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

5-Hydroxymethylfurfural reduces skeletal muscle superoxide production and modifies force production in rats exposed to hypobaric hypoxia

Decreased blood-tissue oxygenation at high altitude (HA) increases mitochondrial oxidant production and reduces exercise capacity. 5-Hydroxymethylfurfural (5-HMF) is an antioxidant that increases hemoglobin's binding affinity for oxygen. For these reasons, we hypothesized that 5-HMF would improve muscle performance in rats exposed to a simulated HA of ~5500 m. A secondary objective was to measure mitochondrial activity and dynamic regulation of fission and fusion because they are linked processes impacted by HA. Fisher 344 rats received 5-HMF (40 mg/kg/day) or vehicle during exposure to sea level or HA for 72 h. Right ankle plantarflexor muscle function was measured pre- and post-exposure. Post-exposure measurements included arterial blood gas and complete blood count, flexor digitorum brevis myofiber superoxide production and mitochondrial membrane potential (ΔΨm), and mitochondrial dynamic regulation in the soleus muscle. HA reduced blood oxygenation, increased superoxide levels and lowered ΔΨm, responses that were accompanied by decreased peak isometric torque and force production at frequencies >75 Hz. 5-HMF increased isometric force production and lowered oxidant production at sea level. In HA exposed animals, 5-HMF prevented a decline in isometric force production at 75-125 Hz, prevented an increase in superoxide levels, further decreased ΔΨm, and increased mitochondrial fusion 2 protein expression. These results suggest that 5-HMF may prevent a decrease in hypoxic force production during submaximal isometric contractions by an antioxidant mechanism.

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.

Systemic Oxygen Transport with Rest, Exercise, and Hypoxia: A Comparison of Humans, Rats, and Mice

The objective of this article is to compare and contrast the known characteristics of the systemic O₂ transport of humans, rats, and mice at rest and during exercise in normoxia and hypoxia. This analysis should help understand when rodent O₂ transport findings can-and cannot-be applied to human responses to similar conditions. The O₂ -transport system was analyzed as composed of four linked conductances: ventilation, alveolo-capillary diffusion, circulatory convection, and tissue capillary-cell diffusion. While the mechanisms of O₂ transport are similar in the three species, the quantitative differences are naturally large. There are abundant data on total O₂ consumption and on ventilatory and pulmonary diffusive conductances under resting conditions in the three species; however, there is much less available information on pulmonary gas exchange, circulatory O₂ convection, and tissue O₂ diffusion in mice. The scarcity of data largely derives from the difficulty of obtaining blood samples in these small animals and highlights the need for additional research in this area. In spite of the large quantitative differences in absolute and mass-specific O₂ flux, available evidence indicates that resting alveolar and arterial and venous blood PO₂ values under normoxia are similar in the three species. Additionally, at least in rats, alveolar and arterial blood PO₂ under hypoxia and exercise remain closer to the resting values than those observed in humans. This is achieved by a greater ventilatory response, coupled with a closer value of arterial to alveolar PO₂ , suggesting a greater efficacy of gas exchange in the rats. © 2018 American Physiological Society. Compr Physiol 8:1537-1573, 2018.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Rat strain differences in pulmonary artery smooth muscle Ca(2+) entry following chronic hypoxia

Effects of chronic hypoxia (CH) on store- and receptor-operated Ca(2+) entry (SOCE, ROCE) in pulmonary vascular smooth muscle (VSM) are controversial, although whether genetic variation explains such discrepancies in commonly studied rat strains is unclear. Since protein kinase C (PKC) can inhibit Ca(2+) permeable nonselective cation channels, we hypothesized that CH differentially alters PKC-dependent inhibition of SOCE and ROCE in pulmonary VSM from Sprague-Dawley and Wistar rats. To test this hypothesis, we examined SOCE and endothelin-1 (ET-1)-induced ROCE in endothelium-disrupted, pressurized pulmonary arteries from control and CH Sprague-Dawley and Wistar rats. Basal VSM Ca(2+) was elevated in CH Wistar, but not Sprague-Dawley, rats. Further, CH attenuated SOCE in VSM from Sprague-Dawley rats, while augmenting this response in Wistar rats. CH reduced ROCE in arteries from both strains. PKC inhibition restored SOCE in CH Sprague-Dawley arteries to control levels, while having no effect on SOCE in Wistar arteries or on ROCE in either strain. We conclude that effects of CH on pulmonary VSM SOCE are strain dependent, whereas inhibitory effects of CH on ROCE are strain independent. Further, PKC inhibits SOCE following CH in Sprague-Dawley, but not Wistar, rats but does not contribute to ET-1-induced ROCE in either strain.

