Contribution of polycythaemia to pulmonary hypertension in simulated high altitude in rats

The impact of a PDA on tissue oxygenation and haemodynamics following a blood transfusion in preterm infants

BACKGROUND

To examine the impact of PRBC transfusion on pulmonary vascular resistance (PVR), systemic vascular resistance and myocardial function using echocardiography and cerebral and splanchnic tissue oxygenation using near-infrared spectroscopy (NIRS) in premature babies with and without a PDA.

METHODS

A prospective observational study of premature infants born <1500 g in receipt of PRBC transfusions beyond 10 days of age. Echocardiography and NIRS monitoring were performed at baseline, during the transfusion and 24 h after transfusion.

RESULTS

Thirty infants with a median gestation of 26.4 [24.8-28.0] weeks were enrolled. Ten infants had a PDA. Following transfusion, a significant decrease in PVR markers occurred in all infants. Right ventricular (RV) function increased following transfusion in the PDA closed group only. Cerebral oxygen saturation increased following transfusion in all infants. Babies in the PDA open group had significantly lower splanchnic oxygen saturations at baseline compared to the PDA closed group which persisted over the study period and were unaltered by transfusion.

CONCLUSIONS

PRBC transfusion lowers PVR irrespective of PDA status. Those with a PDA demonstrated a lack of improvement in RV function and splanchnic oxygenation highlighting the impact a PDA has on the neonatal circulation.

IMPACT

The presence or absence of the PDA imposes differential effects on splanchnic oxygenation during red blood cell (PRBC) transfusion in the premature population. This is the first study to assess the impact of the PDA on splanchnic oxygenation via near-infrared spectroscopy (NIRS) during red blood cell transfusion in premature neonates. New insights have been found into the impact of PRBC transfusion on pulmonary vascular resistance, right ventricular function, cerebral and splanchnic oxygenation in the presence and absence of a PDA and emphasises the ongoing impact of ductal patency on gut oxygenation.

Pulmonary Hypertension in Acute and Chronic High Altitude Maladaptation Disorders

Alveolar hypoxia is the most prominent feature of high altitude environment with well-known consequences for the cardio-pulmonary system, including development of pulmonary hypertension. Pulmonary hypertension due to an exaggerated hypoxic pulmonary vasoconstriction contributes to high altitude pulmonary edema (HAPE), a life-threatening disorder, occurring at high altitudes in non-acclimatized healthy individuals. Despite a strong physiologic rationale for using vasodilators for prevention and treatment of HAPE, no systematic studies of their efficacy have been conducted to date. Calcium-channel blockers are currently recommended for drug prophylaxis in high-risk individuals with a clear history of recurrent HAPE based on the extensive clinical experience with nifedipine in HAPE prevention in susceptible individuals. Chronic exposure to hypoxia induces pulmonary vascular remodeling and development of pulmonary hypertension, which places an increased pressure load on the right ventricle leading to right heart failure. Further, pulmonary hypertension along with excessive erythrocytosis may complicate chronic mountain sickness, another high altitude maladaptation disorder. Importantly, other causes than hypoxia may potentially underlie and/or contribute to pulmonary hypertension at high altitude, such as chronic heart and lung diseases, thrombotic or embolic diseases. Extensive clinical experience with drugs in patients with pulmonary arterial hypertension suggests their potential for treatment of high altitude pulmonary hypertension. Small studies have demonstrated their efficacy in reducing pulmonary artery pressure in high altitude residents. However, no drugs have been approved to date for the therapy of chronic high altitude pulmonary hypertension. This work provides a literature review on the role of pulmonary hypertension in the pathogenesis of acute and chronic high altitude maladaptation disorders and summarizes current knowledge regarding potential treatment options.

RNA-seq analyses the effect of high-salt diet in hypertension

OBJECTIVES

High-salt diet is one of the major risk factors in the development of hypertension. Previous studies have observed a relationship between high-salt induced blood pressure levels and cardiac-cerebral disease. However, the molecular mechanism of high-salt diet induced hypertension and the serious complications in cardiovascular system still remain unknown.

MATERIALS AND METHODS

We built high-salt diet induced hypertension rat models, and investigated the transcriptomic alteration in four hypertension affected tissues, i.e. ventricle and atrium in heart as well as cortex and medulla in kidney. Differential expression gene (DEG) analysis and further functional annotation including Ingenuity Pathway Analysis (IPA) was performed to reveal the molecular mechanism of high-salt induced hypertension and organ injury.

RESULTS

We observed that several genes associated with cardiovascular development and organ injury were significantly dysregulated rat fed with high-salt diet, such as Mmp-15, Igfbp7, Rgs18 and Hras. We demonstrated that differential expressed genes were functionally related to the increased levels of alkaline phosphatase (ALP).

CONCLUSION

Our study provided new insight about molecular mechanism of high-salt induced hypertension and heart and kidney damage.

Relationship between polycythemia and in-hospital mortality in chronic obstructive pulmonary disease patients with low-risk pulmonary embolism

BACKGROUNDS

Pulmonary embolism (PE) is frequent in subjects with chronic obstructive pulmonary disease (COPD) and associated with high mortality. This multi-center retrospective study was performed to investigate if secondary polycythemia is associated with in-hospital mortality in COPD patients with low-risk PE.

