Pulmonary ultrastructural changes in hypoxic rats

Pathological features of the lung in fatal high altitude pulmonary edema occurring at moderate altitude in Japan

In order to characterize the pathological features of high altitude pulmonary edema (HAPE) occurring at moderate altitude in Japan, we performed routine hematoxylin and eosin (HE) staining in lung materials from HAPE autopsied cases. We also undertook advanced immunohistochemical staining for observation of type II pneumocytes, pulmonary surfactant (PS), and mast cells in the lung of HAPE cases to examine the biological changes within the lung parenchyma. The pathological findings of HAPE were characterized by alveolar edema, congestion of pulmonary vessels, alveolar hyaline membranes, alveolar hemorrhage, and multithrombi and fibrin clots, but maintained alveolar structure. The immunostaining results showed that the type II pneumocytes were cellular fusion, deformity, and exfoliation from the walls of alveoli; the PS not only lined the alveolar surface, but was also patchily distributed within alveoli; and the number of mast cells were increased (9.0 +/- 0.9 cells/mm(2)) compared to that in controls (1.1 +/- 0.4 cells/mm(2)) (p < 0.01). We conclude that the pathological features of HAPE at moderate altitude in Japan are similar to others reported worldwide, and that the type II pneumocytes, PS, and mast cells may contribute to some extent to pathophysiological parts in the development and progression of HAPE.

Inhaled nitric oxide improves survival in the rat model of high-altitude pulmonary edema

OBJECTIVES

High-altitude pulmonary edema (HAPE) afflicts certain individuals after a rapid gain in elevation. Those susceptible demonstrate an exaggerated hypoxic pulmonary vasoconstrictive response. This causes pulmonary hypertension, which may disrupt vascular integrity. This experiment was designed to test whether inhaled nitric oxide would affect development of HAPE in a rat model.

METHODS

Subjects were exposed in a hypobaric chamber to a simulated altitude of 6200 m (barometric pressure = 380 mm Hg, fraction of inspired oxygen = 0.19) for 24 hours. Control animals (n = 48) spontaneously breathed a mixture of 90% room air and 10% nitrogen, whereas the nitric oxide group (n = 48) received a similar mixture containing 83 ppm nitric oxide. Postmortem examination of lungs was performed for light microscopy, total hemoglobin, and gravimetric estimates of water content.

RESULTS

Mortality was 39.5% (n = 19) in control animals and 6.2% (n = 3) in the nitric oxide group (P < .001). Both groups significantly increased their lung weight-body weight ratio. Percentage of lung water was similar in both groups despite increases in lung weight, which is consistent with the protein-rich edema characteristic of HAPE. Light microscopic examination of survivors' lungs in both groups revealed scattered alveolar hemorrhage. No significant cellular inflammatory response was present.

CONCLUSIONS

We conclude that inhaled nitric oxide improves survival in the rat model of HAPE.

Impairment of transalveolar fluid transport and lung Na(+)-K(+)-ATPase function by hypoxia in rats

We examined whether hypoxic exposure in vivo would influence transalveolar fluid transport in rats. We found a significant decrease in alveolar fluid clearance of the rats exposed to 10% oxygen for 48 h. Terbutaline did not stimulate alveolar fluid clearance, and alveolar fluid cAMP levels were lower than those determined in normoxia experiment. Hypoxia did not influence the alveolar fluid lactate dehydrogenase levels, Evans blue dye fluid-to-serum concentration ratio, or lung wet-to-dry weight ratio, indicating no significant change in the permeability of alveolar-capillary barrier. Histological examination showed no significant fluid accumulation into the interstitium and the alveolar space. Hypoxia did not reduce lung ATP content; however, we found significant decrease in Na(+)-K(+)-ATPase hydrolytic activity in lung tissue preparations and isolated alveolar type II cells. Our data indicate that hypoxic exposure in vivo impairs transalveolar fluid transport, and this impairment is related to the decrease in alveolar epithelial Na(+)-K(+)-ATPase hydrolytic activity but is not secondary to the alteration of cellular energy source.

