Downregulation of hypoxic vasoconstriction by chronic hypoxia in rabbits: effects of nitric oxide

Hypoxic pulmonary vasoconstriction (HPV) matches lung perfusion to ventilation for optimizing pulmonary gas exchange. Chronic alveolar hypoxia results in vascular remodeling and pulmonary hypertension. Previous studies have reported conflicting results of the effect of chronic alveolar hypoxia on pulmonary vasoreactivity and the contribution of nitric oxide (NO), which may be related to species and strain differences as well as to the duration of chronic hypoxia. Therefore, we investigated the impact of chronic hypoxia on HPV in rabbits, with a focus on lung NO synthesis. After exposure of the animals to normobaric hypoxia (10% O(2)) for 1 day to 10 wk, vascular reactivity was investigated in ex vivo perfused normoxic ventilated lungs. Chronic hypoxia induced right heart hypertrophy and increased normoxic vascular tone within weeks. The vasoconstrictor response to an acute hypoxic challenge was strongly downregulated within 5 days, whereas the vasoconstrictor response to the thromboxane mimetic U-46619 was maintained. The rapid downregulation of HPV was apparently not linked to changes in the lung vascular NO system, detectable in the exhaled gas and by pharmacological blockage of NO synthesis. Treatment of the animals with long-term inhaled NO reduced right heart hypertrophy and partially maintained the reactivity to acute hypoxia, without any impact on the endogenous NO system being noted. We conclude that chronic hypoxia causes rapid downregulation of acute HPV as a specific event, preceding the development of major pulmonary hypertension and being independent of the lung vascular NO system. Long-term NO inhalation partially maintains the strength of the hypoxic vasoconstrictor response.     

Leukotriene C4 production during hypoxic pulmonary vasoconstriction in isolated rat lungs

Leukotriene inhibitors preferentially inhibit hypoxic pulmonary vasoconstriction in isolated rat lungs. If lipoxygenase products are involved in the hypoxic pressor response they might be produced during acute alveolar hypoxia and a leukotriene inhibitor should block both the vasoconstriction and leukotriene production that occurs in response to hypoxia. We investigated in isolated blood perfused rat lungs whether leukotriene C4 (LTC4) could be recovered from whole lung lavage fluid during ongoing hypoxic vasoconstriction. Lung lavage from individual rats had slow reacting substance (SRS)-like myotropic activity by guinea pig ileum bioassay. Pooled lavage (10 lungs) as analyzed by reverse phase high performance liquid chromatography had an ultraviolet absorbing component at the retention time for LTC4. At radioimmunoassay, and SRS myotropic activity by bioassay. LTC4 was not found during normoxic ventilation, during normoxic ventilation after a hypoxic pressor response, or during vasoconstriction elicited by KCl. Diethylcarbamazine citrate, a leukotriene synthesis blocker, concomitantly inhibited the hypoxic vasoconstriction and LTC4 production. Thus 5-lipoxygenase products may play a role in the sequence of events leading to hypoxic pulmonary vasoconstriction.     

Hydrogen sulfide mediates hypoxic vasoconstriction through a production of mitochondrial ROS in trout gills

Hypoxic pulmonary vasoconstriction (HPV) is an adaptive response that diverts pulmonary blood flow from poorly ventilated and hypoxic areas of the lung to more well-ventilated parts. This response is important for the local matching of blood perfusion to ventilation and improves pulmonary gas exchange efficiency. HPV is an ancient and highly conserved response, expressed in the respiratory organs of all vertebrates, including lungs of mammals, birds, and reptiles; amphibian skin; and fish gills. The mechanism underlying HPV and how cells sense low Po(2) remains elusive. In perfused trout gills (Oncorhynchus mykiss), acute hypoxia, as well as H(2)S, caused an initial and transient constriction of the vasculature. Inhibition of the enzymes cystathionine-β-synthase and cystathionine-γ-lyase, which blocks H(2)S production, abolished the hypoxic response. Individually blocking the four complexes in the electron transport chain abolished both the hypoxic and the H(2)S-mediated constriction. Glutathione, an antioxidant and scavenger of superoxide, attenuated the vasoconstriction in response to hypoxia and H(2)S. Furthermore, diethyldithiocarbamate, an inhibitor of superoxide dismutase, attenuated the hypoxic and H(2)S constriction. This strongly suggests that H(2)S mediates the hypoxic vasoconstriction in trout gills. H(2)S may stimulate the mitochondrial production of superoxide, which is then converted to hydrogen peroxide (H(2)O(2)). Thus, H(2)O(2) may act as the "downstream" signaling molecule in hypoxic vasoconstriction.     