METHODS

We identified COPD patients with proven PE between October, 2005 and October, 2015. Patients in risk classes III-V on the basis of the PESI score were excluded. We extracted demographic, clinical and laboratory information at the time of admission from medical records. All subjects were followed until hospital discharge to identify all-cause mortality.

RESULTS

We enrolled 629 consecutive patients with COPD and PE at low risk: 132 of them (21.0%) with and 497 (79.0%) without secondary polycythemia. Compared with those without polycythemia, the polycythemia group had significantly lower forced expiratory volume in one second (FEV₁) level (0.9±0.3 vs. 1.4±0.5, P=0.000), lower PaO₂ and SpO₂ as well as higher PaCO₂ (P=0.03, P=0.03 and P=0.000, respectively). COPD patients with polycythemia had a higher proportion of arrhythmia in electrocardiogram (ECG) (49.5% vs. 35.7%, P=0.02), a longer hospital duration time (15.3±10.1 vs. 9.7±9.1, P=0.001), a higher mechanical ventilation rate (noninvasive and invasive, 51.7% vs. 30.3%, P=0.04 and 31.0% vs. 7.9%, P=0.04, respectively), and a higher in-hospital mortality (12.1% vs. 6.6%, P=0.04). Multivariate logistic regression analysis revealed that polycythemia was associated with mortality in COPD patients with low-risk PE (adjusted OR 1.11; 95% CI, 1.04-1.66).

CONCLUSIONS

Polycythemia is an independent risk factor for all-cause in-hospital mortality in COPD patients with PE at low risk.

Impact of increased hematocrit on right ventricular afterload in response to chronic hypoxia

Chronic hypoxia causes chronic mountain sickness through hypoxia-induced pulmonary hypertension (HPH) and increased hematocrit. Here, we investigated the impact of increased hematocrit and HPH on right ventricular (RV) afterload via pulmonary vascular impedance. Mice were exposed to chronic normobaric hypoxia (10% oxygen) for 10 (10H) or 21 days (21H). After baseline hemodynamic measurements, ∼500 μl of blood were extracted and replaced with an equal volume of hydroxyethylstarch to normalize hematocrit and all hemodynamic measurements were repeated. In addition, ∼500 μl of blood were extracted and replaced in control mice with an equal volume of 90% hematocrit blood. Chronic hypoxia increased input resistance (Z0 increased 82% in 10H and 138% in 21H vs. CTL; P < 0.05) and characteristic impedance (ZC increased 76% in 10H and 109% in 21H vs. CTL; P < 0.05). Hematocrit normalization did not decrease mean pulmonary artery pressure but did increase cardiac output such that both Z0 and ZC decreased toward control levels. Increased hematocrit in control mice did not increase pressure but did decrease cardiac output such that Z0 increased. The paradoxical decrease in ZC with an acute drop in hematocrit and no change in pressure are likely due to inertial effects secondary to the increase in cardiac output. A novel finding of this study is that an increase in hematocrit affects the pulsatile RV afterload in addition to the steady RV afterload (Z0). Furthermore, our results highlight that the conventional interpretation of ZC as a measure of proximal artery stiffness is not valid in all physiological and pathological states.

Hypoxaemia affects male reproduction: a case study of how to differentiate between primary and secondary hypoxic testicular toxicity due to chemical exposure

Classification for fertility is based on two conditions, namely on evidence of an adverse effect on sexual function and fertility and that the effect is not secondary to other toxic effects. To decide on an adverse effect is a relatively simple day-to-day decision in toxicology but whether this effect is secondary often leads to serious controversy. As the seminiferous epithelium operates on the verge of hypoxia, oxygen deficit can lead to secondary impairment of testicular function. This is well known from healthy mountaineers exposing themselves to high altitude. They have reduced blood oxygen content that goes in parallel with impairment of testicular function and this effect remains for some time in spite of a compensatory polycythaemia. Similar findings are described for experimental animals exposed to hypobaric oxygen/high altitude. In addition, testicular function is affected in severe diseases in humans associated with systemic oxygen deficit like chronic obstructive pulmonary disease, sickle cell disease or beta-thalassaemia as well as in transgenic animals simulating haemolytic anaemia or sickle cell disease. The problem of insufficient oxygen supply as the underlying cause for testicular impairment has received relatively little attention in toxicology, mainly because blood oxygen content is generally not measured in these animal experiments. The difficulties associated with the decision whether testicular toxicity is primary or secondary to hypoxia are exemplified by the results of inhalation studies with nickel subsulphide and gallium arsenide (GaAs). Both of these particulate substances lead to severe lung toxicity that might impair oxygen uptake, but testicular toxicity is only observed with GaAs. This may first be explained by different effects on the blood: nickel subsulphide inhalation leads to a compensatory erythropoiesis that may mitigate pulmonary lack of oxygen uptake. In contrast, GaAs exposure is associated with microcytic haemolytic anaemia thereby aggravating any possible oxygen undersupply. Furthermore, the predominant pulmonary effect caused by GaAs (but not by nickel subsulphide) is alveolar proteinosis. Pulmonary alveolar proteinosis is also known as a severe disease in humans associated with hypoxaemia. Therefore, we conclude that the testicular effects observed after GaAs are secondary to hypoxaemia caused by the combination of pulmonary proteinosis and haemolytic anaemia. This publication tries to raise awareness to the severe consequences of hypoxaemia on testicular function that may already be caused by reduced oxygen pressure at high altitude without any chemical exposure.

