The inhibition of angiotensin converting enzyme attenuates the effects of chronic hypoxia on pulmonary blood vessels in the rat

The effect of chronic administration of angiotensin converting enzyme inhibitor on the development of hypoxic pulmonary hypertension was studied in rats. Male Wistar rats were-exposed for 3 weeks to isobaric hypoxia (10% O2) and treated with 10 mg/kg b.w. of Ramipril daily. The haemodynamic properties of the pulmonary vasculature were then measured in isolated blood-perfused lung preparation. Ramipril administration during the sojourn in hypoxia resulted in lower baseline perfusion pressure and lower slope of perfusion pressure-flow relationship compared to non-treated hypoxic rats. Partitioning of the distribution of pulmonary vascular resistance across the vascular bed by the occlusion technique showed that it was mainly due to a decrease of arterial and venous vascular resistances to blood flow. It is suggested that Ramipril attenuates the process of morphological reconstruction of pulmonary vasculature by chronic hypoxia rather than the level of vascular smooth muscle tone.

Production of hydrogen peroxide by alveolar macrophages from rats exposed to subacute and chronic hypoxia

We have studied in vitro alveolar macrophages (AMs) obtained by tracheobronchial lavage from rats exposed to subacute (3 hours and 3 days) and chronic (3 weeks) hypoxia (FiO2 = 0.1) and from rats recovering from chronic hypoxia. Hydrogen peroxide production by AMs was measured by luminol-dependent chemiluminescence after AMs adhered to the walls of the measuring cuvette, after stimulation with phorbol-myristate-acetate (PMA), and when N-formyl-methionyl-leucyl-phenylanine (FMLP) was added subsequently to the cells which had been previously stimulated by adherence or PMA. H2O2 production after cell adherence and adherence combined with FMLP stimulation did not differ between the groups. The increase of H2O2 production after adding PMA, and FMLP in addition to PMA was significantly higher in AMs from rats exposed to hypoxia for 3 days than in the controls. Other experimental groups did not differ from their controls. It is concluded that 3 days' hypoxia primes AMs for enhanced production of H2O2 upon stimulation. The mechanism is probably at the level of synthesis of proteins involved in H2O2 production, or the shift to a more reactive phenotype of alveolar macrophages subpopulations.

Long-term effects of prenatal indomethacin administration on the pulmonary circulation in rats

Mechanical properties of the adult pulmonary vasculature are affected by perinatal experience of hypoxic pulmonary hypertension. In the present study, we followed the long-term effects of perinatal pulmonary hypertension induced by means other than hypoxia in rats. Daily injections of indomethacin (1 mg.kg-1 body weight (BW)) were given to the parturient rats. Their newborn pups had significantly increased number of muscularized peripheral pulmonary vessels. Pulmonary hypertension, however, did not persist to adulthood (mean pulmonary arterial pressure (Ppa) was 17.2 +/- 1.3 torr in the experimental group and 16.4 +/- 0.8 torr in controls). Pulmonary hypertension induced in adult rats by exposure to chronic hypoxia or by acute hypoxic challenges was similar in indomethacin-treated and control rats. Normoxic perfusion pressure/flow (P/Q) plots in isolated lungs were less steep in indomethacin-treated than in control rats. Acute hypoxia increased the slope of P/Q plots in indomethacin treated rats but not in controls. The described changes in the pulmonary vasculature induced by indomethacin are similar to those found previously in adult rats born in hypoxia. We conclude that perinatal pulmonary hypertension permanently modifies the pulmonary vasculature.

Almitrine in low dose potentiates vasoconstrictor responses of isolated rat lungs to moderate hypoxia