Simultaneous measurement of O2 radicals and pulmonary vascular reactivity in rat lung

The role of endogenous radicals in the regulation of pulmonary vascular tone was evaluated by simultaneous measurement of pulmonary artery pressure and lung radical levels during exposure of isolated rat lungs to varying inspired O2 concentrations (0-95%) and angiotensin II. Lung radical levels, measured "on-line" using luminol and lucigenin-enhanced chemiluminescence, decreased in proportion to the degree of alveolar hypoxia. Radical levels fell during hypoxia before the onset of pulmonary vasoconstriction and promptly returned to basal levels with restoration of normoxic ventilation. Mild alveolar hypoxia (10% O2), which failed to decrease chemiluminescence, did not trigger pulmonary vasoconstriction. Although chemiluminescence tended to decrease more as the hypoxic response strengthened, there was not a simple correlation between the magnitude of the change in chemiluminescence induced by hypoxia and the strength of the hypoxic pressor response. Normoxic chemiluminescence was largely inhibited by superoxide dismutase but not catalase. Superoxide dismutase also increased normoxic pulmonary vascular tone and the strength of the pressor response to hypoxia and angiotensin II. Thus the predominant activated O2 species in the lung, during normoxia, was the superoxide anion or a closely related substance. Alteration of endogenous radical levels can result in changes in vascular tone. It remains uncertain whether the decrease in lung radical production during hypoxia caused pulmonary vasoconstriction or was merely associated with hypoxic ventilation.     

Effects of hypercapnia and NO synthase inhibition in sustained hypoxic pulmonary vasoconstriction

Background

Acute respiratory disorders may lead to sustained alveolar hypoxia with hypercapnia resulting in impaired pulmonary gas exchange. Hypoxic pulmonary vasoconstriction (HPV) optimizes gas exchange during local acute (0-30 min), as well as sustained (> 30 min) hypoxia by matching blood perfusion to alveolar ventilation. Hypercapnia with acidosis improves pulmonary gas exchange in repetitive conditions of acute hypoxia by potentiating HPV and preventing pulmonary endothelial dysfunction. This study investigated, if the beneficial effects of hypercapnia with acidosis are preserved during sustained hypoxia as it occurs, e.g in permissive hypercapnic ventilation in intensive care units. Furthermore, the effects of NO synthase inhibitors under such conditions were examined.

Method

We employed isolated perfused and ventilated rabbit lungs to determine the influence of hypercapnia with or without acidosis (pH corrected with sodium bicarbonate), and inhibitors of endothelial as well as inducible NO synthase on acute or sustained HPV (180 min) and endothelial permeability.

Results

In hypercapnic acidosis, HPV was intensified in sustained hypoxia, in contrast to hypercapnia without acidosis when HPV was amplified during both phases. L-N^G-Nitroarginine (L-NNA), a non-selective NO synthase inhibitor, enhanced acute as well as sustained HPV under all conditions, however, the amplification of sustained HPV induced by hypercapnia with or without acidosis compared to normocapnia disappeared. In contrast 1400 W, a selective inhibitor of inducible NO synthase (iNOS), decreased HPV in normocapnia and hypercapnia without acidosis at late time points of sustained HPV and selectively reversed the amplification of sustained HPV during hypercapnia without acidosis. Hypoxic hypercapnia without acidosis increased capillary filtration coefficient (Kfc). This increase disappeared after administration of 1400 W.

Conclusion

Hypercapnia with and without acidosis increased HPV during conditions of sustained hypoxia. The increase of sustained HPV and endothelial permeability in hypoxic hypercapnia without acidosis was iNOS dependent.     