[Bronchopulmonary dysplasia-associated pulmonary arterial hypertension of very preterm infants]

Bronchopulmonary dysplasia (BPD) of very preterm infants is a multifactorial chronic lung disease and its incidence has not decreased despite improvements in neonatal intensive care, including lung protective strategies. Pulmonary hypertension (PH) can complicate the course of BPD. Mortality in infants with BPD-associated PH is thought to be very high, but its incidence is unknown and a standard diagnostic and therapeutic strategy has not been well defined. In this article, we will first describe the current knowledge on the BPD-associated PH and the current treatments available for this pathology. We will then present the HTP-DBP Study, carried out in Paris (France) starting in 2012. The diagnosis of PH is suspected on echocardiographic criteria, but cardiac catheterization is considered the gold standard for diagnosis and evaluation of the severity of PH. Moreover, pulmonary vasoreactivity testing is used to guide the management of patients with PH. The pathogenesis of BPD-associated PH is poorly understood and even less is known about appropriate therapy. Today, optimizing ventilation and reducing the pulmonary vascular tone with specific pulmonary vasodilatator drugs are the main goals in treating HTP-associated DBP. Animal studies and a few clinical studies suggest that medications targeting the nitric oxide (NO) signaling pathway (NO inhalation, oral sildenafil citrate) could be effective treatments for BPD-associated PH, but they have not been approved for this indication. The HTP-DBP study is a French multicenter prospective observational study. The objective is to evaluate the frequency of BPD-associated PH, to describe its physiopathology, its severity (morbidity and mortality), and the effectiveness of current treatments.

Impact of pulmonary vascular stiffness and vasodilator treatment in pediatric pulmonary hypertension: 21 patient-specific fluid-structure interaction studies

Recent clinical studies of pulmonary arterial hypertension (PAH) have found correlations between increased pulmonary vascular stiffness (PVS) and poorer disease outcomes. However, mechanistic questions remain about the relationships amongst PVS, RV power, and vascular hemodynamics in the setting of progressive PAH that are difficult or impossible to answer using direct measurements. Clinically validated patient-specific computational modeling may allow exploration of these issues through perturbation-based predictive testing. Here we use a simple patient-specific model to answer four questions: how do hemodynamics change as PAH worsens? How does increasing PVS impact hemodynamics and RV power? For a patient with moderate PAH, what are the consequences if the pressures increase modestly yet sufficiently to engage collagen in those vessels? What impact does pressure-reducing vasodilator treatment have on hemodynamics? Twenty-one sets of model-predicted impedance and mean PA pressure (mPAP) show good agreement with clinical measurements, thereby validating the model. Worsening was modeled using data from three PAH outcomes groups; these show not only the expected increase in mPAP, but also an increase in pressure pulsatility. Interestingly, chronically increasing mPAP decreased WSS, suggesting that increased PA cross-sectional area affected WSS greater than increased PVS. For a patient with moderately high PVR (12.7 WU) with elastin-based upstream vascular remodeling, moving from elastin-dominant vessel behavior to collagen-dominant behavior caused substantial increases in mPAP, pressure and WSS pulsatility. For the same patient, reducing PVR through a simulated vasodilator to a value equivalent to mild PAH did not decrease pressure pulsatility and dramatically increased WSS pulsatility. Overall, these results suggest a close association between PVS and hemodynamics and that hemodynamics may play an important role in progressing PAH. These support the hypothesis that treatments should target decreasing or reversing upstream vascular remodeling in addition to decreasing mean pressures.

Pulmonary vascular resistance and viscosity: the forgotten factor

Calculating pulmonary vascular resistance is important in many fields of medicine. Although the influence of hematocrit on calculated resistance has been known for many years, it is rare to find the appropriate corrections in published articles. This review discusses the relationship between viscosity and resistance and shows how the effect of viscosity can be allowed for by calculating hindrance or relative viscosity.

Time course of ventilatory acclimatisation to hypoxia in a model of anemic transgenic mice

We questioned the assumption that polycythemia is essential for adaptation to chronic hypoxia. Thus, the objective of our study was to determine if anemic Epo-TAg(h) mice could survive in hypoxia despite low oxygen carrying capacity. We explored the possibility that ventilatory acclimatisation is involved in the strategy used by anemic transgenic mice to adapt to chronic hypoxia. Epo-TAg(h) and Wild Type mice were exposed during 2 weeks at a barometric pressure of 450 Torr. After 1, 5 and 14 days of exposure, ventilation at different inspired oxygen fraction was measured in both groups. Ventilation during acclimatisation to hypoxia was significantly greater in Epo-TAg(h) than in Wild Type. The difference was mainly due to a higher tidal volume that could explain a higher arterial PO2 in Epo-TAg(h) mice. Epo-Tag(h) mice did not develop right ventricle hypertrophy after 2 weeks of exposure to hypoxia while Wild Type did. Hemoglobin concentration was 60% lower in anemic mice versus Wild Type after acclimatisation. In conclusion, ventilatory acclimatisation contributed to the adaptation of Epo-Tag(h) mice in chronic hypoxia despite low arterial oxygen carrying capacity.