Hypoxia-specific upregulation of calpain activity and gene expression in pulmonary artery endothelial cells

The effects of exposure to hypoxia on the catalytic activity and mRNA expression of calpain, a calcium-regulated neutral cysteine protease, were examined in porcine pulmonary artery endothelial cells (PAECs). Specificity of the response to hypoxia was determined by comparing the effects of hypoxic exposure with exposure to oxidants such as nitrogen dioxide (NO2) and nitric oxide (NO), as well as to the sulfhydryl reactive chemical acrolein. Exposure of cells to hypoxia (0% O2) for 1 and 12 h significantly increased catalytic activity (P < 0.01 for both 1 and 12 h vs. control cells), as well as mRNA expression (P < 0.01 for 1 h and P < 0.05 for 12 h vs. control cells) of calpain. With more prolonged exposure to 24 h of hypoxia, calpain activity remained significantly elevated, whereas calpain mRNA expression returned to the control level. Calpain activities in cells exposed to NO2 [5 parts/million (ppm)] or NO (7.5 ppm) for 1 h or to acrolein (5 microM) for 1 and 24 h were unchanged. However, calpain activities in cells exposed to NO2 or NO for 24 h were significantly (P < 0.05) reduced compared with control cells. The hypoxia-induced increases in calpain mRNA content were prevented by the transcriptional inhibitor actinomycin D and by calpain inhibitor I. In addition, hypoxia increased the degradation of nuclear factor-kappaB (NF-kappaB) inhibitor IkappaB and enhanced the translocation of the p50 subunit of NF-kappaB to the nuclear membrane. Pretreatment with the calpain-specific inhibitor E-64d prevented hypoxia-induced mRNA expression and degradation of IkappaBalpha, as well as translocation of p50 subunit to the nuclear membrane. These results demonstrate for the first time that hypoxia upregulates calpain activity and mRNA expression in PAECs and that the upregulation is specific to hypoxia. Upregulation appears to involve activation of the transcription factor NF-kappaB.

Hypoxia downregulates expression and activity of epithelial sodium channels in rat alveolar epithelial cells

Decrease in alveolar oxygen tension may induce acute lung injury with pulmonary edema. We investigated whether, in alveolar epithelial cells, expression and activity of epithelial sodium (Na) channels and Na,K-adenosine triphosphatase, the major components of transepithelial Na transport, were regulated by hypoxia. Exposure of cultured rat alveolar cells to 3% and 0% O2 for 18 h reduced Na channel activity estimated by amiloride-sensitive 22Na influx by 32% and 67%, respectively, whereas 5% O2 was without effect. The decrease in Na channel activity induced by 0% O2 was time-dependent, significant at 3 h of exposure and maximal at 12 and 18 h. It was associated with a time-dependent decline in the amount of mRNAs encoding the alpha-, beta-, and gamma-subunits of the rat epithelial Na channel (rENaC) and with a 42% decrease in alpha-rENaC protein synthesis as evaluated by immunoprecipitation after 18 h of exposure. The 0% O2 hypoxia also caused a time-dependent decrease in (1) ouabain-sensitive 86Rubidium influx in intact cells, (2) the maximal velocity of Na,K-ATPase on crude homogenates, and (3) alpha1- and beta1-Na,K-ATPase mRNA levels. Levels of rENaC and alpha1-Na,K-ATPase mRNA returned to control values within 48 h of reoxygenation, and this was associated with complete functional recovery. We conclude that hypoxia induced a downregulation of expression and activity of epithelial Na channels and Na,K-ATPase in alveolar cells. Subsequent decrease in Na reabsorption by alveolar epithelium could participate in the maintenance of hypoxia-induced alveolar edema.

Exaggerated pulmonary hypertension with monocrotaline in rats susceptible to chronic mountain sickness