Leukotriene C4 production during hypoxic pulmonary vasoconstriction in isolated rat lungs

Leukotriene inhibitors preferentially inhibit hypoxic pulmonary vasoconstriction in isolated rat lungs. If lipoxygenase products are involved in the hypoxic pressor response they might be produced during acute alveolar hypoxia and a leukotriene inhibitor should block both the vasoconstriction and leukotriene production that occurs in response to hypoxia. We investigated in isolated blood perfused rat lungs whether leukotriene C₄ (LTC₄) could be recovered from whole lung lavage fluid during ongoing hypoxic vasoconstriction. Lung lavage from individual rats had slow reacting substance (SRS)-like myotropic activity by guinea pig ileum bioassay. Pooled lavage (10 lungs)_as analyzed by reverse phase high performance liquid chromatography had an ultraviolet absorbing component at the retention time for LTC₄. At this retention time the element had both LTC₄ immunoreactivitiy by radioimmunoassay, and SRS myotropic activity by bioassay. LTC₄ was not found during normoxic ventilation, during normoxic ventilation after a hypoxic pressor response, or during vasoconstriction elicited by KCL. Diethylcarbamazine citrate, a leukotriene synthesis blocker, concomitantly inhibited the hypoxic vasoconstriction and LTC₄ production. Thus 5-lipoxygenase products may play a role in the sequence of events leading to hypoxic pulmonary vasoconstriction.     

Effect of endotoxin pretreatment on the pulmonary vascular response to hypoxia in O2-exposed lambs

We recently reported that endotoxin infusion before O2 exposure significantly reduced or delayed the onset of pulmonary edema formation and respiratory failure by reducing the oxidant stress of O2 exposure. Despite these beneficial effects of endotoxin treatment, lung microvascular permeability eventually increased, but postmortem lung water content was less than expected. Prolonged O2 breathing blunts or abolishes the pulmonary constrictor response to alveolar hypoxia in some species, and it is possible that the loss of this response could contribute further to edema formation. To determine whether the reduction in lung edema observed in endotoxin-treated, O2-exposed lambs was linked to the preservation of hypoxic pulmonary vasoconstriction (HPV), we measured pulmonary vascular resistance before and after 8 min of isocarbic hypoxia (inspired O2 fraction 0.12) during each day of O2 exposure. In six control lambs, the pressor response to hypoxia was abolished after 72 h in O2, and the lambs developed respiratory failure shortly thereafter. In six endotoxin-treated lambs, HPV was preserved for as long as 144 h of O2 exposure. In two control O2-exposed lambs in whom HPV was abolished, the infusion of either angiotensin or prostaglandin H2 analogue increased pulmonary vascular resistance by greater than 75%. We conclude that in lambs 1) hyperoxia abolishes the pulmonary vascular response to hypoxia, 2) endotoxin pretreatment reduces acute O2-induced lung injury and preserves the pulmonary constrictor response to hypoxia, and 3) the loss of HPV during O2 exposure may be the result of oxidant-mediated injury to the hypoxia response itself and not the result of diffuse damage to the vasoconstrictor effector mechanism.     

Oxygen radicals and antioxidant enzymes alter pulmonary vascular reactivity in the rat lung

It has been postulated that changes in the availability of partially reduced O2 species, such as O2 radicals, could serve as a link between PO2 in the alveolus and pulmonary vascular tone (Herz 11: 127-141, 1986). To assess this hypothesis, the hemodynamic effects of acute changes in the balance between the production of O2 radicals and availability of antioxidant enzymes were studied in the isolated perfused rat lung. Intravascular generation of O2 radicals, by administration of xanthine-xanthine oxidase, decreased the pulmonary vascular pressor response to alveolar hypoxia (-55 +/- 5%) and angiotensin II (-58 +/- 10%, P less than 0.01 for each) in isolated perfused rat lungs without increasing the lung wet-to-dry weight ratio. Decreases in pulmonary vascular reactivity were inhibited by pretreatment of the lung with desferrioxamine or a mixture of catalase and superoxide dismutase. Catalase and superoxide dismutase preserved the hypoxic pressor response whether given in liposomes or in dissolved form. Superoxide dismutase administered free in solution, or combined with catalase in liposomes, increased the normoxic pulmonary arterial pressure and enhanced vascular reactivity to angiotensin II and hypoxia. Lungs treated with antioxidant enzymes in liposomes had 50% higher lung catalase levels than control lungs (P less than 0.05). These findings demonstrate that exogenous partially reduced O2 species can decrease pulmonary vascular reactivity and suggest that endogenous radicals, superoxide radical in particular, might be important in modulating pulmonary vascular tone.     