To test whether the effect of almitrine on hypoxic pulmonary vasoconstriction was dose-dependent, two series of experiments were performed on isolated rat lungs perfused with constant flow of blood. In the first series, the effects of different doses of almitrine on perfusion pressure were measured. Baseline perfusion pressure was not changed by solvent or by 0.25 micrograms.ml-1 almitrine, but it was increased by 0.5 and 2.0 micrograms.ml-1 almitrine. The increase in perfusion pressure in response to 10 min ventilation with hypoxic gas mixture (5% O2) was significantly (p less than 0.05) higher after 0.25 micrograms.ml-1 almitrine (12.0 +/- 0.8 torr) than before addition of the drug (5.43 +/- 1.8 torr). Responses to hypoxia were insignificant after higher doses (0.5 and 2.0 micrograms.ml-1) of almitrine. In the second series of experiments the responses to varying degrees of hypoxia were measured after administration of one dose of almitrine (0.25 micrograms.ml-1). Almitrine, compared to solvent alone, significantly altered the shape of the dose-response curve to hypoxia. Increases in perfusion pressure in response to moderate degrees of hypoxia were potentiated (10% O2: 8.7 +/- 1.8 torr after almitrine, 2.1 +/- 0.6 torr after solvent, p less than 0.05), whereas responses to severe hypoxia (3% O2) were not changed by almitrine. Reactivity to angiotensin II was decreased by 0.25 micrograms.ml-1 almitrine. We conclude that almitrine in low but not in high dose augments pulmonary vasoconstriction induced by mild degrees of hypoxia.

Almitrine in low dose potentiates vasoconstrictor responses of isolated rat lungs to moderate hypoxia

To test whether the effect of almitrine on hypoxic pulmonary vasoconstriction was dose-dependent, two series of experiments were performed on isolated rat lungs perfused with constant flow of blood. In the first series, the effects of different doses of almitrine on perfusion pressure were measured. Baseline perfusion pressure was not changed by solvent or by 0.25 micrograms.ml-1 almitrine, but it was increased by 0.5 and 2.0 micrograms.ml-1 almitrine. The increase in perfusion pressure in response to 10 min ventilation with hypoxic gas mixture (5% O2) was significantly (p less than 0.05) higher after 0.25 micrograms.ml-1 almitrine (12.0 +/- 0.8 torr) than before addition of the drug (5.43 +/- 1.8 torr). Responses to hypoxia were insignificant after higher doses (0.5 and 2.0 micrograms.ml-1) of almitrine. In the second series of experiments the responses to varying degrees of hypoxia were measured after administration of one dose of almitrine (0.25 micrograms.ml-1). Almitrine, compared to solvent alone, significantly altered the shape of the dose-response curve to hypoxia. Increases in perfusion pressure in response to moderate degrees of hypoxia were potentiated (10% O2: 8.7 +/- 1.8 torr after almitrine, 2.1 +/- 0.6 torr after solvent, p less than 0.05), whereas responses to severe hypoxia (3% O2) were not changed by almitrine. Reactivity to angiotensin II was decreased by 0.25 micrograms.ml-1 almitrine. We conclude that almitrine in low but not in high dose augments pulmonary vasoconstriction induced by mild degrees of hypoxia.

Perinatal hypoxia increases hypoxic pulmonary vasoconstriction in adult rats recovering from chronic exposure to hypoxia

The possibility that perinatal exposure to hypoxia influences the pulmonary vasculature in adults was tested. Rats born in a hypoxic environment were kept in hypoxia for an additional week after birth. The rats were then raised in atmospheric air, and when adult, they were compared with the rats born and raised in air. Rats (10 wk old) of both groups were exposed to 10% O2 for 2 wk. They were then studied immediately after the exposure and after 2 wk of recovery from the sojourn in the hypoxic environment. The experience of perinatal hypoxia did not affect mean pulmonary arterial blood pressure, right ventricle weight, or the number of muscularized peripheral pulmonary vessels. During exposure to chronic hypoxia in adulthood, both groups developed pulmonary hypertension, which was not affected by previous perinatal hypoxia. The pulmonary vascular responses to acute hypoxic challenges were studied in the preparation of isolated perfused lungs. In both groups of rats, perinatally hypoxic and normoxic, the acute hypoxic vasoconstriction was attenuated immediately after the exposure of adult animals to chronic hypoxia. However, during the recovery from this hypoxic sojourn, the rats born in hypoxia were significantly more reactive to acute lung hypoxia than all other groups of rats studied. It is concluded that the experience of a short period of perinatal hypoxia did not affect the development of hypoxic pulmonary hypertension induced in adulthood. It increased, however, the pulmonary vascular reactivity to acute hypoxic stimuli during the period of recovery from a sojourn in the hypoxic environment in adulthood.