Exercise training improves lung gas exchange and attenuates acute hypoxic pulmonary hypertension but does not prevent pulmonary hypertension of prolonged hypoxia

Our laboratory has previously shown an attenuation of hypoxic pulmonary hypertension by exercise training (ET) (Henderson KK, Clancy RL, and Gonzalez NC. J Appl Physiol 90: 2057–2062, 2001), although the mechanism was not determined. The present study examined the effect of ET on the pulmonary arterial pressure (Pap) response of rats to short- and long-term hypoxia. After 3 wk of treadmill training, male rats were divided into two groups: one (HT) was placed in hypobaric hypoxia (380 Torr); the second remained in normoxia (NT). Both groups continued to train in normoxia for 10 days, after which they were studied at rest and during hypoxic and normoxic exercise. Sedentary normoxic (NS) and hypoxic (HS) littermates were exposed to the same environments as their trained counterparts. Resting and exercise hypoxic arterial Po2 were higher in NT and HT than in NS and HS, respectively, although alveolar ventilation of trained rats was not higher. Lower alveolar-arterial Po2 difference and higher effective lung diffusing capacity for O2 in NT vs. NS and in HT vs. HS suggest ET improved efficacy of gas exchange. Pap and Pap/cardiac output were lower in NT than NS in hypoxia, indicating that ET attenuates the initial vasoconstriction of hypoxia. However, ET had no effect on chronic hypoxic pulmonary hypertension: Pap and Pap/cardiac output in hypoxia were similar in HS vs HT. However, right ventricular weight was lower in HT than in HS, although Pap was not different. Because ET attenuates the initial pulmonary vasoconstriction of hypoxia, development of pulmonary hypertension may be delayed in HT rats, and the time during which right ventricular afterload is elevated may be shorter in this group. ET effects may improve the response to acute hypoxia by increasing efficacy of gas exchange and lowering right ventricular work.

Hypoxia: unique myocardial morphology?

OBJECTIVE

The objective of this study was to investigate the effects of chronic and intermittent hypoxia on myocardial morphology.

METHODS

Rats randomly divided into 3 groups (n = 14 per group) were exposed to room air (Fio(2) = 0.21), chronic hypoxia (Fio(2) = 0.10), and intermittent hypoxia (chronic hypoxia with 1 hour per day of room air) for 2 weeks. Weight, blood gas analysis, hematocrit, hemoglobin, red cells, and right and left ventricular pressures were measured. Hearts excised for morphologic examination were randomly divided into 2 groups (9 per group for gross morphologic measurements and 5 per group for histologic and morphometric analysis). The weight ratio of right to left ventricles plus interventricular septum, myocyte diameter, cross-sectional area, and free wall thickness in right and left ventricles were measured.

RESULTS

Despite the same polycythemia, the right ventricle pressure (P <.05) and ratio of right to left ventricle pressures (P <.02) were higher after chronic hypoxia than intermittent hypoxia. The ratio of heart weight to total body weight and the ratio of right to left ventricles plus interventricular septum was higher (P <.01) in chronic and intermittent hypoxia than in normoxia. Myocyte diameter was not different between the right and left ventricles in normoxia, whereas right ventricle myocytes were larger than left ventricle myocytes in chronic hypoxia (P <.05) and intermittent hypoxia (P <.0005). There was marked dilatation of right ventricle size (P <.001) and marked reduction of left ventricle (P <.001) size in chronic and intermittent hypoxia compared with normoxia. The total ventricular area (right ventricle plus left ventricle area) remained the same in all groups. The wall thickness ratio in chronic hypoxia and intermittent hypoxia was increased (P <.001) compared with normoxia in the right ventricle but not in the left ventricle.

CONCLUSIONS

Intermittent reoxygenation episodes do not induce a lesser ventricular hypertrophic response than observed with chronic hypoxia. The functional myocardial preconditioning consequence of intermittent reoxygenation is not supported by structural differences evident with the available techniques.

Syndromes of Subacute Mountain Sickness

Two clinical syndromes, acute and chronic mountain sickness, have traditionally been associated with high altitude. Recently, two separate entities of subacute nature have been described in infants and adults. In this paper, we review the published literature on these conditions. Subacute infantile mountain sickness is a condition seen predominantly in Han Chinese infants living in Tibet, although it has been described in other high altitude communities as well. It came into prominence only after the large-scale migration of Chinese population from the low altitude of mainland China to the high altitudes of the Qinghai-Tibetan plateau. The condition is characterized by features of severe hypoxic pulmonary hypertension and heart failure. Pulmonary histology is consistent with muscularization of the pulmonary arterioles, but no intimal proliferation or plexiform lesions are seen. The second syndrome, adult subacute mountain sickness, has been described almost exclusively in Indian soldiers living at extreme altitude for prolonged periods of time. In this condition also, hypoxic pulmonary hypertension appears to be the dominant factor responsible for severe congestive heart failure. Both these conditions have several similarities with brisket disease in cattle; hypoxic pulmonary vasoconstriction plays an important role in the pathogenesis, and removal from high altitude results in complete resolution. Thus, it appears that both these syndromes are human counterparts of brisket disease in cattle.

