Accumulation of histamine and cyclic nucleotides in lung tissue during chronic hypobaric hypoxia

Alveolar Hypoxia-Induced Pulmonary Inflammation: From Local Initiation to Secondary Promotion by Activated Systemic Inflammation

Pulmonary hypertension (PH) is a pathological condition with high mortality and morbidity. Hypoxic PH (HPH) is a common form of PH occurring mainly due to lung disease and/or hypoxia. Most causes of HPH are associated with persistent or intermittent alveolar hypoxia, including exposure to high altitude and chronic obstructive respiratory disease. Recent evidence suggests that inflammation is a critical step for HPH initiation and development. A detailed understanding of the initiation and progression of pulmonary inflammation would help in exploring potential clinical treatments for HPH. In this review, the mechanism for alveolar hypoxia-induced local lung inflammation and its progression are discussed as follows: (1) low alveolar PO2 levels activate resident lung cells, mainly the alveolar macrophages, which initiate pulmonary inflammation; (2) systemic inflammation is induced by alveolar hypoxia through alveolar macrophage activation; (3) monocytes are recruited into the pulmonary circulation by alveolar hypoxia-induced macrophage activation, which then contributes to the progression of pulmonary inflammation during the chronic phase of alveolar hypoxia, and (4) alveolar hypoxia-induced systemic inflammation contributes to the development of HPH. We hypothesize that a combination of alveolar hypoxia-induced local lung inflammation and the initiation of systemic inflammation ("second hit") is essential for HPH progression.

Inhaled nitric oxide and nifedipine have similar effects on lung cGMP levels in rats

Inhaled nitric oxide (NO) may downregulate the endogenous NO/cyclic guanosine monophosphate (cGMP) pathway, potentially explaining clinical rebound pulmonary hypertension. We determined if inhaled NO decreases pulmonary cGMP levels, if the possible down-regulation is the same as with nifedipine, and if regulation also occurs with the cyclic adenosine monophosphate (cAMP) pathway. Rats were exposed to 3 wk of normoxia, hypoxia (10% O2), or monocrotaline (MCT; single dose = 60 mg/kg) and treated with either nothing (control), inhaled NO (20 ppm), or nifedipine (10 mg x kg(-1) x day(-1). The lungs were then isolated and perfused with physiologic saline. Perfusate cGMP, prostacyclin, and cAMP levels were measured. Perfusate cGMP was not altered by inhaled NO or nifedipine in normoxic or MCT rats. Although hypoxia significantly increased cGMP by 128%, both inhaled NO and nifedipine equally prevented the hypoxic increase. Inhibition of the NO/cGMP pathway with N(G)-nitro-L-arginine methyl ester (L-NAME) decreased cGMP by 72% and 88% in normoxic and hypoxic lungs. Prostacyclin and cAMP levels were not altered by inhaled NO or nifedipine. L-NAME significantly decreased cGMP levels, whereas inhaled NO had no effect on cGMP in normoxic or MCT lungs, suggesting that inhaled NO does not inhibit the NO/cGMP pathway. Inhaled NO decreased cGMP in hypoxic lungs, however, nifedipine had the same effect, which indicates the decrease is not specific to inhaled NO.

IMPLICATIONS

High pulmonary pressure after discontinuation of inhaled nitric oxide (NO) may be secondary to a decrease in the natural endogenous NO vasodilator. This rat study suggests that inhaled NO either does not alter endogenous NO or that it has similar effects as nifedipine.

Time-dependently enhanced pressor responses in rat lungs after chronic exposure to high altitude

To investigate the de-acclimatization process with respect to pulmonary vasoreactivity after high-altitude exposure, we performed a study of changes in pressor responses over time in salt-perfused lungs from rats that had been exposed to high altitude (HA, n = 41, 20 weeks at actual 2400 m). Comparisons were made with the lungs of rats kept at low altitude (n = 8, kept at 610 m). We studied HA rats from the time just after removal from high altitude to 2 weeks after the exposure. The pressor responses of HA lungs to both 0.5 microgram-angiotensin II and 5 min airway hypoxia (3% O2) just after the exposure did not differ from those of controls, but their responses began to increase within 12 h, and reached a maximum 3 days after exposure ended. The increase was time-dependent. We conclude that pulmonary vasopressor responses are enhanced time-dependently during the very early stages of de-acclimatization after high-altitude exposure.