Dexamethasone potentiates hypoxic vasoconstriction in salt solution-perfused rat lungs

Isolated rat lungs perfused with blood or plasma develop good hypoxic pressor reactivity, but those flushed of blood and perfused solely with physiological salt solution (PSS) show decreased or no hypoxic vasoconstriction. Because the handling of rats during collection of blood increases the plasma corticosterone levels, we wondered whether the difference between blood and PSS perfusion was due partly to higher glucocorticoid activity with blood than with PSS. Treatment of lung donors with the synthetic glucocorticoid dexamethasone (2 mg/kg im 3 h before isolation) or addition of dexamethasone (4.2 X 10(-6) M) to the perfusate increased markedly the hypoxic pressor reactivity of rat lungs perfused with PSS plus albumin (4 g/100 ml). In comparison, dexamethasone did not cause a significant increase in the already vigorous hypoxic pressor reactivity of blood-perfused lungs, but it reversed the blunted pressor reactivity of lungs perfused with blood collected from adrenalectomized rats. The potentiation of hypoxic vasoconstriction by dexamethasone required greater than 40 min to develop and was inhibited by a 100-fold higher concentration of progesterone, a glucocorticoid receptor antagonist. Although dexamethasone and the cyclooxygenase blocker meclofenamate (3.1 X 10(-5) M) caused similar decreases in perfusate levels of cyclooxygenase metabolites, the glucocorticoid was much more effective in inducing hypoxic vasoconstriction in PSS-perfused lungs. Thus a reduction of cyclooxygenase metabolism might have contributed to, but did not fully account for, the glucocorticoid effect. Dexamethasone also caused small increases in base-line perfusion pressure and in pressor reactivity to angiotensin II.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroxine treatment increases the hypoxic pulmonary vasoconstriction in isolated lungs from thyroidectomized rats

The possibility that changes in energy metabolism are involved in oxygen sensing during hypoxic pulmonary vasoconstriction was tested indirectly by measurement of hypoxic-pressor reactivity in lungs isolated from rats with low and high levels of plasma thyroxine. In the first study, male rats were treated for one week after thyroidectomy with 50 micrograms (n = 6) or 100 micrograms (n = 6) thyroxine per 100 g body weight (b.w.) daily or with solvent (n = 6). The lungs were isolated and perfused at constant flow with salt-albumin solution. They were ventilated with air +5% CO2 in a humid chamber at 38 degrees C. The dose-pressor response to hypoxia and angiotensin II were measured. In the second study, thyroidectomized male rats were treated similarly with 100 micrograms thyroxine (n = 7) or solvent (n = 6) and isolated lungs were perfused with homologous blood obtained from thyroidectomized blood donors treated in the same manner. Then the dose-pressor responses to hypoxia and K+ were elicited. The hypoxic-pressor responses were bigger in thyroxine than in solvent-treated rats. The response to angiotensin II and K+ was not affected by thyroxine treatment. The results are consistent with the idea that hypoxic-pressor reactivity varies directly with the metabolic rate of lung tissue.

Effects of ouabain, low K+, and aldosterone on hypoxic pressor reactivity of rat lungs

It can be postulated that inhibition of lung tissue Na+-K+-ATPase might potentiate hypoxic pulmonary vasoconstriction by depolarizing some excitable cell or, in contrast, that it might blunt the hypoxic response by reducing cellular metabolic rate and sensitivity to hypoxia. Thus the purpose of this study was to test in isolated rat lungs whether hypoxic pressor reactivity was related inversely or positively to Na+-K+-ATPase activity. Dose-pressor response curves to hypoxia, angiotensin II, or KCl were measured under control conditions and after exposure either to one of two inhibitors of Na+-K+-ATPase, ouabain, and low-K+ solution or to a stimulator of Na+-K+ pumping, aldosterone. Ouabain and low K+ depressed the response to hypoxia but had little effect on that to angiotensin II. The response to KCl was increased by ouabain. Aldosterone potentiated the hypoxic response. These results do not support the idea that membrane depolarization due to inhibition Na+-K+ pumping is a component of hypoxic vasoconstriction. They do suggest a positive relationship between Na+-K+-ATPase activity and hypoxic pressor reactivity and are consistent with the idea that Na+-K+-ATPase activity might influence hypoxic reactivity indirectly by altering cellular energy metabolism. It is also possible that the results were somehow due to changes in intracellular [Na+] or transmembrane Na+ gradient, rather than to changes in energy metabolism.

