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.     

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.     

Decreased pulmonary vascular responsiveness in rats raised on an essential fatty acid deficient diet

Leukotriene C₄ is produced during hypoxic pulmonary vasoconstriction and leukotriene inhibitors preferentially inhibit the hypoxic pressor response in rats. If lipoxygenase products are important in hypoxic vasoconstriction, then an animal deficient in arachidonic acid should have a blunted hypoxic pressor response. We investigated if vascular responsiveness was decreased in vascular rings and isolated perfused lungs from rats raised on an essential fatty acid deficient diet (EFAD) compared to rats raised on a normal diet. Rats raised on the EFAD diet had decreased esterified plasma arachidonic acid and increased 5-, 8-, 11- eicosatrieonic acid compared to rats raised on the normal diet (control). Compared to the time matched responses in control isolated perfused lungs the pressor responses to angiotensin II and alveolar hypoxia were blunted in lungs from the arachidonate deficient rats. This decreased pulmonary vascular responsiveness was not affected by the addition of indomethacin or arachidonic acid to the lung perfusate. Similarly, the pulmonary artery rings from arichidonate deficient rats demonstrated decreased reactivity to norepinephrine compared to rings from control rats. In contrast, the tension increases to norepinephrine were greater in aortic rings from the arachidonate deficient rats compared to control. Stimulated lung tissue from the arachidonate deficient animals produced less slow reacting substance and platelet activating factor like material but the same amount of 6-keto-PGF_1α and TXB₂ compared to control lungs. Thus there is an associated between altered vascular responsiveness and impairment of stimulated production of slow reacting substance and platelet activating factor like materiali rat raised on an EFAD diet.     

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)     

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.     

Does leukotriene C4 mediate hypoxic vasoconstriction in isolated ferret lungs?

To evaluate leukotriene (LT) C4 as a mediator of hypoxic pulmonary vasoconstriction, we examined the effects of FPL55712, a putative LT antagonist, and indomethacin, a cyclooxygenase inhibitor, on vasopressor responses to LTC4 and hypoxia (inspired O2 tension = 25 Torr) in isolated ferret lungs perfused with a constant flow (50 ml.kg-1.min-1). Pulmonary arterial injections of LTC4 caused dose-related increases in pulmonary arterial pressure during perfusion with physiological salt solution containing Ficoll (4 g/dl). FPL55712 caused concentration-related inhibition of the pressor response to LTC4 (0.6 micrograms). Although 10 micrograms/ml FPL55712 inhibited the LTC4 pressor response by 61%, it did not alter the response to hypoxia. At 100 microgram/ml, FPL55712 inhibited the responses to LTC4 and hypoxia by 73 and 71%, respectively, but also attenuated the vasoconstrictor responses to prostaglandin F2 alpha (78% at 8 micrograms), phenylephrine (68% at 100 micrograms), and KCl (51% at 40 mM). At 0.5 microgram/ml, indomethacin significantly attenuated the pressor response to arachidonic acid but did not alter responses to LTC4 or hypoxia. These results suggest that in isolated ferret lungs 1) the vasoconstrictor response to LTC4 did not depend on release of cyclooxygenase products and 2) LTC4 did not mediate hypoxic vasoconstriction.     

Developmental changes in vascular responses to histamine in normoxic and hypoxic lamb lungs

