretreated with meclofenamate (3 microM), the endothelium-dependent vasodilator responses to acetylcholine (10(-9)-10(-6) M) and ionophore A23187 (10(-9)-10(-7) M) were examined during conditions of increased tone by U46619 (50 pmol/min). Acetylcholine or A23187 produced dose-dependent vasodilation in control lungs, this response was reduced in group 1W-H (P less than 0.02), abolished in group 3W-H (P less than 0.001), and restored in group 3WH + R. In contrast, the endothelium-independent vasodilator agent sodium nitroprusside remained fully active in group 3W-H. The pressor response to 300 pM endothelin was greater in group 3W-H than in controls (6.8 +/- 0.5 mmHg vs. 1.6 +/- 0.2 mmHg, P less than 0.001) but was not potentiated by the endothelium-dependent relaxing factor (EDRF) antagonists: hydroquinone (10(-4) M); methylene blue (10(-4) M); and pyrogallol (3 x 10(-5) M) as it was in controls. It was similar to controls in group 3W-H + R. Our results demonstrate that hypoxia-induced pulmonary hypertension is associated with a loss of EDRF activity in pulmonary vessels, with a rapid recovery on return to a normoxic environment.

Effects of hypoxia on isolated intrapulmonary arteries from the sheep

The effects of hypoxia on contraction of sheep pulmonary artery rings (large = 2.2-4.1 mm diameter, small = 0.32-0.64 mm diameter) has been investigated following precontraction or with the artery rings set at their optimal resting force. Hypoxia (PO2 4 mmHg) caused a marked contraction of pulmonary artery rings precontracted with 5-HT at its EC85 (small arteries 40 +/- 8 g cm-2) but not when precontracted with KCl. At optimal resting force hypoxia caused a small contraction (small arteries 11 +/- 2 g cm-2). Larger artery rings gave a smaller contraction in response to hypoxia at optimal resting force than did small artery rings (2 +/- 0.2 g cm-2 at PO2 = 4 mmHg). Large, unlike the small, artery rings did not contract in response to hypoxia when precontracted with 5-HT at its EC35. Lowering the PO2 to 40 mmHg caused contraction in arteries precontracted with 5-HT at its EC85 but not in arteries at their optimal resting force. Removal of the endothelium abolished all hypoxia-induced contractile responses in sheep pulmonary artery rings. Hypoxia reversibly abolished acetylcholine-induced relaxation and augmented the 5-HT contraction (206 +/- 28 to 255 +/- 34 g cm-2) in small rings. It is concluded that hypoxia may produce contraction in sheep pulmonary artery rings at least, in part, by reducing the output of vasodilator mediators from the endothelium.

Endothelium-derived relaxing factors and the human pulmonary circulation

The vasodilator effect of acetylcholine on the pulmonary circulation was first described over 30 years ago, however, the mechanism remained unknown until Furchgott described the endothelium-dependent relaxation of certain vasodilators. It was not until 1987 that endothelium-derived relaxant factor (EDRF) was demonstrated to dilate human pulmonary arteries in vitro. Despite this work, the physiologic role of EDRF in the pulmonary circulation is not known. It has been suggested that hypoxia-induced inhibition of EDRF action or release from pulmonary artery endothelial cells may have a role in hypoxic pulmonary vasoconstriction (HPV) but present evidence suggests that loss of EDRF activity is not directly involved in the phenomenon of HPV. It is more likely that EDRF is released from pulmonary artery endothelial cells during hypoxia and this released EDRF then modulates HPV. If EDRF does modulate HPV in vivo then the role of EDRF in the altered HPV found in disease merits attention. It is known that in disease states such as acute lung injury and pneumonia there is loss or attenuation of HPV which inevitably leads to increased V/Q mismatch and hypoxemia. Whether this attenuation of HPV is due to release of an endogenous vasodilator such as EDRF is presently being investigated. Additionally, there is in vitro evidence that loss of EDRF activity may be important in the genesis of pulmonary hypertension such as found in severe cystic fibrosis. During the next decade the role of EDRF in the human pulmonary circulation in both health and disease will undoubtedly be elucidated.