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

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

EDHF contributes to strain-related differences in pulmonary arterial relaxation in rats

Pulmonary arteries from the Madison (M) strain relax more in response to acetylcholine (ACh) than those from the Hilltop (H) strain of Sprague-Dawley rats. We hypothesized that differences in endothelial nitric oxide (NO) synthase (eNOS) expression and function, metabolism of ACh by cholinesterases, release of prostacyclin, or endothelium-derived hyperpolarizing factor(s) (EDHF) from the endothelium would explain the differences in the relaxation response to ACh in isolated pulmonary arteries. eNOS mRNA and protein levels as well as the NO-dependent relaxation responses to thapsigargin in phenylephrine (10(-6) M)-precontracted pulmonary arteries from the M and H strains were identical. The greater relaxation response to ACh in M compared with H rats was also observed with carbachol, a cholinesterase-resistant analog of ACh, a response that was not modified by pretreatment with meclofenamate (10(-5) M). N(omega)-nitro-L-arginine (10(-4) M) completely abolished carbachol-induced relaxation in H rat pulmonary arteries but not in M rat pulmonary arteries. Precontraction with KCl (20 mM) blunted the relaxation response to carbachol in M rat pulmonary arteries and eliminated differences between the M and H rat pulmonary arteries. NO-independent relaxation present in the M rat pulmonary arteries was significantly reduced by 17-octadecynoic acid (2 microM) and was completely abolished by charybdotoxin plus apamin (100 nM each). These findings suggest that EDHF, but not NO, contributes to the strain-related differences in pulmonary artery reactivity. Also, EDHF may be a metabolite of cytochrome P-450 that activates Ca(2+)-dependent K(+) channels.

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

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

Renovascular adaptive changes in chronic hypoxic polycythemia

BACKGROUND

Chronic hypoxia in rats produces polycythemia, and the plasma fraction falls, reducing renal plasma flow (RPF) relative to renal blood flow (RBF). Polycythemia also causes increased blood viscosity, which tends to reduce RBF and renal oxygen delivery. We studied how renal regulation of electrolyte balance and renal tissue oxygenation (which is crucial for erythropoietin regulation) are maintained in rats during hypoxic exposure.

METHODS

Rats of two strains with differing polycythemic responses, with surgically implanted catheters in the urinary bladder, femoral artery, and left renal and right external jugular veins, were exposed to a simulated high altitude (0.5 atm) for 0, 1, 3, 14, and 30 days, after which RPF (para-aminohippurate clearance), glomerular filtration rate (GFR, polyfructosan clearance), hematocrit and blood gases were measured, and RBF, renal vascular resistance and hindrance (resistance/viscosity), renal oxygen delivery, and renal oxygen consumption were calculated.

RESULTS

During chronic hypoxia RBF increased, but RPF decreased because of the polycythemia. GFR remained normal because the filtration fraction (FF) increased. Renal vascular resistance decreased, and renal vascular hindrance decreased more markedly. Renal oxygen delivery and consumption both increased.

CONCLUSIONS

During chronic hypoxia GFR homeostasis apparently took precedence over RBF autoregulation. The large decrease in renal vascular hindrance suggested that renal vascular remodeling contributes to GFR regulation. The reduced hindrance also prevented a vicious cycle of increasing polycythemia and blood viscosity, decreasing RBF, and increasing renal hypoxia and erythropoietin release.

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.

The relationship between systemic hypertension and obstructive sleep apnea: facts and theory

This article provides an in-depth overview of the relationship between primary hypertension and adult obstructive sleep apnea syndrome. The background data and research are taken from the English-language literature through 1993. Primary hypertension is a common cause of major medical illnesses, including stroke, heart disease, and renal failure, in middle-aged males. Its prevalence in the United States is around 20%, with the rate of newly diagnosed hypertensive patients being about 3% per year. Sleep apnea syndrome is common in the same population. It is estimated that up to 2% of women and 4% of men in the working population meet criteria for sleep apnea syndrome. The prevalence may be much higher in older, non-working men. Many of the factors predisposing to hypertension in middle age, such as obesity and the male sex, are also associated with sleep apnea. Recent publications describe a 30% prevalence of occult sleep apnea among middle-aged males with so called "primary hypertension." Is this association fortuitous, related to a high prevalence of both diseases in the same population, or is it caused by a factor common to both diseases, such as obesity? Should the diagnosis of apnea be actively sought with sleep studies in hypertensive populations? If a diagnosis of "asymptomatic" sleep apnea is made in a hypertensive person, should the apnea be treated? Current research data provide only partial answers to these and other questions regarding the association of apnea and hypertension. Logic dictates that clinically symptomatic patients in hypertensive clinics should receive appropriate evaluation for apnea, but broad populations of hypertensive individuals should not be referred for sleep studies.