This study of newborn (3-10 day old) and juvenile (6-8 mo old) in situ isolated lamb lungs was undertaken to determine whether 1) histamine receptor blockade accentuates hypoxic pulmonary vasoconstriction more in newborns than in juveniles, 2) histamine infusion causes a decrease in both normoxic pulmonary vascular resistance and hypoxic pulmonary vasoconstriction in newborns, and 3) the H1-mediated dilator response to infused histamine in newborns is due to enhanced dilator prostaglandin release. Pulmonary arterial pressure (Ppa) was determined at baseline and in response to histamine (infusion rates of 0.1-10.0 micrograms.kg-1 min-1) in control, H1-blocked, H2-blocked, combined H1- and H2-blocked, and cyclooxygenase-inhibited H2-blocked lungs under "normoxic" (inspired O2 fraction 0.28) and hypoxic (inspired O2 fraction 0.04) conditions. In newborns, H1-receptor blockade markedly accentuated baseline hypoxic Ppa, and H2-receptor blockade caused an increase in baseline normoxic Ppa. In juveniles, neither H1 nor H2 blockade altered baseline normoxic or hypoxic Ppa. Histamine infusion caused both H1- and H2-mediated decreases in Ppa in normoxic and hypoxic newborn lungs. In juvenile lungs, histamine infusion also caused H2-mediated decreases in Ppa during both normoxia and hypoxia. During normoxia, histamine infusion caused an H1-mediated increase in normoxic Ppa in juveniles as previously seen in mature animals; however, during hypoxia there was an H1-mediated decrease in Ppa at low doses of histamine followed by an increase in Ppa. Combined histamine-receptor blockade markedly reduced both dilator and pressor responses to histamine infusion. Indomethacin failed to alter the H1-mediated dilator response to histamine in newborns.(ABSTRACT TRUNCATED AT 250 WORDS)     

Decreased pulmonary vascular responsiveness in rats raised on an essential fatty acid deficient diet

Leukotriene C4 is produced during hypoxic pulmonary vasoconstriction and leukotriene inhibitors preferentially inhibit the hypoxic pressor response in rats. If lipoxygenase products are important in hypoxic vasoconstriction, then an animal deficient in arachidonic acid should have a blunted hypoxic pressor response. We investigated if vascular responsiveness was decreased in vascular rings and isolated perfused lungs from rats raised on an essential fatty acid deficient diet (EFAD) compared to rats raised on a normal diet. Rats raised on the EFAD diet had decreased esterified plasma arachidonic acid and increased 5-, 8-, 11-eicosatrienoic acid compared to rats raised on the normal diet (control). Compared to the time matched responses in control isolated perfused lungs the pressor responses to angiotensin II and alveolar hypoxia were blunted in lungs from the arachidonate deficient rats. This decreased pulmonary vascular responsiveness was not affected by the addition of indomethacin or arachidonic acid to the lung perfusate. Similarly, the pulmonary artery rings from arachidonate deficient rats demonstrated decreased reactivity to norepinephrine compared to rings from control rats. In contrast, the tension increases to norepinephrine were greater in aortic rings from the arachidonate deficient rats compared to control. Stimulated lung tissue from the arachidonate deficient animals produced less slow reacting substance and platelet activating factor like material but the same amount of 6-keto-PGF1 alpha and TXB2 compared to control lungs. Thus there is an association between altered vascular responsiveness and impairment of stimulated production of slow reacting substance and platelet activating factor like material in rats raised on an EFAD diet.     

Lipoxygenase and cyclooxygenase blockade by BW 755C enhances pulmonary hypoxic vasoconstriction

Lipoxygenase products (leukotrienes) have been proposed as the mediators of pulmonary hypoxic vasoconstriction. However, the supporting data are inconclusive because the lipoxygenase and leukotriene receptor blockers that reduce hypoxic vasoconstriction (such as diethylcarbamazine and the FPL's) have confounding effects. We investigated BW 755C, a potent inhibitor of both lipoxygenase and cyclooxygenase, in eight intact anesthetized dogs with acute left lower lobe atelectasis. We examined two manifestations of hypoxic vasoconstriction: shunt fraction, as an inverse indicator of regional constriction in response to local hypoxia, and the pulmonary pressor response to global alveolar hypoxia, as an index of general hypoxic vasoconstriction. During normoxia, shunt fraction, measured using a sulfur hexafluoride infusion, was 32.0 +/- 7.0%. The pulmonary pressor response to hypoxia, defined as the increase in pulmonary end-diastolic gradient produced by 10% O2 inhalation, averaged 4.5 +/- 1.8 mmHg. Then, during normoxia, BW 755C was administered. Shunt fraction fell in all eight dogs from the previous mean of 32% to 25.5 +/- 6.1% (t = 6.5, P less than 0.0005). The hypoxic pressor response rose in all dogs, from the previous 4.5 mmHg to 9.0 +/- 3.5 mmHg (t = 4.5, P less than 0.005). BW 755C enhances hypoxic vasoconstriction, an effect consistent with its activity as a cyclooxygenase inhibitor. These data do not support a substantive role for the lipoxygenase pathway in hypoxic vasoconstriction.     