Hilltop (H) strain Sprague-Dawley rats are more susceptible to chronic mountain sickness than are the Madison (M) strain rats. It is unclear what role pulmonary vascular remodeling, polycythemia, and hypoxia-induced vasoconstriction play in mediating the more severe pulmonary hypertension that develops in the H rats during chronic hypoxia. It is also unclear whether the increased sensitivity of the H rats to chronic mountain sickness is specific for a hypoxia effect or, instead, reflects a general propensity toward the development of pulmonary hypertension. Monocrotaline (MCT) causes pulmonary vascular remodeling and pulmonary hypertension. We hypothesized that the difference in the pulmonary vascular response to chronic hypoxia between H and M rats reflects an increased sensitivity of the H rats to any pulmonary hypertensive stimuli. Consequently, we expected the two strains to also differ in their susceptibility to MCT-induced pulmonary hypertension. Pulmonary arterial pressures in conscious H and M rats were measured 3 wk after a single dose of MCT, exposure to a simulated high altitude of 18,000 ft (barometric pressure = 380 mmHg), and administration of a single dose of saline as a placebo. The H rats had significantly higher pulmonary arterial pressures and right ventricular weights after MCT and chronic hypoxia than did the M rats. The H rats also had more pulmonary vascular remodeling, i.e., greater wall thickness as a percentage of vessel diameter, after MCT and chronic hypoxia than did the M rats. The H rats had significantly lower arterial PO2 than did the M rats after MCT, but the degree of hypoxemia was mild [arterial PO2 of 72.5 +/- 0.8 (SE) Torr for H rats vs. 77.4 +/- 0.8 Torr for M rats after MCT]. The H rats had lower arterial PCO2 and larger minute ventilation values than did the M rats after MCT. These ventilatory differences suggest that MCT caused more severe pulmonary vascular damage in the H rats than in the M rats. These data support the hypothesis that the H rats have a general propensity to develop pulmonary hypertension and suggest that differences in pulmonary vascular remodeling account for the increased susceptibility of H rats, compared with M rats, to both MCT and chronic hypoxia-induced pulmonary hypertension.

Acute hypoxia increases intracellular L-arginine content in cultured porcine pulmonary artery endothelial cells

Exposure to hypoxia (0% O2) for 4-24 h resulted in increased intracellular L-arginine content and increased activity of calpain, a calcium-dependent neutral cysteine protease, in pulmonary artery endothelial cells. Calpain-inhibitor I abolished the increased L-arginine content in hypoxic cells. When endothelial cell proteins were labeled with [3H]-L-arginine and the cells exposed to hypoxia, we observed an increase in free [3H]-L-arginine and a decrease in [3H]-L-arginine-labeled proteins. Once again, calpain-inhibitor I prevented the increases in free [3H]-L-arginine and the decreases in [3H]-L-arginine-labeled proteins in hypoxic cells. Hypoxia also inhibited the synthesis of L-arginine-containing proteins. Thus, the increase in intracellular L-arginine content in hypoxic pulmonary artery endothelial cells is caused by an increase in proteolysis secondary to calpain and a decrease in protein synthesis. These results indicate that hypoxia can modulate the availability of free intracellular L-arginine, the exclusive precursor of nitric oxide (NO) and the primary substrate of NO synthase, by affecting the synthesis and degradation of cellular proteins.

Hypoxia-induced increases in pulmonary transvascular protein escape in rats. Modulation by glucocorticoids

Pulmonary edema after ascent to altitude is well recognized but its pathogenesis is poorly understood. To determine whether altitude exposure increases lung vascular permeability, we exposed rats to a simulated altitude of approximately 14,500 feet (barometric pressure [Pb] 450 Torr) and measured the pulmonary transvascular escape of radiolabeled 125I-albumin corrected for lung blood content with 51Cr-tagged red blood cells (protein leak index = PLI). Exposures of 24 and 48 h caused significant increases in PLI (2.30 +/- 0.08 and 2.40 +/- 0.06) compared with normoxic controls (1.76 +/- 0.06), but brief hypoxic exposures of 1-13 h produced no increase in PLI, despite comparable increases in pulmonary artery pressure. There were associated increases in gravimetric estimates of lung water in the altitude-exposed groups and perivascular edema cuffs on histologic examination. Normobaric hypoxia (48 h; fractional inspired oxygen concentration [FIO2] = 15%) also increased lung transvascular protein escape and lung water. Dexamethasone has been used to prevent and treat altitude-induced illnesses, but its mechanism of action is unclear. Dexamethasone (0.5 or 0.05 mg/kg per 12 h) started 12 h before and continued during 48 h of altitude exposure prevented the hypoxia-induced increases in transvascular protein escape and lung water. Hemodynamic measurements (mean pulmonary artery pressure and cardiac output) were unaffected by dexamethasone, suggesting that its effect was not due to a reduction in pulmonary artery pressure or flow. The role of endogenous glucocorticoid activity was assessed in adrenalectomized rats that showed augmented hypoxia-induced increases in transvascular protein escape, which were prevented by exogenous glucocorticoid replacement. In summary, subacute hypoxic exposures increased pulmonary transvascular protein escape and lung water in rats. Dexamethasone prevented these changes independent of reductions of mean pulmonary artery pressure or flow, whereas adrenalectomy increased pulmonary vascular permeability and edema at altitude. Increases in vascular permeability in hypoxia could contribute to the development of high-altitude pulmonary edema and endogenous glucocorticoids may have an important influence on pulmonary vascular permeability in hypoxia.