Effect of alpha-methyldopa on lung microembolism in the rat

The effect of chronic pretreatment with alpha-methyldopa on acute pulmonary hypertension induced by lung microembolization with Sephadex particles in the rat was studied. Alpha-methyldopa was administered twice a day for 10 days by oesophageal gavage (20 mg X kg-1 of body weight) prior to embolization. Rats were anaesthetized with urethane. A catheter was positioned in closed chest rats in the pulmonary artery and 50 microliter of Sephadex suspension in saline were injected. The control group was not pretreated with alpha-methyldopa. In a subgroup of control and pretreated rats, the cardiac output was measured by the dye dilution technique before and after lung microembolization. Rats pretreated with alpha-methyldopa had less pulmonary hypertension, higher cardiac output and more severe arterial hypoxaemia 15 min after lung microembolization than untreated animals. There was no difference in platelet counts prior and after embolization. We suggest that pulmonary hypertension after lung microembolization in the rat includes a vasoconstrictor component which can be suppressed by alpha-methyldopa, and that severe arterial hypoxaemia in pretreated rats is probably the result of a decrease of the ventilation/perfusion ratio in unobstructed areas, which in untreated rats may be prevented by active vasoconstriction.

Enlargement of carotid bodies in rats with lung emphysema or silicosis

Volume of carotid bodies was measured morphometrically in rats with experimental lung emphysema or silicosis. The mean volume of the left carotid body was 19.9 +/- (SD) 12.4 in 13 control rats, 30.7 +/- 12.5 in 9 rats with lung emphysema, and 41 +/- 18.9 . 10(6) micrometer 3 in 10 rats with lung silicosis. There was no correlation between carotid body volume and pulmonary arterial blood pressure. A good correlation was found between the volume of carotid body and partial pressure of oxygen in arterial blood. The functional meaning of the increase of volume of carotid bodies in rats with chronic respiratory insufficiency remains unclear.

Pulmonary hypertension induced by repeated pulmonary inflammation in the rat

The effect of repeated lung inflammation on the pulmonary vascular bed was studied in rats. Nonbacterial lung inflammation was induced by repeated carrageenan instillations into the lungs. Three days after the single carrageenan injection, the mean pulmonary arterial blood pressure was only slightly raised [16.3 +/- 0.6 (mean +/- SE) Torr in controls and 19.5 +/- 0.5 Torr in rats with lung inflammation, P less than 0.001]. A substantial pulmonary hypertension was found in rats 3 days after the sixth lung inflammation (24.6 +/- 1 Torr). In this group, arterial hypoxemia, hypercapnia, and right-heart hypertrophy were found. In the 14th day of recovery after the last injection of carrageenan, the mean pulmonary artery blood pressure decreased (18.5 +/- 0.9 Torr) but remained higher than in the control group. There was no difference in cardiac output measured by dye-dilution technique between the experimental and control groups. After repeated inflammation, the media of distal pulmonary vessels thickened and the number of pulmonary arterioles with distinct media increased.

Pulmonary hypertension in silicotic rats

In male rats, pulmonary hypertension was found 9, 17 to 18, and 30 wk after an intrapulmonary injection of 50 mg quartz dust suspended in saline. Pulmonary arterial blood pressure increased progressively with time, and in the oldest group, the weight of the right heart ventricle was increased.

Pulmonary hypertension in rats with papain emphysema

Lung emphysema was produced in 80 rats by tracheal chronic constriction and repeated instillations of 0.1% papain solution intratracheally. 17 animals survived 90 days of the experiment; a complete examination was performed on 14 experimental and 11 control animals. 9 rats of the experimental group had pulmonary hypertension, in 5 other rats the pulmonary arterial blood pressure was not different from that in the controls. The experimental animals with hypertension had arterial hypoxemia and increased weight of the right ventricle. All experimental rats (with and without pulmonary hypertension) had increased air spaces in the lung and thickened media of distal pulmonary vessels. After breathing 100% oxygen for 20 min, the pulmonary arterial blood pressure in animals with pulmonary hypertension decreased but did not reach the control level. The decrease of the mean pulmonary arterial blood pressure after oxygen breathing correlated well with the initial level of pulmonary hypertension and with the degree of hypoxemia during air breathing.