Distribution of pulmonary blood flow in conscious resting rats

The pattern of pulmonary blood flow (PBF) distribution was determined in the rat, in which lung gravitational forces are minimal. Microspheres were infused into the inferior vena cava of 15 conscious, and 5 anesthetized rats. Relative scatter of specific PBF [(sample activity/sample weight)/(total activity/total weight)] in 28 lung samples was calculated. In 5 of the conscious rats, consecutive determinations were made 30 min apart. In 5 anesthetized rats, PBF was determined in prone and supine positions. Relative scatter of specific PBF varied from 0.84 to 1.12, with PBF being distributed preferentially to the hilar, central regions. There was a high correlation between consecutive measurements: y = 0.88 x +0.11 (n = 140, r = 0.92). By changing from prone to supine position, PBF to the topmost regions increased, and that to the lowermost regions decreased, by only 3 percent. The results indicate that in the conscious resting rat, PBF has a small but significant preferential distribution to the hilar, central regions, with lower blood flow to the peripheral regions of the lung.

Hematologic responses and the early development of hypoxic pulmonary hypertension in rats

We have previously described the development of greater right ventricular hypertrophy after 7 days of hypoxia in the altitude-susceptible H strain compared to the resistant M strain of Sprague-Dawley rat. Greater polycythemia also occurs in the H strain after 2-3 weeks of hypoxia and is characterized by increased mean red cell volume (MCV), reticulocyte count (Retic), and blood viscosity after 4 weeks of hypoxia. In the present study, we determined the time course of development of these hematologic responses, whether differences in MCV are associated with differences in red cell deformability, and whether the hematologic differences might contribute to the early cardiopulmonary differences between the strains. We found that although hematocrit (Hct) did not differ between the strains until 21 days of hypoxia, MCV and Retic were greater in the H strain after only 3 days and whole blood viscosity was greater after 7 days. However, no differences in the viscosity or deformability of reconstituted red cells (Hcts 10% and 25%) were apparent at any time during hypoxic exposure. Furthermore, pressure-flow curves obtained using blood and lungs isolated from 7-day hypoxic rats suggested that the largest component of pressure elevation in the H rats was related to pulmonary vascular rather than hematologic factors. We conclude that although H rats have exaggerated hematologic responses to hypoxia, differences in pulmonary vascular structure and tone are more likely to be responsible for the strain differences in cardiopulmonary responses occurring after 7 days of hypoxia.

Exogenous erythropoietin fails to augment hypoxic pulmonary hypertension in rats

In two rat strains (H and M) with differing susceptibilities to chronic hypoxia we examined the role of polycythemia in the differing hypoxic pulmonary hemodynamic responses. We hypothesized that augmentation of hematocrit (Hct) during hypoxia in the resistant M strain would render cardiopulmonary responses similar to those obtained in the susceptible H strain. Administration of human recombinant erythropoietin (EPO) in doses of 100, 250 and 500 U.kg-1 s.c. thrice weekly for three weeks raised Hct similarly in both strains indicating that normoxic rats had similar sensitivities to EPO. In rats exposed to hypobaric hypoxia (0.5 atm) for 21 days, EPO (500 U.kg-1 thrice weekly) significantly increased Hct and whole blood viscosity as expected. Surprisingly, right ventricular (RV) to body weight (BW) ratio as an index of right ventricular hypertrophy (RVH) and RV peak systolic pressure did not increase in EPO-injected rats of either strain compared to hypoxic controls. Among hypoxic animals, Hct correlated highly with viscosity but not with RV/BW. We conclude, contrary to our hypothesis, that polycythemia does not appear to be responsible for the strain difference in RVH and pulmonary hypertension.

Effects of atrial natriuretic factor in chronic hypoxic spontaneously hypertensive rats

The present study was designed first to investigate the pulmonary hypertensive effects of chronic hypoxia in spontaneously hypertensive rats and second to compare the cardiovascular effects of atrial natriuretic factor on rats exposed to hypoxia and on control rats kept at sea level. Catheters were placed in the femoral and pulmonary arteries for measurement of mean systemic arterial pressure and mean pulmonary arterial pressure. The cardiac output was measured by thermodilution method. It was found that 4 weeks of simulated 18,000-foot hypoxia led to polycythemia, right ventricular hypertrophy, and pulmonary hypertension, which resulted from an increased pulmonary vascular resistance. However, systemic arterial pressure was not significantly different between the two groups of rats. Atrial natriuretic factor administration decreased systemic arterial pressure and pulmonary arterial pressure to a lesser extent in the hypoxic group compared with the sea level control group. It is concluded that these animals showed an impaired response to atrial natriuretic factor after long-term exposure to hypoxia.