Selected Contribution: Pulmonary hypertension in mice following intermittent hypoxia

Sleep apnea (intermittent periods of hypoxia with or without hypercapnia) is associated with systemic hypertension and increased mortality from cardiovascular disease, but the relationship to pulmonary hypertension is uncertain. Previous studies on intermittent hypoxia (IH) in rats that demonstrated pulmonary hypertension utilized relatively long periods of hypoxia. Recent studies that utilized brief periods of hypoxia have conflicting reports of right ventricular (RV) hypertrophy. In addition, many studies have not measured pulmonary hemodynamics to asses the severity of pulmonary hypertension in vivo. Given the increasing availability of genetically engineered mice and the need to establish a rodent model of IH-induced pulmonary hypertension, we studied the effect of IH (2-min cycles of 10% and 21% O2, 8 h/day, 4 wk) on wild-type mice, correlating in vivo measurements of pulmonary hypertension with RV mass and pulmonary vascular remodeling. RV systolic pressure was increased after IH (36 +/- 0.9 mmHg) compared with normoxia (29.5 +/- 0.6) but was lower than continuous hypoxia (44.2 +/- 3.4). RV mass [RV-to-(left ventricle plus septum) ratio] correlated with pressure measurements (IH = 0.27 +/- 0.02, normoxia = 0.22 +/- 0.01, and continuous hypoxia = 0.34 +/- 0.01). Hematocrits were also elevated after IH and continuous hypoxia (56 +/- 1.6 and 54 +/- 1.1 vs. 44.3 +/- 0.5%). Evidence of neomuscularization of the distal pulmonary circulation was found after IH and continuous hypoxia. We conclude that mice develop pulmonary hypertension following IH, representing a possible animal model of pulmonary hypertension in response to the repetitive hypoxia-reoxygenation of sleep apnea.

Upregulation of nitric oxide synthase in mice with severe hypoxia-induced pulmonary hypertension

BACKGROUND

The importance of nitric oxide (NO) in hypoxic pulmonary hypertension has been demonstrated using nitric oxide synthase (NOS) knockout mice. In that model NO from endothelial NOS (eNOS) plays a central role in modulating pulmonary vascular tone and attenuating hypoxic pulmonary hypertension. However, the normal regulation of NOS expression in mice following hypoxia is uncertain. Because genetically engineered mice are often utilized in studies of NO, we conducted the present study to determine how hypoxia alters NOS expression in wild-type mice.

METHOD

Mice were exposed to sea level, ambient conditions (5280 feet) or severe altitude (17,000 feet) for 6 weeks from birth, and hemodynamics and lung NOS expression were assessed.

RESULTS

Hypoxic mice developed severe pulmonary hypertension (right ventricular systolic pressure [RVsP] 60 mmHg) as compared with normoxic mice (27 mmHg). Using quantitative reverse-transcription PCR, it was found that expressions of eNOS and inducible NOS (iNOS) increased 1.5-fold and 3.5-fold, respectively, in the lung. In addition, the level of lung eNOS protein was increased, neuronal NOS (nNOS) protein was unchanged, and iNOS was below the limit of detection. Immunohistochemistry demonstrated no change in lung iNOS or nNOS staining in either central or peripheral areas, but suggested increased eNOS in the periphery following hypoxia.

CONCLUSION

In mice, hypoxia is associated with increases in lung eNOS, possibly in iNOS, but not in nNOS; this suggests that the pattern of lung NOS expression following hypoxia must be considered in studies using genetically engineered mice.

Maintained upregulation of pulmonary eNOS gene and protein expression during recovery from chronic hypoxia

We previously demonstrated augmented endothelium-derived nitric oxide (EDNO)-dependent pulmonary arterial dilation and increased arterial endothelial nitric oxide synthase (eNOS) levels in chronic hypoxic (CH) and monocrotaline (nonhypoxic) models of pulmonary arterial hypertension. Therefore, we hypothesized that the long-term elevation of arterial eNOS levels associated with CH is related to pulmonary hypertension or some factor(s) associated with hypertension and not directly to hypoxia. To test this hypothesis, we examined responses to the EDNO-dependent dilator ionomycin in U-46619-constricted, isolated, saline-perfused lungs from control rats, CH (4 wk at 380 mmHg) rats, and rats previously exposed to CH but returned to normoxia for 4 days or 2 wk. Microvascular pressure was assessed by double-occlusion technique, allowing calculation of segmental resistances. In addition, vascular eNOS immunoreactivity was assessed by quantitative immunohistochemistry, and eNOS mRNA abundance was determined by RT-PCR assays. Our findings indicate that 4-day and 2-wk posthypoxic rats exhibit persistent pulmonary hypertension, likely due to maintained arterial remodeling and polycythemia associated with prior exposure to CH. Furthermore, arterial dilation to ionomycin was augmented in lungs from each experimental group compared with controls. Finally, arterial eNOS immunoreactivity and whole lung eNOS mRNA levels remained elevated in posthypoxic animals. These findings suggest that altered vascular mechanical forces or vascular remodeling contributes to enhanced EDNO-dependent arterial dilation and upregulation of arterial eNOS in various models of established pulmonary hypertension.