Resolution of pulmonary hypertension and other features induced by chronic hypoxia in rats during complete and intermittent normoxia

Rats subjected to 10% O2 (hypoxic rats) for various periods and recovery regimens were compared with control animals with respect to pulmonary artery pressure (Ppa), right ventricular hypertrophy (RVH), and muscularisation of small pulmonary vessels. Mean Ppa was measured in anaesthetised animals spontaneously breathing air and rose from 16 mmHg in controls to 36 mmHg in rats exposed to hypoxia for three weeks. Ppa had returned to normal after 20 weeks' recovery in air. RVH regressed a little more quickly, but muscularisation of small pulmonary vessels. Mean Ppa was measured in anaesthetised animals spontaneously breathing air and rose from 16 mmHg in controls to 36 mmHg in rats exposed to hypoxia for three weeks. Ppa had returned to normal after 20 weeks' recovery in air. RVH regressed a little more quickly, but muscularisation of small pulmonary vessels was still apparent after 20 weeks. Some hypoxic rats were subjected to an intermittent normoxic recovery regimen for either 40 or 80 hours a week in air, the remainder in 10% O2. Some reduction in RVH probably occurred after six weeks on the 80-hour regimen, but there was no reduction in Ppa or muscularisation of small pulmonary vessels. These results suggest that the pulmonary hypertension of chronic alveolar hypoxia resolves very slowly and is probably related to structural changes in the pulmonary vessels. Their relevance to human cor pulmonale and intermittent long-term oxygen treatment for these patients is discussed.

The effect of acute and chronic hypoxia on thoracic gas volume in anaesthetized rats

1. Thoracic gas volume at end expiration (functional residual capacity, FRC) was measured in chronically and acutely hypoxic anaesthetized rats by a plethysmograph method. 2. FRC, measured during air breathing, was 34-62% larger in rats which had been kept in an environmental chamber in 8, 10 or 12% O2 for 3 weeks than in littermate controls. FRC returned to normal after the rats had returned to air for 9 days. There was no constant difference in the pattern of breathing between control and chronically hypoxic rats. 3. Pressure-volume curves measured post mortem showed no difference in the volume of the lung at 25 cm H2O pressure or in the compliance of the lung between chronically hypoxic and control rats. Thus there was no gross mechanical change in the lung to account for the increase in FRC. 4. Acute hypoxia caused by breathing 12% O2 increased FRC in control but not in chronically hypoxic rats. The increase in FRC in control rats was abolished by combined blockade of the vagus nerves and carotid bodies (with procaine) but not by vagal blockade alone. 5. The combined vagal and carotid body blockade reduced FRC significantly in rats which had been in 10% O2 for 3 days but not in those which had been in 10% O2 for 21 days. 6. Lung area measured from radiographs was not reduced by a muscle relaxant in chronically hypoxic rats. Electromyograms from anterior intercostal muscles and the diaphragm showed no electrical activity in expiration in chronically hypoxic rats which might indicate an active muscular basis for their increased FRC. However when FRC was raised by acute hypoxia in control animals there was also no increase in electrical activity in expiration which could have explained their increase in lung volume. 7. We concluded that the increase in FRC during acute hypoxia in control rats was probably due to a reflex from the carotid body. The increase in FRC in chronically hypoxic rats, which was present while they breathed air, may have had an active muscular component in the early stages but later on there was possibly a structural factor in the chest wall.

Effect of alpha-methyldopa on the pulmonary vascular changes induced by chronic hypoxia in rats

1. Groups of young rats were kept in a hypoxic chamber or in air as control animals for 28 days. 2. Hypoxic and control animals were treated with either alpha-methyl-3,4-dihydroxyphenylalanine (alpha-methyldopa) or distilled water. 3. alpha-Methyldopa significantly reduced the increase in pulmonary artery pressure and right ventricular hypertrophy induced by chronic hypoxia and partially prevented the histological changes in the small pulmonary vessels.