The adrenergic innervation of pulmonary vasculature in the normal and pulmonary hypertensive rat

It would appear that susceptibility to chronic proliferative pulmonary hypertension in response to chronic alveolar hypoxia is most severe in species in which adrenergic innervation of pulmonary arteries is reduced or lacking. Intrapulmonary arteries of the rat have been reported to lack adrenergic innervation by some workers but not others. Since the rat develops severe proliferative pulmonary hypertension in response to prolonged alveolar hypoxia, the different divisions of the lung vasculature of Sprague-Dawley rats were thoroughly examined to determine the presence or absence of an adrenergic innervation. The degree of innervation in normal rats was compared with that of rats developing pulmonary hypertension. Both in normal and experimental pulmonary hypertensive rats the pulmonary arteries, all their branches and the small pulmonary veins with a smooth muscle media were found to be devoid of adrenergic innervation. In contrast, the cardiac-like muscle in the media of large pulmonary veins, the bronchial arteries and the vasa vasorum of larger vessels were richly innervated by adrenergic nerves. Thus the increase in medial smooth muscle which occurs in pulmonary arteries during chronic alveolar hypoxia is independent of a pre-existing adrenergic innervation or of such an innervation newly derived from that of adjacent vessels or structures. This is in contrast to systemic vessels where it has been suggested that increased adrenergic activity and density of innervation may augment hypertrophy of the media in hypertensive animals. Adrenergic nerves are suggested to have a protective action on pulmonary vessels.

Experimental post-traumatic lung insufficiency in dogs: ultrastructure of lung lesions

The ultrastructure of developing lung lesions in two groups of dogs exposed to a combination of haemorrhagic hypotension and liver trauma was studied with particular attention to changes at the alveolar level and lung micro-vessels. Lung samples were obtained every four hours and at collapse in one group and 12 hrs after initiation of the trauma in the other. An interstitial oedema was recognized in biopsies obtained 4 hrs after initiation of the trauma, and before marked lesions were observed at the ultrastructural level in endothelial cells. Endothelial damage was, however, evident in biopsies obtained at 8 hrs and at collapse. Aggregates of degranulated and degenerated leucocytes and platelets were occasionally found to obstruct respiratory capillaries together with erythrocytes, some of which seemed to be haemolysing. A considerable amount of protein-rich oedema, cellular debris and fibrinoid material was found in alveolar lumina at collapse. The present experiments indicate that increased vascular permeability in lung micro-vessels is of importance for the development of the characteristic lesions seen in shock lung. Possible pathogenetic mechanisms, initiating the lung lesions, are discussed with special emphasis on the significance of kinin activation and the presence of polymorphonuclear leucocytes and microthrombi.

Lung morphometry in guinea pigs acclimated to hypoxia during growth

The effect of chronic hypoxic exposure on lung development has been assessed in growing guinea pigs (Cavia porcellus). Weanling males of initial W = 229 g were acclimated to a PO2 of 80 Torr for 2-14 weeks before sacrifice (range of W = 244-965 g). Growth was the same in hypoxic animals as in controls maintained at a PO2 of 133 Torr (range of W in controls = 89-1274 g). Lungs were fixed by tracheal instillation of glutaraldehyde and examined morphometrically with the electron microscope. Within 3 weeks of exposure, lung volume (VL) and alveolar surface area (Sa) were significantly increased by 32% and 27% respectively in the hypoxia acclimated animals compared to controls of similar W. However, these differences were progressively reduced with increasing time of exposure, and mean values of VL and Sa were not different between groups when W greater than 900 g. Chronic hypoxia accelerated lung development towards normal adult dimensions to a degree remarkably similar to that reported in cold acclimated guinea pigs. These findings are compatable with the theory of adaptive lung growth mediated by increased pulmonary blood flow, and suggest anatomical limitations to such growth related to an animal's age.