Hypoxic pulmonary vasoconstriction in reptiles: a comparative study of four species with different lung structures and pulmonary blood pressures

Low O2 levels in the lungs of birds and mammals cause constriction of the pulmonary vasculature that elevates resistance to pulmonary blood flow and increases pulmonary blood pressure. This hypoxic pulmonary vasoconstriction (HPV) diverts pulmonary blood flow from poorly ventilated and hypoxic areas of the lung to more well-ventilated parts and is considered important for the local matching of ventilation to blood perfusion. In the present study, the effects of acute hypoxia on pulmonary and systemic blood flows and pressures were measured in four species of anesthetized reptiles with diverse lung structures and heart morphologies: varanid lizards (Varanus exanthematicus), caimans (Caiman latirostris), rattlesnakes (Crotalus durissus), and tegu lizards (Tupinambis merianae). As previously shown in turtles, hypoxia causes a reversible constriction of the pulmonary vasculature in varanids and caimans, decreasing pulmonary vascular conductance by 37 and 31%, respectively. These three species possess complex multicameral lungs, and it is likely that HPV would aid to secure ventilation-perfusion homogeneity. There was no HPV in rattlesnakes, which have structurally simple lungs where local ventilation-perfusion inhomogeneities are less likely to occur. However, tegu lizards, which also have simple unicameral lungs, did exhibit HPV, decreasing pulmonary vascular conductance by 32%, albeit at a lower threshold than varanids and caimans (6.2 kPa oxygen in inspired air vs. 8.2 and 13.9 kPa, respectively). Although these observations suggest that HPV is more pronounced in species with complex lungs and functionally divided hearts, it is also clear that other components are involved.     

Contribution of oxygen radicals to altered NO-dependent pulmonary vasodilation in acute and chronic hypoxia

Chronic hypoxia (CH) increases pulmonary arterial endothelial nitric oxide (NO) synthase (NOS) expression and augments endothelium-derived nitric oxide (EDNO)-dependent vasodilation, whereas vasodilatory responses to exogenous NO are attenuated in CH rat lungs. We hypothesized that reactive oxygen species (ROS) inhibit NO-dependent pulmonary vasodilation following CH. To test this hypothesis, we examined responses to the EDNO-dependent vasodilator endothelin-1 (ET-1) and the NO donor S-nitroso-N-acetyl penicillamine (SNAP) in isolated lungs from control and CH rats in the presence or absence of ROS scavengers under normoxic or hypoxic ventilation. NOS was inhibited in lungs used for SNAP experiments to eliminate influences of endogenously produced NO. Additionally, dichlorofluorescein (DCF) fluorescence was measured as an index of ROS levels in isolated pressurized small pulmonary arteries from each group. We found that acute hypoxia increased DCF fluorescence and attenuated vasodilatory responses to ET-1 in lungs from control rats. The addition of ROS scavengers augmented ET-1-induced vasodilation in lungs from both groups during hypoxic ventilation. In contrast, upon NOS inhibition, DCF fluorescence was elevated and SNAP-induced vasodilation diminished in arteries from CH rats during normoxia, whereas acute hypoxia decreased DCF fluorescence, which correlated with augmented reactivity to SNAP in both groups. ROS scavengers enhanced SNAP-induced vasodilation in normoxia-ventilated lungs from CH rats similar to effects of hypoxic ventilation. We conclude that inhibition of NOS during normoxia leads to greater ROS generation in lungs from both control and CH rats. Furthermore, NOS inhibition reveals an effect of acute hypoxia to diminish ROS levels and augment NO-mediated pulmonary vasodilation.     