Hypoxic contraction of cultured pulmonary vascular smooth muscle cells

The cellular events involved in generating the hypoxic pulmonary vasoconstriction response are not clearly understood, in part because of the multitude of factors that alter pulmonary vascular tone. The goal of the present studies was to determine if a cell culture preparation containing vascular smooth muscle (VSM) cells could be made to contract when exposed to a hypoxic atmosphere. Cultures containing only fetal bovine pulmonary artery VSM cells were assessed for contractile responses to hypoxic stimuli by two methods. In the first, tension forces generated by cells grown on a flexible growth surface (polymerized polydimethyl siloxane) were manifested as wrinkles and distortions of the surface under the cells. Wrinkling of the surface was noted to progressively increase with time as the culture medium bathing the cells was made hypoxic (PO2 approximately 25 mmHg). The changes were sometimes reversible upon return to normoxic conditions and appeared to be enhanced in cells already exhibiting evidence of some baseline tone. Repeated passage in culture did not diminish the hypoxic response. Evidence for contractile responses to hypoxia was also obtained from measurements of myosin light chain (MLC) phosphorylation. Conversion of MLC to the phosphorylated species is an early step in the activation of smooth muscle contraction. Lowering the PO2 in the culture medium to 59 mmHg caused a 45% increase in the proportion of MLC in the phosphorylated form as determined by two-dimensional gel electrophoresis. Similarly, cultures preincubated for 4 h with 32P and then exposed to normoxia or hypoxia for a 5-min experimental period showed more than twice as much of the label in MLCs of the hypoxic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoxic pulmonary vasoconstriction is enhanced by inhibition of the synthesis of an endothelium derived relaxing factor

Inhibition of the synthesis of endothelium derived relaxing factor by NG-monomethyl-L-arginine, a competitive inhibitor of the synthesis of nitric oxide from L-arginine, enhances hypoxic pulmonary vasoconstriction in pulmonary artery rings and isolated, Krebs albumin perfused rat lungs. L-arginine rapidly reduces hypoxic vasoconstriction, particularly in lungs treated with NG-monomethyl-L-arginine. Following administration of NG-monomethyl-L-arginine, bradykinin-induced vasodilatation is inhibited (p less than 0.01) and a bradykinin-induced vasoconstriction develops (p less than 0.001). NG-monomethyl-L-arginine does not significantly diminish acetylcholine-induced vasodilatation in the isolated lung. NG-monomethyl-L-arginine causes an endothelium-dependent vasoconstriction in pulmonary artery rings.

Prostacyclin contributes to inhibition of hypoxic pulmonary vasoconstriction by alkalosis

The mechanism by which extracellular alkalosis inhibits hypoxic pulmonary vasoconstriction is unknown. We investigated whether the inhibition was due to intrapulmonary production of a vasodilator prostaglandin such as prostacyclin (PGI2). Hypoxic vasoconstriction in isolated salt-solution-perfused rat lungs was blunted by both hypocapnic and NaHCO3-induced alkalosis (perfusate pH increased from 7.3 to 7.7). The NaHCO3-induced alkalosis was accompanied by a significant increase in the perfusate level of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), an hydrolysis product of PGI2. Meclofenamate, an inhibitor of cyclooxygenase, counteracted both the blunting of hypoxic vasoconstriction and the increased level of 6-keto-PGF1 alpha. In intact anesthetized dogs, hypocapnic alkalosis (blood pH increased from 7.4 to 7.5) blunted hypoxic pulmonary vasoconstriction before but not after administration of meclofenamate. In separate cultures of bovine pulmonary artery endothelial and smooth muscle cells stimulated by bradykinin, the incubation medium levels of 6-keto-PGF1 alpha were increased by both hypocapnic and NaHCO3-induced alkalosis (medium pH increased from 7.4 to 7.7). These results suggest that inhibition of hypoxic pulmonary vasoconstriction by alkalosis is mediated at least partly by PGI2.