Temporal, spatial, and oxygen-regulated expression of hypoxia-inducible factor-1 in the lung

Hypoxia-inducible factor (HIF)-1 is a basic helix-loop-helix transcription factor that transactivates genes encoding proteins that participate in homeostatic responses to hypoxia. Several of these downstream gene products, such as erythropoietin, vascular endothelial growth factor, heme oxygenase-1, and inducible nitric oxide synthase, may contribute to the pathogenesis of pulmonary hypertension. Previous studies demonstrated increased HIF-1 mRNA levels in rats and mice subjected to hypoxia. In this study, we have demonstrated spatial, temporal, and O2-dependent expression of HIF-1 protein. Immunoblot analysis revealed hypoxic induction of HIF-1 in all cultured pulmonary cell types assayed, including those derived from pulmonary arterial endothelium and smooth muscle, bronchial epithelium, alveolar macrophages, alveolar epithelium, and microvascular endothelium. In contrast to all other cell types, pulmonary arterial smooth muscle cells expressed HIF-1 under nonhypoxic conditions. Immunohistochemistry and immunoblot analysis of ferret lungs demonstrated pulmonary expression of HIF-1 in vivo. HIF-1 protein expression was induced maximally when lungs were ventilated with 0 or 1% O2 for 4 h. On reoxygenation, HIF-1 was rapidly degraded, with a half-life of <1 min. These findings demonstrate that HIF-1 expression is tightly coupled to O2 concentration in vivo and are consistent with the involvement of HIF-1 in the physiological and pathophysiological responses to hypoxia in the lung.

Sustained pulmonary hypertension and right ventricular hypertrophy after chronic hypoxia in mice with congenital deficiency of nitric oxide synthase 3

Chronic hypoxia induces pulmonary hypertension and right ventricular (RV) hypertrophy. Nitric oxide (NO) has been proposed to modulate the pulmonary vascular response to hypoxia. We investigated the effects of congenital deficiency of endothelial NO synthase (NOS3) on the pulmonary vascular responses to breathing 11% oxygen for 3-6 wk. After 3 wk of hypoxia, RV systolic pressure was greater in NOS3-deficient than in wild-type mice (35+/-2 vs 28+/-1 mmHg, x+/-SE, P < 0.001). Pulmonary artery pressure (PPA) and incremental total pulmonary vascular resistance (RPI) were greater in NOS3-deficient than in wild-type mice (PPA 22+/-1 vs 19+/-1 mmHg, P < 0.05 and RPI 92+/-11 vs 55+/-5 mmHg.min.gram.ml-1, P < 0.05). Morphometry revealed that the proportion of muscularized small pulmonary vessels was almost fourfold greater in NOS3-deficient mice than in wild-type mice. After 6 wk of hypoxia, the increase of RV free wall thickness, measured by transesophageal echocardiography, and of RV weight/body weight ratio were more marked in NOS3-deficient mice than in wild-type mice (RV wall thickness 0.67+/-0.05 vs 0.48+/-0.02 mm, P < 0.01 and RV weight/body weight ratio 2.1+/-0.2 vs 1.6+/-0.1 mg. gram-1, P < 0.05). RV hypertrophy produced by chronic hypoxia was prevented by breathing 20 parts per million NO in both genotypes of mice. These results suggest that congenital NOS3 deficiency enhances hypoxic pulmonary vascular remodeling and hypertension, and RV hypertrophy, and that NO production by NOS3 is vital to counterbalance pulmonary vasoconstriction caused by chronic hypoxic stress.

Exploration of the pulmonary circulation. Festschrift to Professor Donald Heath

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Pulmonary vascular responsiveness in rats following neonatal exposure to high altitude or carbon monoxide

Exposure of adult and neonatal rats to high altitude increases pulmonary vascular responsiveness during the exposure. A study was undertaken to determine if a short exposure of neonatal rats to either high-altitude or carbon monoxide (CO) hypoxia would cause persistent alterations in pulmonary vascular responsiveness postexposure. One-day-old male Sprague-Dawley rats were obtained as 16 litters of 10-12 pups each. At 2 days of age, 4 litters were exposed to CO (500 ppm) for 32 days, and 4 litters were exposed to ambient air (AIR) in Detroit (200 m). Another 4 litters were exposed to 3500 m altitude (ALT) in a chamber for 32 days, and 3 litters were exposed to ambient conditions in Fort Collins (CON, 1524 m). After the exposures, all rats were maintained at 1524 m. At 2, 40, 76, and 112 days postexposure, lungs were isolated and perfused with Earle's salt solution (+Ficoll, 4 g%). Pulmonary vascular responsiveness was assessed by dose responses to angiotensin II (AII, 0.025-0.40 micrograms) and acute hypoxia (3% O2 for 3 min). AII responses were higher in ALT vs CON rats at all ages, but no differences were noted between CO and AIR rats. Acute hypoxic responses were higher in ALT versus CON rats at 2 and 40 days postexposure, but no differences were noted between CO and AIR rats. Baseline pulmonary vascular resistance and pulmonary arterial pressure (in isolated lungs) were higher in ALT rats at all four ages compared to the other three groups. Both the ALT and CO rats displayed hypertrophy of the right ventricle (RV) and the left ventricle (LV) at the termination of treatment and elevated hematocrit. LV hypertrophy and polycythemia regressed with time, but RV hypertrophy remained significant in the ALT rats through 112 days postexposure. The results indicate that neonatal exposure to ALT, but not CO, causes a persistent increase in pulmonary vascular responsiveness and RV hypertrophy for at least 112 days after termination of the exposure.