Stiffened erythrocytes augment the pulmonary hemodynamic response to hypoxia

Isolated rat lungs were perfused with suspensions containing normal and stiffened erythrocytes (RBCs) during normoxic and hypoxic ventilation to assess the effect of reduced RBC deformability on the hypoxic pressor response. RBC suspensions were prepared with cells previously incubated in isotonic phosphate-buffered saline with or without 0.0125% glutaraldehyde. The washed RBCs were resuspended in isotonic bicarbonate-buffered saline (with 4% albumin) to hematocrits of approximately 35%. The lungs were perfused with control and experimental cell suspensions in succession while pulmonary arterial pressure was measured during normoxic (21% O2) and hypoxic (3% O2) ventilation. On the attainment of a peak hypoxic pressor response, flow rate was changed so that pressure-flow curves could be constructed for each suspension. RBC deformability was quantified by a filtration technique using 4.7-microns-pore filters. Glutaraldehyde treatment produced a 10% decrease in RBC deformability (P less than 0.05). Over the range of flow rates, Ppa was increased by 15-17% (P less than 0.05) and 26-31% (P less than 0.05) during normoxic and hypoxic ventilation, respectively, when stiffened cells were suspended in the perfusate. The magnitude of the hypoxic pressor response was 50-54% greater with stiffened cells over the three flow rates. In a separate set of experiments, normoxic and hypoxic arterial blood samples from conscious unrestrained rats were used to investigate the effects of acute hypoxia on RBC deformability. Deformability was measured with the same filtration technique. There was no difference in the deformability of hypoxic compared with normoxic RBCs. We conclude that the presence of stiffened RBCs enhances the hemodynamic response to hypoxia but acute hypoxia does not affect RBC deformability.     

Prolonged pulmonary hypertension caused by platelet-activating factor and leukotriene C4 in the rat lung.

Platelet-activating factor (PAF) and leukotrienes (LTs) are potent pulmonary hypertensive and inflammatory mediators produced by the lung. Previously we showed that a rapid injection of PAF into the pulmonary artery of an isolated rat lung produced an extended elevation in mean pulmonary arterial pressure (PAP). The objective of the present study was to determine whether the extended pressor response induced by PAF was caused by prolonged activation of the 5-lipoxygenase pathway or slow clearance of LTs from the lung parenchyma. Rat lungs were perfused with a nonrecirculating physiological salt solution that contained indomethacin and albumin. Five minutes after a rapid injection of PAF into the pulmonary artery catheter, the following elevations (mean % above baseline) were observed: PAP (83%), LTB4 (3,260%), LTC4 (1,490%), LTD4 (970%), and LTE4 (1,500%). At 20 min these levels declined but were still significantly elevated above baseline. The 5-lipoxygenase inhibitor diethylcarbamazine (DEC), administered before the PAF injection, inhibited the elevations of PAP and all LTs. DEC administration that began 5 min after PAF reduced PAP and only LTC4 levels at 20 min in comparison to lungs with no DEC. The 5-lipoxygenase-activating protein inhibitor MK886, administered orally 2-6 h before perfusion, also inhibited the pressor response to PAF as well as LT production, as did DEC. We conclude that 1) the extended pulmonary hypertension induced by PAF was caused mainly by prolonged activation of 5-lipoxygenase with LTC4 production, 2) the relative overall lung clearance of LTB4, LTD4, and LTE4 was slower than that of LTC4, and 3) LTB4, LTD4, and LTE4 had no appreciable pressor effect.     