Effect of cyclic guanosine monophosphate on hypoxic and angiotensin-II-induced pulmonary vasoconstriction

We examined, in isolated blood perfused rat lungs, the effect of the cell permeable 8-bromo derivative of cGMP on pulmonary vasoconstriction induced by either alveolar hypoxia or angiotensin II. 8-Bromo cGMP dose-dependently reduced both hypoxia-(IC50 = 2.2 X 10(-5) M) and angiotensin-II-induced pulmonary vasoconstriction (IC50 = 5.0 X 10(-5) M). This effect of 8-bromo cGMP on pulmonary vasoconstriction was not affected by cyclooxygenase blockade. M & B 22948 (0.1 mM), an inhibitor of cGMP-phosphodiesterase, reduced synergistically with 8-bromo cGMP the hypoxia or angiotensin-II-induced vasoconstriction. The cGMP-phosphodiesterase inhibitor M & B 22948, by itself, selectively reduced hypoxia-induced vasoconstriction, suggesting a modulating effect of endogenous cGMP during hypoxic vasoconstriction.

Effect of chronic in vivo exposure to hypoxia on serotonin uptake by isolated rats lungs

1. Previously, we have shown that exposure to hypoxia stimulates serotonin uptake by cultured bovine pulmonary arterial endothelial cells. 2. In the present study, lungs isolated from rats exposed to 24 h, 48 h or 14 days of hypobaric hypoxia (0.43 or 0.5 atm) manifested no alteration of serotonin uptake in comparison to lungs from normoxic controls. 3. In addition, hypoxic stimulation of serotonin uptake by cultured rat epididymal endothelial cells was much less than that occurring in bovine pulmonary artery endothelial cells. 4. We conclude that chronic hypoxia does not stimulate serotonin uptake by rat lungs exposed in vivo as it does in endothelial cells exposed in vitro.

Glycolytic activity and enhancement of serotonin uptake by endothelial cells exposed to hypoxia/anoxia

Serotonin (5-HT) uptake by bovine pulmonary artery endothelial cells in culture was stimulated several fold by exposure of cells for 24 hours to 0-3% O2. The cells appeared normal morphologically, excluded trypan blue dye, showed normal levels of release of lactate dehydrogenase, and contained normal amounts of adenosine triphosphate (ATP). Incubation of cells with iodoacetate (3-5 microM), an inhibitor of glycolytic metabolism, prevented the stimulation of 5-HT uptake but did not impair cellular proliferation. A similar, but less pronounced, effect was produced by 1 mM deoxyglucose. On the other hand, the mitochondrial inhibitors antimycin A, sodium azide, potassium cyanide, and 2,4-dinitrophenol inhibited cellular replication similar to the effect of anoxia but did not alter the stimulation of 5-HT uptake. 5-HT uptake correlated strongly with lactate production by the cells exposed to anoxia (r = 0.98). We conclude that stimulation of 5-HT uptake by prolonged exposure of the endothelial cell to hypoxia/anoxia causes alteration of a membrane function (serotonin transport) of the endothelial cell. This alteration is associated with enhanced glycolytic activity of the cell during exposure to hypoxia. The mechanism for translation of enhanced glycolysis to later stimulation of 5-HT uptake remains to be determined. Endothelial cellular replication is dependent on aerobic metabolism.

Pulmonary vascular actions of the antihistamine oxatomide during hypoxia

The role of histamine in the cardiovascular responses to hypoxia has not been resolved. Oxatomide, an orally active anti-allergic drug, has been reported to inhibit mast cell discharge and to antagonize any histamine which is released. This drug was administered to six dogs to determine the effect of inhibiting histamine release on the pulmonary vascular and cardiac responses to hypoxia. Oxatomide potentiated hypoxic pulmonary vasoconstriction, but reduced the cardiac output and stroke volume responses to hypoxia. These findings suggest that histamine is released during hypoxia and acts to oppose hypoxic vasoconstriction. In addition, histamine released from peripheral tissues may play an important role in mediating the increased cardiac output observed during acute hypoxia. By utilizing infusions of histamine, oxatomide was also found to be a histamine H1-receptor antagonist.

Adrenalectomy prevents hypoxic lung histamine accumulation

The lung histamine concentration of rats exposed to chronic hypobaric hypoxia was higher than in normoxic sea-level control rats. This hypoxia-induced lung histamine increase was absent in adrenalectomized chronically hypoxic rats. Adrenalectomy, per se, did not influence lung histamine content. The results suggest that the lung histamine content of chronically hypoxic rats is under hormonal control. This control mechanism is not evident under normoxic conditions.