Introduction
Recent studies have shown that intermittent normobaric hypoxia (INH) lowers blood pressure in patients with essential arterial hypertension [1] and normalises cerebral blood flow while correcting cognitive impairment and dyssomnia [2]. The mechanisms underlying these effects are currently being investigated intensively, including the effects of INH on vascular tone regulation.
Current understanding assigns a key role in essential hypertension to endothelial dysfunction and injury [3]. These are caused in part by deficient antioxidant enzymes [4], leading to increased accumulation of reactive oxygen species, or oxidative stress, neutralisation of nitric oxide by these species, and peroxidative injury to the vascular wall. We previously showed that INH sessions increase reactive oxygen species production and thereby induce an adaptive increase in the activity of the principal intracellular antioxidant enzymes in healthy people [5]. Together with evidence that hypoxia induces biosynthesis of angiogenic factors activating endothelial cell proliferation [6], these findings suggested that adaptive circulatory changes under INH in hypertension may involve reduced reactive oxygen species damage to endothelial cells and activation of endothelial repair.
The study therefore aimed to evaluate the effects of INH on markers of endothelial injury and repair and on oxidative homeostasis in essential hypertension.
Materials and methods
Participants were patients with grade I essential hypertension, nine men and 21 women aged 44±2 years, with no more than one or two additional cardiovascular risk factors according to the DAG 1, 2000 classification. They received no antihypertensive medication for at least three weeks before enrolment or during the study. No patient withdrew early for clinical reasons. The control group comprised 30 normotensive volunteers, ten men and 20 women aged 40±4 years, without documented chronic circulatory disease.
INH was administered using the Bio-Nova-204 system. The course comprised 20 daily sessions, each containing six ten-minute cycles alternating inhalation of a hypoxic gas mixture containing 10–12 vol.% oxygen with atmospheric air. During the first three or four sessions, hypoxic inhalation was increased from one to five minutes; in subsequent sessions, five minutes of hypoxic inhalation alternated with five minutes of atmospheric air.
Before INH and one, two, four and fourteen days after the final session, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) concentrations were determined in all participants by enzyme-linked immunosorbent assay using Cytimmune Sciences Inc. reagents (USA). In hypertensive patients, measurements before and after INH included blood pressure by the Korotkoff method and bulbar conjunctival microcirculation by noninvasive microscopy [7]. Plasma tests assessed total prooxidant activity (TPA), thiobarbituric acid-reactive products (TBARP) [8] and endothelial microparticles (EMP) by flow cytometry using FITC-labelled anti-CD31 antibodies from Caltag Laboratories (USA). Erythrocyte superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) activities were also measured [8].
Results and discussion
After INH, both systolic and diastolic blood pressure decreased: respectively, from 155.6±1.5 to 137.5±1.2 mmHg (p<0.05) and from 101.5±1.0 to 87.7±0.9 mmHg (p<0.05). Bulbar conjunctival biomicroscopy showed accompanying microcirculatory changes, graded by pathological signs and summed as the conjunctival index (CI). Before INH, CI was higher in hypertensive patients than in healthy people: 12.2±1.1 versus 2.7±0.1 points (p<0.001). INH reduced CI in patients to 8.2±0.7 points (p<0.05), mainly through fewer areas of perivascular oedema, more uniform venular calibre and less tortuosity, an increased arteriolar-to-venular ratio, fewer nonperfused capillary areas and reduced sludging.
Consistent with earlier findings of a mismatch between antioxidant enzyme capacity and reactive oxygen species production in hypertension, pre-INH activities were reduced: SOD by 18% (p<0.05), CAT twofold and GPx by 14% (p<0.05). Four days after INH, all studied enzymes had increased activity, possibly reflecting substrate induction of their biosynthesis by reactive oxygen species and lipid peroxidation products. This mechanism is consistent with intermittent moderate oxidative stress during INH [5].
Before INH, plasma oxidative stress markers, TPA and TBARP concentration, were substantially elevated (Table 1); TBARP could serve as an indirect measure of peroxidative vascular wall injury. Plasma EMP, a marker of endothelial injury, was also increased. Four days after INH, TPA, TBARP and EMP concentrations decreased (Table 1).