Contributions of nitric oxide synthase isozymes to exhaled nitric oxide and hypoxic pulmonary vasoconstriction in rabbit lungs

We investigated the source(s) for exhaled nitric oxide (NO) in isolated, perfused rabbits lungs by using isozyme-specific nitric oxide synthase (NOS) inhibitors and antibodies. Each inhibitor was studied under normoxia and hypoxia. Only nitro-L-arginine methyl ester (L-NAME, a nonselective NOS inhibitor) reduced exhaled NO and increased hypoxic pulmonary vasoconstriction (HPV), in contrast to 1400W, an inhibitor of inducible NOS (iNOS), and 7-nitroindazole, an inhibitor of neuronal NOS (nNOS). Acetylcholine-mediated stimulation of vascular endothelial NOS (eNOS) increased exhaled NO and could only be inhibited by L-NAME. Selective inhibition of airway and alveolar epithelial NO production by nebulized L-NAME decreased exhaled NO and increased hypoxic pulmonary artery pressure. Immunohistochemistry demonstrated extensive staining for eNOS in the epithelia, vasculature, and lymphatic tissue. There was no staining for iNOS but moderate staining for nNOS in the ciliated cells of the epithelia, lymphoid tissue, and cartilage cells. Our findings show virtually all exhaled NO in the rabbit lung is produced by eNOS, which is present throughout the airways, alveoli, and vessels. Both vascular and epithelial-derived NO modulate HPV.     

Differences in acute hypoxic pulmonary vasoresponsiveness between rat strains: role of endothelium.

Intact Madison (M) rats have greater pulmonary pressor responses to acute hypoxia than Hilltop (H) rats. We tested the hypothesis that the difference in pressor response is intrinsic to pulmonary arteries and that endothelium contributes to the difference. Pulmonary arteries precontracted with phenylephrine (10(-7) M) from M rats had greater constrictor responses [hypoxic pulmonary vasoconstriction (HPV)] to acute hypoxia (0% O(2)) than those from H rats: 473 +/- 30 vs. 394 +/- 29 mg (P < 0.05). Removal of the endothelium or inhibition of nitric oxide (NO) synthase by N(omega)-nitro-L-arginine (L-NA, 10(-3) M) significantly blunted HPV in both strains. Inhibition of cyclooxygenase by meclofenamate (10(-5) M) or blockade of endothelin type A and B receptors by BQ-610 (10(-5) M) + BQ-788 (10(-5) M), respectively, had no effect on HPV. Constrictor responses to phenylephrine, endothelin-1, and prostaglandin F(2alpha) were similar in pulmonary arteries from both strains. The relaxation response to ACh, an NO synthase stimulator, was significantly greater in M than in H rats (80 +/- 3 vs. 62 +/- 4%, P < 0.01), but there was no difference in response to sodium nitroprusside, an NO donor. L-NA potentiated phenylephrine-induced contraction to a greater extent in pulmonary arteries from M than from H rats. These findings indicate that at least part of the strain-related difference in acute HPV is attributable to differences in endothelial function, possibly related to differences in NO production.     

Comparison of the hemodynamic effects of nitric oxide and endothelium-dependent vasodilators in intact lungs

The effects of endothelium-dependent vasodilation on pulmonary vascular hemodynamics were evaluated in a variety of in vivo and in vitro models to determine 1) the comparability of the hemodynamic effects of acetylcholine (ACh), bradykinin (BK), nitric oxide (NO), and 8-bromo-guanosine 3',5'-cyclic monophosphate (cGMP), 2) whether methylene blue is a useful inhibitor of endothelium-dependent relaxing factor (EDRF) activity in vivo, and 3) the effect of monocrotaline-induced pulmonary hypertension on the responsiveness of the pulmonary vasculature to ACh. In isolated rat lungs, which were preconstricted with hypoxia, ACh, BK, NO, and 8-bromo-cGMP caused pulmonary vasodilation, which was not inhibited by maximum tolerable doses of methylene blue. Methylene blue did not inhibit EDRF activity in any model, despite causing increased pulmonary vascular tone and responsiveness to various constrictor agents. There were significant differences in the hemodynamic characteristics of ACh, BK, and NO. In the isolated lung, BK and NO caused transient decreases of hypoxic vasoconstriction, whereas ACh caused more prolonged vasodilation. Pretreatment of these lungs with NO did not significantly inhibit ACh-induced vasodilation but caused BK to produce vasoconstriction. Tachyphylaxis, which was agonist specific, developed with repeated administration of ACh or BK but not NO. Tachyphylaxis probably resulted from inhibition of the endothelium-dependent vasodilation pathway proximal to NO synthesis, because it could be overcome by exogenous NO. Pretreatment with 8-bromo-cGMP decreased hypoxic pulmonary vasoconstriction and, even when the hypoxic pressor response had largely recovered, subsequent doses of ACh and NO failed to cause vasodilation, although BK produced vasoconstriction. These findings are compatible with the existence of feedback inhibition of the endothelium-dependent relaxation by elevation of cGMP levels. Responsiveness to ACh was retained in lungs with severe monocrotaline-induced pulmonary hypertension. Many of these findings would not have been predicted based on in vitro studies and illustrate the importance for expanding studies of EDRF to in vivo and ex vivo models.     

