Cellular responses to hypoxia in the pulmonary circulation

Nocturnal Hypoxemia, But Not Sleep Apnea, Is Associated With a Poor Prognosis in Patients With Pulmonary Arterial Hypertension

BACKGROUND

Sleep apnea (SA) can cause repeated nocturnal arterial oxygen desaturation and result in acute increase in pulmonary arterial pressure (PAP). The presence of SA is associated with a poor prognosis in patients with chronic left-sided heart failure, but little is known for patients with pulmonary arterial hypertension (PAH). Methods and Results: We enrolled 151 patients with PAH (44±16 years old, male/female=37/114). They were all in the Nice Classification group 1 (idiopathic PAH/associated PAH=52/48%, mean PAP of 46±16 mmHg). They underwent right-heart catheterization and a sleep study with simplified polysomnography. Averaged percutaneous oxygen saturation (SpO₂) during sleep was measured and an apnea-hypopnea index >5 was defined as SA. SA was noted in 58 patients (obstructive SA/central SA: 29/29). Over an average follow-up of 1,170±763 days, 32 patients died. By Kaplan-Meier analysis, there was no significant difference in deaths of patients with and without SA (χ²=2.82, P=0.093). On the other hand, the mortality in patients with lower averaged SpO₂ was significantly higher than in those with higher averaged SpO₂ (χ²=14.7, P<0.001) and that was the only independent variable related to death in multivariate Cox proportional hazards analysis.

CONCLUSIONS

SA in patients with PAH was not associated with worse prognosis, unlike left ventricular heart failure, but nocturnal hypoxemia was related to poor prognosis.

Oxygen sensing strategies in mammals and bacteria

The ability to sense and adapt to changes in pO2 is crucial for basic metabolism in most organisms, leading to elaborate pathways for sensing hypoxia (low pO2). This review focuses on the mechanisms utilized by mammals and bacteria to sense hypoxia. While responses to acute hypoxia in mammalian tissues lead to altered vascular tension, the molecular mechanism of signal transduction is not well understood. In contrast, chronic hypoxia evokes cellular responses that lead to transcriptional changes mediated by the hypoxia inducible factor (HIF), which is directly controlled by post-translational hydroxylation of HIF by the non-heme Fe(II)/αKG-dependent enzymes FIH and PHD2. Research on PHD2 and FIH is focused on developing inhibitors and understanding the links between HIF binding and the O2 reaction in these enzymes. Sulfur speciation is a putative mechanism for acute O2-sensing, with special focus on the role of H2S. This sulfur-centered model is discussed, as are some of the directions for further refinement of this model. In contrast to mammals, bacterial O2-sensing relies on protein cofactors that either bind O2 or oxidatively decompose. The sensing modality for bacterial O2-sensors is either via altered DNA binding affinity of the sensory protein, or else due to the actions of a two-component signaling cascade. Emerging data suggests that proteins containing a hemerythrin-domain, such as FBXL5, may serve to connect iron sensing to O2-sensing in both bacteria and humans. As specific molecular machinery becomes identified, these hypoxia sensing pathways present therapeutic targets for diseases including ischemia, cancer, or bacterial infection.

Predictors of Nocturnal Oxygen Desaturation in Pulmonary Arterial Hypertension

BACKGROUND

Sleep may be associated with significant respiratory compromise in patients with lung disease and can result in hypoxia. In patients with pulmonary arterial hypertension (PAH), nocturnal desaturation may not be reflected in daytime evaluations of oxygenation and can lead to worsening pulmonary hemodynamics. The study was conducted to determine the prevalence and significance of nocturnal oxygen desaturation in patients with PAH.

METHODS

A cross-sectional study conducted at the Cleveland Clinic. Patients were followed up at our institution except for the overnight oximetry study done at home. Data regarding degree of nocturnal desaturation, demographics, hemodynamics, pulmonary function, and functional capacity were collected.

RESULTS

Forty-three patients (mean age, 47.9 +/- 13.5 years [+/- SD]; 36 women and 7 men) underwent nocturnal oximetry. The etiology of PAH included idiopathic PAH (88%) and PAH associated with connective tissue diseases (12%). The majority of patients were New York Heart Association functional class II (42%) or III (53%). Thirty patients (69.7%) spent > 10% of sleep time with oxygen saturation by pulse oximetry < 90%. Desaturators were older (p = 0.024) and had higher hemoglobin (p = 0.002). Sixteen of 27 patients (59%) without desaturation < 90% during a 6-min walk test were nocturnal desaturators. Nocturnal desaturators had higher brain natriuretic protein (p = 0.004), lower cardiac index (p = 0.03), and higher mean right atrial pressure (p = 0.09), mean pulmonary artery pressure, and pulmonary vascular resistance. On echocardiography, desaturators were more likely to have moderate or severe right ventricular dilation (p = 0.04) and pericardial effusion. Only one patient had significant sleep apnea.