Table 1. Oxidative stress and endothelial injury markers (M ± m)
| Parameter | Healthy, n=30 | Before INH, n=30 | After INH, n=30 |
|---|---|---|---|
| TPA, % | 13.3 ± 1.4 | 31.8 ± 2.9; p<0.05 | 22.5 ± 2.3; p<0.05; p1<0.05 |
| TBARP, µM | 3.7 ± 0.2 | 8.6 ± 0.9; p<0.05 | 6.3 ± 0.6; p1<0.05 |
| EMP, ×10⁹/L | 2.14 ± 0.29 | 7.82 ± 0.68; p<0.001 | 4.69 ± 0.43; p1<0.001 |
p: versus healthy participants; p1: versus before INH; Newman–Keuls test.
All participants simultaneously showed an increase in VEGF concentration on the first day after hypoxic therapy, followed by a decrease on day two. A second increase occurred on day four, returning to baseline by day fourteen (Figure 1). These two response phases probably reflect two distinct, sequential mechanisms. Animal experiments [6] suggest that the first peak results from transient mild oxidative endothelial injury, while the second probably reflects increased VEGF biosynthesis. Supporting this possibility is evidence of increased endothelial VEGF mRNA accumulation three to five days after acute hypoxic injury [6].
bFGF concentration followed a different pattern: a slight rise on the first day after INH was followed by a gradual decrease by day four (Figure 2). This may reflect insufficient stimulation of factor biosynthesis in this setting. It agrees with the lower responsiveness of bFGF to various angiogenic stimuli compared with VEGF [9]. Throughout the study, VEGF concentrations in hypertensive patients remained higher than in healthy participants.
Conclusion
The findings suggest that the therapeutic mechanism of INH in essential hypertension includes reduced reactive oxygen species-mediated peroxidative endothelial injury through adaptation of antioxidant enzymes, together with stimulation of endothelial cell proliferation.
References
1. Чижов А.Я., Потиевская В.И. Прерывистая нормобарическая гипоксия в профилактике и лечении гипертонической болезни: Монография. – М.: Изд-во РУДН, 2002. – 187 с.
2. Ельчанинова С.А., Кореняк Н.А., Смагина И.В. и др. Влияние интервальной гипоксии на оксидативный гомеостаз больных дисциркуляторной энцефалопатией на фоне гипертонической болезни // Журнал неврологии и психиатрии им. С.С. Корсакова. – 2002. – № 11, С. 29-32.
3. Гогин Е.Е. Гипертоническая болезнь. – М., 1997. – 400 с.
4. Ельчанинова С.А., Галактионова Л.П., Толмачева Н.В., Варшавский Б.Я. Активность внутриклеточных антиоксидантных энзимов у больных гипертонической болезнью // Тер.архив. – 2000. – № 4. – С. 51-53.
5. Ельчанинова С.А., Смагина И.В., Кореняк Н.А., Варшавский Б.Я. Влияние интервальной гипоксической тренировки на процессы перекисного окисления липидов и активность антиоксидантных ферментов//Физиология человека. – 2003. – том 29. – № 3. – С. 72-75.
6. Ferrara N., Terri D.-S. The Biology of Vascular Endothelial Growth Factor // Еndocrine Reviews. – 1997. – V. 18. – № 1. – Р. 4-25.
7. Волков В.С., Высоцкий Н.Н., Троцюк В.В., Мишин В.И. Оценка состояния микроциркуляции методом коньюктивальной биомикроскопии// Клин. медицина. – 1976. – № 7. – С. 115-118.
8. Ельчанинова С.А., Галактионова Л.П., Молчанов А.В., Варшавский Б.Я. Состояние перекисного окисления у больных с язвенной болезнью желудка и двенадцатиперстной кишки // Клин. лаб. диаг. – 1998. – № 6. – С. 10-14.
9. Breen E.C., Johnson E.C., Wagner H. et al. Angiogenic growth factor mRNA responses in muscle to a single bout of exercise // J. Appl. Physiol. – 1996. – V. 81. – № 1. – Р. 355-361.
Full main-text translation of: Ельчанинова С.А., Кореняк Н.А., Павловская Л.И., Макаренко В.В., Дрягина И.В., Симонова О.Г. К вопросу о механизмах гипотензивного эффекта прерывистой нормобарической гипоксии при артериальной гипертензии. С. 33–39.
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