PAF antagonists inhibit pulmonary vascular remodeling induced by hypobaric hypoxia in rats.

Chronic hypoxia causes pulmonary hypertension and pulmonary vascular remodeling in rats. Because platelet-activating factor (PAF) levels increase in lung lavage fluid and in plasma from chronically hypoxic rats, we examined the effect of two specific, structurally unrelated PAF antagonists, WEB 2170 and BN 50739, on hypoxia-induced pulmonary vascular remodeling. Treatment with either agent reduced hypoxia-induced pulmonary hypertension and right ventricular hypertrophy at 3 wk of hypoxic exposure (simulated altitude 5,100 m) but did not affect cobalt (CoCl2)-induced pulmonary hypertension. The PAF antagonists had no effect on the hematocrit of normoxic or chronically hypoxic rats or CoCl2-treated rats. Hypoxia-induced pulmonary hypertension was associated with an increase in the vessel wall thickness of the muscular arteries and reduction in the number of peripheral arterioles. In WEB 2170-treated rats, these changes were significantly less severe than those observed in untreated chronically hypoxic rats. PAF receptor blockade had no acute hemodynamic effects; i.e., it did not affect pulmonary arterial pressure or cardiac output nor did it affect the magnitude of acute hypoxic pulmonary vasoconstriction in awake normoxic or chronically hypoxic rats. Isolated lungs from chronically hypoxic rats showed a pressor response to the chemotactic tripeptide N-formyl-Met-Leu-Phe (fMLP) and an increase in the number of leukocytes lavaged from the pulmonary circulation. In vivo treatment with WEB 2170 significantly reduced the fMLP-induced pressor response compared with that observed in isolated lungs from untreated chronically hypoxic rats. These results suggest that PAF contributes to the development of chronic pulmonary hypertension induced by chronic hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pulmonary vascular iNOS induction participates in the onset of chronic hypoxic pulmonary hypertension

Pathogenesis of hypoxic pulmonary hypertension is initiated by oxidative injury to the pulmonary vascular wall. Because nitric oxide (NO) can contribute to oxidative stress and because the inducible isoform of NO synthase (iNOS) is often upregulated in association with tissue injury, we hypothesized that iNOS-derived NO participates in the pulmonary vascular wall injury at the onset of hypoxic pulmonary hypertension. An effective and selective dose of an iNOS inhibitor, L-N6-(1-iminoethyl)lysine (L-NIL), for chronic peroral treatment was first determined (8 mg/l in drinking water) by measuring exhaled NO concentration and systemic arterial pressure after LPS injection under ketamine+xylazine anesthesia. A separate batch of rats was then exposed to hypoxia (10% O2) and given L-NIL or a nonselective inhibitor of all NO synthases, N(G)-nitro-L-arginine methyl ester (L-NAME, 500 mg/l), in drinking water. Both inhibitors, applied just before and during 1-wk hypoxia, equally reduced pulmonary arterial pressure (PAP) measured under ketamine+xylazine anesthesia. If hypoxia continued for 2 more wk after L-NIL treatment was discontinued, PAP was still lower than in untreated hypoxic controls. Immunostaining of lung vessels showed negligible iNOS presence in control rats, striking iNOS expression after 4 days of hypoxia, and return of iNOS immunostaining toward normally low levels after 20 days of hypoxia. Lung NO production, measured as NO concentration in exhaled air, was markedly elevated as early as on the first day of hypoxia. We conclude that transient iNOS induction in the pulmonary vascular wall at the beginning of chronic hypoxia participates in the pathogenesis of pulmonary hypertension.     