Blunted pulmonary pressor responses to hypoxia in blood perfused, ventilated lungs isolated from oxygen toxic rats: Possible role of prostaglandins

To determine the effects of high oxygen (O₂) tension on pulmonary vascular reactivity, we exposed rats either to 100% O₂ for 48 hrs or 40% O₂ for 3 to 5 weeks. Lungs from all rats were isolated, blood perfused and ventilated, and pressor responses to airway hypoxia and to infused angiotensin II were measured. We found that chronic subtoxic hyperoxia did not augment subsequent hypoxic vasoconstriction, and that 48 hrs of 100% O₂ markedly blunted hypoxic vasoconstriction. Meclofenamate restored hypoxic vasoconstriction to control levels in the lungs with blunted responses. Evidence for O₂ toxicity in the lungs exposed to 100% O₂ included interstitial swelling with alveolar exudates seen by light microscopy, and lung edema by water content calculations. We conclude that 1) chronic subtoxic hyperoxia does not influence subsequent hypoxic vasoconstriction, and 2) a dilator prostaglandin produced in the lung is a potent inhibitor of hypoxic vasoconstriction in O₂ toxic lungs.     

Cysteinyl leukotriene signaling aggravates myocardial hypoxia in experimental atherosclerotic heart disease

Background

Cysteinyl-leukotrienes (cys-LT) are powerful spasmogenic and immune modulating lipid mediators involved in inflammatory diseases, in particular asthma. Here, we investigated whether cys-LT signaling, in the context of atherosclerotic heart disease, compromises the myocardial microcirculation and its response to hypoxic stress. To this end, we examined Apoe^−/− mice fed a hypercholesterolemic diet and analysed the expression of key enzymes of the cys-LT pathway and their receptors (CysLT1/CysLT2) in normal and hypoxic myocardium as well as the potential contribution of cys-LT signaling to the acute myocardial response to hypoxia.

Methods and principal findings

Myocardial biopsies from Apoe^−/− mice demonstrated signs of chronic inflammation with fibrosis, increased apoptosis and expression of IL-6, as compared to biopsies from C57BL/6J control mice. In addition, we found increased leukotriene C₄ synthase (LTC₄S) and CysLT1 expression in the myocardium of Apoe^−/− mice. Acute bouts of hypoxia further induced LTC₄S expression, increased LTC₄S enzyme activity and CysLT1 expression, and were associated with increased extension of hypoxic areas within the myocardium. Inhibition of cys-LT signaling by treatment with montelukast, a selective CysLT1 receptor antagonist, during acute bouts of hypoxic stress reduced myocardial hypoxic areas in Apoe^−/− mice to levels equal to those observed under normoxic conditions. In human heart biopsies from 14 patients with chronic coronary artery disease mRNA expression levels of LTC₄S and CysLT1 were increased in chronic ischemic compared to non-ischemic myocardium, constituting a molecular basis for increased cys-LT signaling.

Conclusion

Our results suggest that CysLT1 antagonists may have protective effects on the hypoxic heart, and improve the oxygen supply to areas of myocardial ischemia, for instance during episodes of sleep apnea.     