Collagen and elastin metabolism in hypertensive pulmonary arteries of rats

We evaluated the processes controlling the accumulation of collagen and elastin in main pulmonary arteries of rats during an episode of hypoxic pulmonary hypertension. Explant cultures of main pulmonary arteries were incubated with [3H]proline to measure collagen and protein synthesis and percent collagen synthesis. Elastin synthesis was measured by [14C]valine incorporation into insoluble elastin. Relative collagen synthesis increased twofold (from 1.1 +/- 0.2 x 10(3) to 2.0 +/- 1.0 x 10(3) disintegrations per minute [14C]hydroxyproline/vessel/hr/mg protein), relative collagen synthesis doubled (from 2% to 4-5% of total protein synthesis), and elastin synthesis increased ninefold (from 0.4 +/- 0.2 x 10(4) to 3.6 +/- 0.6 x 10(4) dpm [14C]valine/vessel/hr/mg protein) in early hypertension. The level of pro alpha l(I) collagen RNA paralleled the relative collagen synthetic rate during the study period. Within 7 days of recovery from hypoxia, collagen and elastin contents were normal. We conclude that collagen and elastin in main pulmonary arteries are synthesized rapidly during an episode of hypoxic pulmonary hypertension and that collagen and elastin are rapidly removed from the hypertensive vessel during normoxic recovery.

Adult subacute mountain sickness--a syndrome of congestive heart failure in man at very high altitude

A new type of mountain sickness is described. 21 men (age 22.2, standard deviation [SD] 1.8 years) had severe congestive heart failure with oedema and ascites after 10.8 (SD 5.9) weeks at altitudes of 5800-6700 m. Investigation, within 3 days of transfer to 300 m, showed polycythaemia, cardiomegaly with right ventricular enlargement, and (in 17) pericardial effusion. The heart failure resolved rapidly after transfer from high altitude.

Acute and chronic pulmonary pressor responses to hypoxia: the role of blunting in acclimatization

We studied two strains of Sprague-Dawley rats: the Madison (M) that acclimatizes successfully to high altitude; and the Hilltop (H), that manifests signs of chronic mountain sickness at high altitude and has a high mortality rate. Awake, chronically instrumented animals were tested at sea level, at intervals during 30 days at a simulated altitude of 5500 m, and during 10 to 15 days of recovery at sea level. Mean pulmonary artery pressure (PAP) rose at high altitude to reach 60 mm Hg in H and 40 mm Hg in M, but the acute pressor response to hypoxia at sea level was much more pronounced in M than H. Depression of PAP by normoxic exposures in H rats at high altitude was slightly early in the period of stay but was enhanced with further prolongation of high altitude residence. The M rats, in contrast, had a blunted response (normoxia had very little depressant effect on PAP) after the first 24 h at high altitude, and it remained so for the duration of the stay. On return to sea level the response of H rats remained unchanged for 7 days, but the blunted response of the M rats at high altitude reversed at sea level to become exaggerated. We conclude: that responses of PAP to acute hypoxia do not forecast what the chronic response will be; that the appearance of an unidentified mechanism during chronic hypoxia in the M strain attenuates the vasoreactivity of the pulmonary vessels to hypoxia; and that the absence of such a blunting mechanism in H leads to the higher PAP in this strain and its morbid consequences. The hypothesis is put forward that the existence of such a blunting mechanism is an important factor in the adaptability of species to high altitude.

Myocardial capillarity in acclimation to hypoxia

Capillarity, O2 diffusion distances and fiber cross-sectional growth were measured in the hearts of guinea pigs exposed early during growth to hypobaric hypoxia (PB = 430 torr, PO2 = 90 torr). Twelve 5-week old males were maintained in a hypobaric chamber for 4-14 weeks. Their hearts were perfusion-fixed via the aorta with a 2.5% glutaraldehyde, 1% formaldehyde buffered solution; blocks were cut from left (LV) and right (RV) ventricles, post-fixed in OsO4, dehydrated and embedded in Spurr medium. Blocks were cut transversely to fiber orientation, 0.5 micron thick, stained with Toluidine Blue, and photographed at 400 X. Number and location of capillaries and fiber cross-sectional areas (FCSA) were scored from these photographs and from those of normoxic controls. Growth rates were similar for control and hypoxic guinea pigs. As animals grew, LV and RV weights increased linearly with body weight. Hypoxic guinea pigs had LV weights similar to controls but the RV showed varying degrees of hypertrophy. Control and hypoxic guinea pigs showed similar linear increases in FCSA with ventricular weight, suggesting that hypertrophy was due to increased FCSA. Capillary density (CD) decreased and capillary-to-fiber ratio (C:F) increased with FCSA, and O2 diffusion distances lengthened in LV and RV of animals in both groups. CD and C:F were higher and O2 diffusion distances were shorter in most hypoxic animals compared to controls. When RV hypertrophy was large (RV greater than 0.7 g) and failure imminent, CD, C:F and O2 diffusion distances were similar to controls suggesting that in these hearts oxygenation was impaired.