Regional hypoxic pulmonary vasoconstriction in prone pigs.

Hypoxic pulmonary vasoconstriction (HPV) is known to affect regional pulmonary blood flow distribution. It is unknown whether lungs with well-matched ventilation (V)/perfusion (Q) have regional differences in the HPV response. Five prone pigs were anesthetized and mechanically ventilated (positive end-expiratory pressure = 2 cmH2O). Two hypoxic preconditions [inspired oxygen fraction (FI(O2)) = 0.13] were completed to stabilize the animal's hypoxic response. Regional pulmonary blood Q and V distribution was determined at various FI(O2) (0.21, 0.15, 0.13, 0.11, 0.09) using the fluorescent microsphere technique. Q and V in the lungs were quantified within 2-cm3 lung pieces. Pieces were grouped, or clustered, based on the changes in blood flow when subjected to increasing hypoxia. Unique patterns of Q response to hypoxia were seen within and across animals. The three main patterns (clusters) showed little initial difference in V/Q matching at room air where the mean V/Q range was 0.92-1.06. The clusters were spatially located in cranial, central, and caudal portions of the lung. With decreasing FI(O2), blood flow shifted from the cranial to caudal regions. We determined that pulmonary blood flow changes, caused by HPV, produced distinct response patterns that were seen in similar regions across our prone porcine model.     

Atrial natriuretic peptide in acute hypoxia-induced pulmonary hypertension in rats.

To test the hypothesis that exogenous atrial natriuretic peptide (ANP) prevents the acute pulmonary pressor response to hypoxia, ANP (20-micrograms/kg bolus followed by 1-microgram.kg-1.min-1 infusion) or vehicle was administered intravenously to conscious rats beginning 3 min before exposure to hypoxia or room air for 90 min. Exogenous ANP abolished the acute pulmonary pressor response to hypoxia in association with marked and parallel increases in plasma ANP and guanosine 5'-cyclic monophosphate (cGMP) and with a significant increase in lung cGMP content. To examine whether endogenous ANP modulates the acute pulmonary pressor response to hypoxia, rats were pretreated with a monoclonal antibody (Ab) to ANP and exposed to hypoxia. Mean pulmonary arterial pressure (MPAP) in the Ab-treated rats was not different from control over the first 6 h of hypoxic exposure. Thereafter, the Ab-treated group had significantly higher MPAP than control. Our data suggest that 1) exogenous ANP blocks the pulmonary pressor response to acute hypoxia via stimulation of cGMP accumulation in the pulmonary vasculature, and 2) endogenous ANP may modulate the subacute, but not acute, phase of hypoxic pulmonary hypertension.     

一氧化氮对急、慢性缺氧大鼠血流动力学及缺氧性肺血管收缩反应的影响

观察了吸入０．００４％的一氧化氮（ＮＯ）对急、慢性缺氧大鼠血流动力学、缺氧性肺血管收缩反应（ＨＰＶ）、血气及高铁血红蛋白（ＭｅｔＨｂ）的影响。结果表明：（１）常氧吸入ＮＯ时能明显降低慢性缺氧大鼠肺动脉平均压（Ｐｐａ）和肺血管阻力（ＰＶＲ）,但对正常大鼠的Ｐｐａ和ＰＶＲ无明显影响;（２）慢性缺氧大鼠急性缺氧时ＨＰＶ较正常大鼠弱,吸入ＮＯ不但降低两者的急性缺氧肺动脉高压,且完全逆转两者的ＨＰＶ;（３）吸入ＮＯ对急、慢性缺氧大鼠体循环血流动力学、血气及ＭｅｔＨｂ含量无明显影响。提示吸入ＮＯ能选择性降低急、慢性缺氧性肺动脉高压,且逆转ＨＰＶ。