Effects of cyclo- and lipoxygenase inhibitors on hypoxic vasoconstriction in isolated ferret lungs

To evaluate the role of leukotrienes in hypoxic pulmonary vasoconstriction, we measured steady-state pressor responses to graded hypoxia in isolated ferret lungs perfused with autologous blood containing 0.001, 0.03, 1, or 3 mM nordihydroguaiaretic acid (NDGA), 1 mM BW 755C, or 0.02-0.05 mM indomethacin. Untreated lungs served as controls. Perfusate concentrations of thromboxane B2 and 6-ketoprostaglandin F1 alpha, measured by radioimmunoassay, were markedly reduced in all treated lungs, indicating inhibition of cyclooxygenase. The maximum pressor response to hypoxia measured at a blood flow of 50 ml.min-1. kg-1 averaged 26.6 +/- 2.4 Torr in untreated lungs and was not affected by BW 755C or 0.001-0.03 mM NDGA. Because BW 755C and NDGA inhibited cyclooxygenase at concentrations that did not affect hypoxic vasoconstriction and because both agents are thought to inhibit lipoxygenase with a potency greater than or equal to that with which they inhibit cyclooxygenase, these results do not support the possibility that hypoxic pulmonary vasoconstriction was mediated by leukotrienes. At concentrations of 1 and 3 mM, NDGA inhibited the maximum hypoxic pressor response by 57 and 95%, respectively. The mechanism of this attenuation is unknown; however, it was apparently not due to cyclooxygenase inhibition, since indomethacin enhanced the maximum hypoxic pressor response by 45%. Nor was it due to blockade of calcium entry or interference with the contractile process in pulmonary vascular smooth muscle, since 1 mM NDGA did not inhibit vasoconstrictor responses to KCl or prostaglandin F2 alpha.     

The effect of leukotriene D4 on pulmonary and systemic circulation in conscious newborn piglets

In a conscious newborn piglet model, exogenous leukotriene D4 was found to be a potent pulmonary and systemic vasoconstrictor with significant left ventricular depressant effect. The pulmonary pressor effect was seen only in the arterioles and not the veins. In hypoxia the pulmonary response was less. The findings were similar to that in lambs. The role of leukotrienes in hypoxic pulmonary vasoconstriction and the foetal pulmonary circulation needs further elucidation.     

NO and reactive oxygen species are involved in biphasic hypoxic vasoconstriction of isolated rabbit lungs

Hypoxic pulmonary vasoconstriction (HPV) matches lung perfusion with ventilation but may also result in chronic pulmonary hypertension. It has not been clarified whether acute HPV and the response to prolonged alveolar hypoxia are triggered by identical mechanisms. We characterized the vascular response to sustained hypoxic ventilation (3% O(2) for 120-180 min) in isolated rabbit lungs. Hypoxia provoked a biphasic increase in pulmonary arterial pressure (PAP). Persistent PAP elevation was observed after termination of hypoxia. Total blockage of lung nitric oxide (NO) formation by N(G)-monomethyl-L-arginine caused a two- to threefold amplification of acute HPV, the sustained pressor response, and the loss of posthypoxic relaxation. This amplification was only moderate when NO formation was partially blocked by the inducible NO synthase inhibitor S-methylisothiourea. The superoxide scavenger nitro blue tetrazolium and the superoxide dismutase inhibitor triethylenetetramine reduced the initial vasoconstrictor response, the prolonged PAP increase, and the loss of posthypoxic vasorelaxation to a similar extent. The NAD(P)H oxidase inhibitor diphenyleneiodonium nearly fully blocked the late vascular responses to hypoxia in a dose that effected a decrease to half of the acute HPV. In conclusion, as similarly suggested for acute HPV, lung NO synthesis and the superoxide-hydrogen peroxide axis appear to be implicated in the prolonged pressor response and the posthypoxic loss of vasorelaxation in perfused rabbit lungs undergoing 2-3 h of hypoxic ventilation.     

Angiotensin is not required for hypoxic constriction in salt solution-perfused rat lungs

It has been reported that angiotensin II is specifically required for hypoxic vasoconstriction in rat lungs perfused with physiological salt solution. However, studies with other preparations indicate that angiotensin II does not play a necessary role in the mechanism of hypoxic vasoconstriction. In an attempt to resolve this disagreement I investigated in salt solution-perfused rat lungs whether vasoactive agents other than angiotensin II would induce hypoxic vasoconstriction, and, if so, whether the effect was due to selective action on the hypoxic mechanism or to a nonspecific increase in vascular reactivity. The results showed the development of hypoxic pressor responses after addition to perfusate of plasma, angiotensin II, KCl, vanadate, 4-aminopyridine, or norepinephrine plus propranolol. In contrast, addition of saline (control), ouabain, or tetraethylammonium chloride did not induce hypoxic vasoconstriction. Saralasin inhibited the effect of angiotensin II, but not that of plasma. Induction of responsiveness to hypoxia was associated with an increase in normoxic perfusion pressure and with potentiation of pressor responses to KCl. These results suggest that angiotensin II does not play a unique, integral role in the hypoxic mechanism, but instead is only one of many substances that will induce hypoxic pressor reactivity by reversing the vascular hyporeactivity of salt solution-perfused rat lungs.     