The role of pulmonary vascular responses to chronic hypoxia in the development of chronic mountain sickness in rats

A strain of Sprague-Dawley rat obtained from Hilltop Labs, Scottsdale, PA (H rats), develops more severe pulmonary hypertension, right ventricular hypertrophy, and polycythemia than a strain obtained from Madison, WI (M rats), following exposure to simulated high altitude. We sought to determine whether differences in pulmonary vascular responses to chronic hypoxia could explain the differing high altitude susceptibilities of the two strains. Vasoconstrictor responses to hypoxia and angiotensin II were tested in blood perfused lungs isolated from rats of both groups exposed to stimulated high altitude (4 to 5 weeks, 0.5 atm), or from sea level controls. Pressure-flow curves, serving as an index of 'passive' vascular resistance, were also determined in the isolated lungs. Vasoconstrictor responses to hypoxia were blunted in high altitude rats of both the H and M strains compared to sea level controls, and the H sea level rats had blunted vasoconstrictor responses to hypoxia compared to the M sea level rats. Vasoconstrictor responses to angiotensin II were similar among the groups and were unaffected by chronic high altitude exposure. Pressure-flow curves were greater in both high altitude groups than in the sea level groups, and those of the H high altitude rats were slightly greater than those of the M high altitude rats. Thus, differences in vasoconstrictor responses to hypoxia do not explain the greater pulmonary hypertension of H high altitude rats. However, greater 'passive' vascular resistance, probably due to more extensive structural remodeling of pulmonary vessels, does appear to contribute to the greater pulmonary hypertension of the H rats.

Ventilatory responses and blood gases in susceptible and resistant rats to high altitude

On exposure to a stimulated altitude of 5500 m (18 000 ft), the Hilltop (H) strain of Sprague-Dawley rats develops signs of chronic mountain sickness (CMS) (severe polycythemia, severe pulmonary hypertension and right ventricular hypertrophy) associated with a high mortality rate. In contrast, the Madison (M) strain of Sprague-Dawley rats remains healthy with less severe cardiopulmonary and hematological responses. We tested the hypothesis that hypoventilation in the H rats relative to the M rats, leading to greater alveolar hypoxia or hypoxemia, could account for the different hematological and cardiopulmonary responses between the two strains. Ventilatory responses and blood gases were compared under normoxia and acute and chronic hypoxia in fully awake and unrestrained animals of the two strains. There were no differences in VE, Pao2, PaCO2, pHa, P-vO2, PvCO2 and pH-v under either acute or chronic hypoxia between the two strains of rats. It is concluded that relative hypoventilation does not contribute to altitude susceptibility in H rats.

Hypoxic pulmonary vasoconstriction in chronically hypoxic rats

Reactivity of lung vessels to acute hypoxia was found increased or decreased in chronically hypoxic (CH) rats by different authors. We examined severity and duration of hypoxia and age as possible explanations. Isolated blood-perfused lungs of CH rats ventilated with low-O2 mixtures were compared with control (C) rats. Juvenile CH rats (10 or 12% O2, 3 weeks) showed increased reactivity; reactivity of mature CH rats (10% O2, 3 weeks) was not significantly different from controls. Reactivity declined with age in CH but not C rats. Juvenile and mature rats exposed to 10% O2 for 48 h showed reduced reactivity; after 3 weeks but not 48 h hypoxia, pulmonary arterioles are narrowed by new muscle. The pressure/flow relationship differed in CH from C rats in that resistance and critical closing pressure (CCP) were greater during normoxia. Hypoxia caused increased resistance and CCP in C rats but mainly an increase in CCP in CH rats. The difference may be attributable to muscularisation of collapsible alveolar vessels in chronic hypoxia.

Comparison of muscular pulmonary arteries in low and high altitude hamsters and rats

Measurements were made of the ratio of medial area to total arterial area in the muscular pulmonary arteries of a fossorial species, the hamster, and a non-fossorial control, the rat. It was found that the medial area was less in hamsters than in rats. In addition, chronic hypoxic exposure resulted in further medial thickening in rats, while the hamster was unaffected. The hamster has previously been shown to have a blunted hypoxic pulmonary pressor response, and the present data indicate that this relative unresponsiveness is associated with less pulmonary vascular smooth muscle. In addition, the insensitivity to hypoxia of the pulmonary vasculature and a lack of vascular luminal narrowing may tend to minimize increases in pulmonary vascular resistance during chronic hypoxia in the hamster, and thus be beneficial adaptations for a burrowing species.