Effect of heparin and warfarin on chronic hypoxic pulmonary hypertension and vascular remodeling in the guinea pig

Chronic hypoxia produces pulmonary hypertension and pulmonary vascular remodeling. Heparin partially prevents the rise in right ventricular pressure and vascular remodeling in chronically hypoxic mice. To determine if this is due to the anticoagulant property of heparin or another property, we compared the effect of oral warfarin given at an anticoagulating dose (0.5 mg/kg/day) to heparin given by continuous infusion at a dose that does not prolong the partial thromboplastin time (PTT) (20 units/kg/h) on hypoxic pulmonary hypertension and vascular remodeling in the guinea pig. Normoxic control animals either untreated or treated with heparin or Coumadin were all alike in blood gases, pulmonary vascular resistance, right heart weights, and pulmonary histology. Hypoxia (10% 0(2) for 10 days) induced similar and significant increases in mean pulmonary artery (PA) pressure in both the hypoxic control and warfarin groups (19 +/- 1 mm Hg (mean +/- SEM) in both groups versus 11 +/- 0.1 mm Hg in the normoxic control group; p less than 0.05). Total pulmonary vascular resistance (TPR) was also increased from 0.041 +/- 0.002 in the normoxic control group to 0.087 +/- 0.007 and 0.071 +/- 0.003 mm Hg/ml/min/kg in the hypoxic control and warfarin groups, respectively (p less than 0.05). Whereas anticoagulation with warfarin did not protect the guinea pig from developing pulmonary hypertension, heparin markedly reduced PA and TPR (15 +/- 1 mm Hg and 0.052 +/- 0.002 mm Hg/ml/min/kg, respectively; p less than 0.05 versus hypoxic control or warfarin).(ABSTRACT TRUNCATED AT 250 WORDS)

Time course of cardiopulmonary responses to high altitude in susceptible and resistant rat strains

We have identified two strains (H and M) of Sprague-Dawley rat with distinctly different susceptibilities and cardiopulmonary responses to hypoxia. In this study, we studied the development of cardiopulmonary and hematological responses to hypoxia and the post-hypoxic regression of these responses in the two strains over time. Under sea level conditions, there were no differences between the two strains. On exposure to hypobaric hypoxia (0.5 atm), right ventricular peak systolic pressure (RVPP) and right ventricular hypertrophy (RVH) increased more rapidly in the susceptible (H) than in the resistant (M) strain. In contrast, post-hypoxic reversal of these changes occurred at comparable rates. Hematocrits rose at similar rates in the two strains until after two weeks, when that of the H strain slightly exceeded that of the M strain. With the progression of RVH, left ventricular plus septal to body weight ratio (LV + S) g/100 g bw decreased in M rats but increased in the H rats. As a result, a conspicuous overall cardiac hypertrophy developed in the H rats but only a minimal cardiac hypertrophy occurred in the M strain. The data show that susceptibility to hypoxia in H rats is associated with more rapid development of RV systolic hypertension and biventricular hypertrophy than in M rats. The mechanism for the accelerated cardiopulmonary responses in the H rats most likely involves greater hypoxic pulmonary vasoconstriction or pulmonary vascular remodeling. Differences in hematocrit between the strains do not contribute to the early cardiopulmonary responses.

Reticulocytosis, increased mean red cell volume, and greater blood viscosity in altitude susceptible compared to altitude resistant rats

We have identified two strains (H and M) of Sprague-Dawley rat with markedly different susceptibilities and cardiopulmonary responses to chronic hypobaria. To further characterize factors responsible for these differing cardiopulmonary responses to chronic hypobaria, the present study examined differences in hematologic responses between the strains and assessed the contribution of differences in blood viscosity to differences in pulmonary vascular resistance. Following a 4-5 week exposure to simulated high altitude (0.5 atm), hemoglobin, hematocrit, mean red cell volume, and reticulocyte count were all increased in the susceptible H compared to the resistant M rats, whereas red blood cell counts were similar. Sea level controls manifested no differences. Blood viscosity, measured in a capillary viscometer, was 53% greater in chronically hypoxic H than in M rats, and plasma viscosities were similar. Blood from high altitude H rats increased pulmonary vascular resistance more than blood from high altitude M rats when perfused into lungs isolated from high altitude rats of either strain. In conclusion, high altitude H rats have an increased population of immature red cells, leading to a greater mean red cell volume and hematocrit than in high altitude M rats. These hematologic differences contribute to the the increased blood viscosity and greater pulmonary vascular resistance of H compared to M rats after 4 weeks' high altitude exposure.