Positive interaction between leukotriene C4 and histamine and other mediators on vascular tissues

The interaction of leukotriene C₄ (LTC₄) with the contractile activity of histamine (H), serotonin (5HT) and norepinephrine (NE) has been investigated in isolated vascular preparations. Threshold concentration of LTC₄ (5 × 10⁻⁹ M) significantly potentiated the vasoconstricting effect of these compounds on guinea-pig pulmonary artery (GPPA). This phenomenon was long-lasting for H since it was still present 40 min after LTC₄ had been washed. FPL-55712 (10⁻⁵M) counteracted the increased H response on GPPA induced by LTC₄. Potentiation of H activity due to LTC₄ was also observed on guinea-pig thoracic aorta (GPTA) indicating that LTC₄-induced hyperreactivity is not a phenomenon restricted to the pulmonary vascular bed. In the experiments carried out in presence of indomethacin (3 × 10⁻⁶M), LTC₄ still potentiated H-induced vasoconstriction on GPPA, however the time course of the phenomenon was significantly shorter than that observed in absence of the cyclooxygenase inhibitor. The contractile activity of H and NE on guinea-pig portal vein (GPPV) was not potentiated by LTC₄ These results demonstrate that LTC₄ induces hyperreactivity of the arterial vascular tissue to vasoactive compounds and suggest that cysteinyl-leukotrienes may have pathological significance in the hemodynamic changes occurring during anaphylactic reactions. Preliminary experiments carried out on human intralobar pulmonary artery strongly support this hypothesis.     

Leukotriene inhibitors do not block hypoxic pulmonary vasoconstriction in dogs

We studied whether intravenously administered inhibitors of leukotriene synthesis (diethylcarbamazine, DEC) or end-organ effect (FPL-55712) would change the distribution of regional pulmonary blood flow (rPBF) caused by left lower lobe (LLL) alveolar hypoxia in dogs. Both drugs failed to alter rPBF. In addition, the pressor response to whole-lung hypoxia was not blocked by an FPL-55712 infusion. On the other hand, nitroprusside, as a nonspecific vasodilator also administered intravenously, was able to partially reverse the effects of LLL hypoxia on rPBF. Thus our data do not support a role for leukotriene mediation of hypoxic pulmonary vasoconstriction in dogs.     

Effect of hypoxia on erythrocyte deformability in different species

The contribution of erythrocyte deformability to the pulmonary vascular resistance during hypoxia in different animal species has not been examined. We hypothesized that the increase in pulmonary vascular resistance during hypoxia was partially due to erythrocytes (RBC's) becoming less deformable during hypoxia, and therefore their transit in the capillaries becomes restricted. To test this, we measured an index of deformability of RBC's from six animal species (dog, pig, cat, rabbit, hamster, rat) during normoxic and hypoxic condition, and compared the changes in deformability with the pulmonary hypoxic pressor response (HPR) which has been reported in the same species. Deformability was indexed as the resistance that a Hemafil polycarbonate membrane (Nucleopore filter, 4.7 micron pores) offers to a 10% suspension of RBC's. The RBC suspension was either normoxic (PO2 = 150 torr) or hypoxic (PO2 = 50 torr). We found that hypoxia decreased RBC deformability; the largest decrease occurred in rat RBC's, a small but significant decrease was observed in the RBC's of cats, rabbits and hamsters, but no change was detected in RBC's of dogs or pigs. In general, such changes in deformability do not correlate well with the HPR in intact or in isolated lungs, for example the pig, had the largest HPR but the smallest change in RBC deformability. In some species, however, there was a positive correlation between RBC deformability and HPR, for example rats, rabbits and cats are usually better responders than dogs and hamsters, similarly the deformability of RBC's in rats, rabbits and cats were also more influenced by hypoxia than RBC's from dogs. The limiting factors in this relationship are the artificial conditions which were used to measure deformability and HPR, both may be different than in the intact conditions. Nevertheless the present data show that erythrocytes of some species can become less flexible during hypoxia, and hence may impede the transit in the capillaries. Therefore we propose that the hypoxic pressor response in the pulmonary vasculature may be partly due to smooth muscle contraction (vasoconstriction) and partly due to a decrease in erythrocyte deformability (capillary obstruction). Both components are likely to be species dependent.     