CONCLUSIONS

Nocturnal hypoxemia is common in PAH patients and correlates with advanced pulmonary hypertension and right ventricular dysfunction. Approximately 60% patients without exertional hypoxia had nocturnal desaturation. Overnight oximetry should be considered in the routine workup of PAH patients who do not demonstrate exertional desaturation.

Assessment of Tc-99m sestamibi tumor tissue uptake under the influence of increased arterial oxygen saturation

The aim of this study is to investigate the potential effect of increased arterial saturation of oxygen in the magnitude of technetium-99m-hexakis-isobutyl-isonitrile (99mTc-MIBI) uptake in tumor tissue and to compare the results with those of conventional 99mTc-MIBI tumor imaging in the same patient with lung cancer. A total of 26 lung cancer patients underwent initial SPECT (I-SPECT) and after oxygen inhalation SPECT (O2-SPECT). The early (ER) and delayed ratios (DR) of O2-SPECT mean+/-S.D. were significantly higher than those of I-SPECT mean+/-S.D. (2.07+/-0.57 vs. 1.90+/-0.48 and 2.02+/-0.45 vs. 1.83+/-0.39, respectively). There are increased rates for the relative tumor uptake of 99mTc-MIBI by 9% for ER and 10% for DR on the O2-SPECT compared to I-SPECT. We found that 99mTc-MIBI uptake increased significantly in the tumor tissue with O2-SPECT compared to I-SPECT. In conclusion, 99mTc-MIBI scintigraphy is one of the main methods for differentiating viable and nonviable tissue fractions in tumors. We consider that 99mTc-MIBI uptake increase after oxygen inhalation is an indicator of positive acute cellular response of the tumor tissue to the rising tissue oxygen level.

Microvascular and tubulointerstitial injury associated with chronic hypoxia-induced hypertension

BACKGROUND

Rats submitted to chronic hypoxia develop hypertension that persists despite cessation of the hypoxia or correction of the hematocrit. We examined whether chronic hypoxia might induce subtle renal injury since studies in other animal models of hypertension suggest this may cause persistent hypertension.

METHODS

Chronic hypoxia was induced in rats by placement in a hypobaric chamber for up to 24 days. Blood pressure and kidney biopsies were performed at baseline, 6 hours, 24 hours, and 24 days of hypoxia.

RESULTS

Chronic hypoxia induced hypertension and erythrocytosis at 24 days. Acute hypoxia was associated with endothelial cell swelling in arterioles (6 and 24 hours), followed by thickening of the arterioles at 24 days. Subtle tubulointerstitial injury and inflammation occurred and was progressive. The influx of macrophages increased steadily over the 24 days and was associated with a progressive increase in interstitial collagen III deposition. Hypoxia was associated with increased tubular expression of osteopontin as early as 6 hours, the same period when an increase of proximal tubular cell proliferation occurred. Interstitial cell proliferation peaked twice, at 6 hours and at 24 days. Glomerular hypertrophy was manifest at 24 days.

CONCLUSION

Both afferent arteriolar disease and subtle tubulointerstitial inflammation and injury occur early in hypoxic rats. These changes may predispose these animals to persistent hypertension.

Intracellular and extracellular calcium utilization during hypoxic vasoconstriction of cyclostome aortas

Hypoxic vasoconstriction (HV) is an intrinsic response of mammalian pulmonary and cyclostome aortic vascular smooth muscle. The present study examined the utilization of calcium during HV in dorsal aortas (DA) from sea lamprey and New Zealand hagfish. HV was temporally correlated with increased free cytosolic calcium (Ca2+c) in lamprey DA. Extracellular calcium (Ca2+o) did not contribute significantly to HV in lamprey DA, but it accounted for 38.1 +/- 5.3% of HV in hagfish DA. Treatment of lamprey DA with ionomycin, ryanodine, or caffeine added to thapsigargin-reduced HV, whereas HV was augmented by BAY K 8644. Methoxyverapamil (D600) in zero Ca2+o did not affect HV in lamprey DA, nor did it prevent further constriction when Ca2+o was restored during hypoxia in hagfish DA. Removal of extracellular sodium (Na+o) caused a constriction in both species. Lamprey DA relaxed to prehypoxic tension following return to normoxia in zero Na+o, whereas relaxation was inhibited in hagfish DA. Relaxation following HV was inhibited in lamprey DA when Na+o and Ca2+o were removed. These results show that HV is correlated with [Ca2+]c in lamprey DA and that Na+/Ca2+ exchange is used during HV in hagfish but not lamprey DA. Multiple receptor types appear to mediate stored intracellular calcium release in lamprey DA, and L-type calcium channels do not contribute significantly to constriction in either cyclostome.