Age-dependent effects of chronic hypoxia on pulmonary vascular reactivity

Effects of age on the pulmonary vascular responses to histamine (HIST), norepinephrine (NE), 5-hydroxytryptamine (5-HT), and KCl were studied in isolated, perfused lungs from juvenile (7-wk-old), adult (14-wk-old), and mature adult (28-wk-old) normoxic rats and compared with age-matched rats exposed to chronic hypoxia for either 14 or 28 days. Chronic hypoxia changed vasoconstriction to HIST and NE to vasodilation in lungs from juvenile and adult rats. Mature adult lungs only vasoconstricted to these amines in both control and hypoxic animals. Pressor responses to 5-HT were not affected by chronic hypoxia regardless of age group. Pressor responses to KCl were also not altered by hypoxia, but lungs from older rats showed greater control responsiveness to KCl compared with lungs from juveniles. Only lungs from juvenile animals developed significant elevations of base-line resistance as a result of hypoxic exposure. To investigate the contribution of H1-, H2-, and beta-receptors in these changes, we employed chlorpheniramine, metiamide, and propranolol, respectively, as blocking agents in another series of experiments. Chlorpheniramine either reduced vasoconstriction or increased vasodilation to HIST in lungs from both control and hypoxic animals, whereas metiamide was without effect. Propranolol either increased vasoconstriction or reversed vasodilation to HIST and NE in all lungs studied. The present data demonstrate the important interaction between chronic hypoxia and age that can alter pulmonary vascular tone and reactivity. The inverse relationship between age and elevation of pulmonary vascular resistance after chronic hypoxic exposure may be the key element that changes pulmonary vascular reactivity observed during hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vascular reactivity is increased in rat lungs injured with alpha-naphthylthiourea

We investigated the effects of lung injury due to alpha-naphthylthiourea (ANTU) on pulmonary vascular reactivity. Rats were treated with ANTU (10 mg/kg ip) or the vehicle Tween 80. Four hours later, lungs from ANTU-treated rats had increased wet-to-dry weight ratios, bronchial lavage protein concentrations, and perivascular edema. To test vascular reactivity, lungs were isolated and perfused with blood at constant flow rate, while mean pulmonary arterial pressure was monitored. ANTU-treated lungs vasoconstricted earlier than Tween-treated lungs in response to severe airway hypoxia (fractional inspired O2 0%). ANTU-treated lungs vasoconstricted in response to 10% O2, while Tween-treated lungs failed to respond to 10% O2, indicating that the threshold for hypoxic vasoconstriction was decreased by ANTU. ANTU also decreased the threshold for and increased the magnitude of angiotensin II pressor responses, indicating that the increased vasoreactivity was not specific for hypoxia. Addition of meclofenamate to perfusates increased the rate and magnitude of responses to 0% O2 in Tween-treated lungs, but did not change the responses of ANTU-treated lungs. Light microscopy of ANTU-treated lungs showed no pulmonary arterial obstruction, and electron microscopy revealed mild capillary endothelial cell injury. We conclude that enhanced pulmonary vascular reactivity accompanies the increased-permeability pulmonary edema caused by ANTU. A similar increase in vasoreactivity might contribute to pulmonary hypertension observed in patients with the adult respiratory distress syndrome.