Hypoxia-induced inhibition of converting enzyme activity: role in vascular regulation

A pilot investigation into the effects of acute normobaric hypoxia, high altitude exposure and exercise on serum angiotensin-converting enzyme, aldosterone and cortisol

Introduction:

Aldosterone decreases at high altitude (HA) but the effect of hypoxia on angiotensin-converting enzyme (ACE), a key step in the renin-angiotensin-aldosterone system, is unclear.

Methods:

We investigated the effects of exercise and acute normobaric hypoxia (NH, ~11.0% FiO₂) on nine participants and six controls undertaking the same exercise at sea level (SL). NH exposure lasted 5 hours with 90 minutes of submaximal treadmill walking. Blood samples for aldosterone, ACE and cortisol were taken throughout exposure and at rest during a trek to HA (5140 m) in eight separate participants.

Results:

There was no difference in cortisol or aldosterone between groups pre-exercise. Aldosterone rose with exercise to a greater extent at SL than in NH (post-exercise: 700 ± 325 versus 335 ± 238 pmol/L, mean ± SD, p = 0.044). Conversely, cortisol rose to a greater extent in NH (post-exercise: 734 ± 165 versus 344 ± 159 nmol/L, mean ± SD, p = 0.001). There were no differences in ACE activity. During the trek to HA, resting aldosterone and cortisol reduced with no change in ACE.

Conclusions:

Acute NH subdues the exercise-associated rise in aldosteroe but stimulates cortisol, whereas prolonged exposure at HA reduces both resting aldosterone and cortisol. As ACE activity was unchanged in both environments, this is not the mechanism underlying the fall in aldosterone.

Budesonide, but not dexamethasone, blunted the response of aldosterone to renin elevation by suppressing angiotensin converting enzyme upon high-altitude exposure

INTRODUCTION

Inhaled budesonide is a novel approach to prevent acute mountain sickness (AMS). However, its mechanism is not completely understood. We aimed to investigate the effects of budesonide and dexamethasone on renin-angiotensin-aldosterone system in AMS prevention.

MATERIALS AND METHODS

Data were obtained from a randomised controlled trial including 138 participants. The participants were randomly assigned to receive budesonide, dexamethasone or placebo as prophylaxis before they travelled to 3450 m altitude from 400 m by car. Their plasma concentrations of renin, angiotensin-converting enzyme (ACE) and aldosterone were measured at both altitudes.

RESULTS

All parameters were comparable among the three groups at 400 m. After high-altitude exposure of 3450, renin in all groups increased significantly; the ACE, aldosterone concentrations, as well as the aldosterone/renin ratio, rose markedly in the dexamethasone and placebo groups but not in the budesonide group. Moreover, the aldosterone/renin ratio correlated closely with ACE concentration.

CONCLUSIONS

Upon acute high-altitude exposure, budesonide, but not dexamethasone, blunted the response of aldosterone to renin elevation by suppressing angiotensin converting enzyme.

Insertion/deletion polymorphism of the angiotensin I-converting enzyme gene and arterial oxygen saturation at high altitude

There is a significant genetic influence on arterial oxygen saturation (Sa(O(2))) in high-altitude (HA) residents. It is not known whether this is true of lowlanders ascending to HA. The I allele of the angiotensin-converting enzyme (ACE) gene is associated with low ACE activity and elite endurance performance. An excess of the I allele has also been reported in South American natives living over 3,000 m and among elite HA mountaineers who demonstrate extreme endurance in a hypoxic environment, where maintenance of Sa(O(2)) is crucial to performance. We postulated that the I allele may confer an advantage at HA through genotype-dependent alterations in Sa(O(2)). Rapid ascent (n = 32) and slow ascent groups (n = 40), ascending to approximately 5,000 m over 12.0 and 18.5 days, respectively, had their Sa(O(2)) assessed throughout and compared with their ACE genotype. Resting Sa(O(2)) was independent of the ACE genotype and remained so for the slow ascent group, in whom the fall in Sa(O(2)) with ascent was genotype independent. However, Sa(O(2)) with ascent was significantly associated with the ACE genotype in the rapid ascent group (p = 0.01) with a relatively sustained Sa(O(2)) in the II subjects. These data are the first to report an association of the I allele with the maintenance of Sa(O(2)) at HA.

Angiotensin-converting enzyme and genetics at high altitude

As part of the renin-angiotensin system (RAS), angiotensin-converting enzyme (ACE) plays a key role in circulatory homeostasis. ACE degrades vasodilator kinins and generates angiotensin II. A polymorphism in intron 16 of the human ACE gene has been identified in which the presence (insertion, I allele) rather than the absence (deletion, D allele) of a 287 bp fragment is associated with lower serum and tissue ACE activity. The I allele has been associated with some aspects of endurance performance, being found with excess frequency in elite distance runners, rowers, and other elite athletes. Mountaineers also demonstrate an allele skew with a significant excess of the I allele and II genotype in elite, male, British mountaineers who have ascended beyond 7000 m without the use of supplemental oxygen. This review evaluates the evidence for and against an association of the I allele with human endurance, and performance at high altitude. We conclude that the I allele does confer an advantage, most likely mediated via improved muscle efficiency with secondary benefits in terms of conservation of non-fat mass.

Correlation of HO-1 expression with onset and reversal of hypoxia-induced vasoconstrictor hyporeactivity

Rats exposed to chronic hypoxia (CH; 4 wk at 0.5 atm) exhibit attenuated renal vasoconstrictor reactivity to phenylephrine (PE). Preliminary studies from our laboratory suggest that this response is mediated by hypoxic induction of heme oxygenase (HO) and subsequent release of the endogenous vasodilator carbon monoxide. Because vascular HO mRNA is increased within hours of hypoxic exposure, we hypothesized that the onset of reduced reactivity may occur fairly rapidly and correlate with HO expression. Therefore, we examined the onset of attenuated vasoconstriction on CH exposure as well as the duration of hyporeactivity on return to a normoxic environment. Renal vascular resistance (RVR) responses to graded intravenous infusion of PE were measured in conscious rats under control conditions and after 24 h, 48 h, and 4 wk of CH exposure. Vasoreactivity responses were also determined in 4-wk CH rats 1, 5, 24, and 96 h after return to normoxia. We found that RVR responses to PE were significantly blunted after 48 h and 4 wk but not after 24 h of hypoxic exposure. Inhibition of HO with zinc protoporphyrin IX increased RVR and decreased renal blood flow in 48-h CH rats but not controls. Although reactivity to PE was gradually restored after 4 wk of CH, responsiveness was still slightly blunted at 96 h after return to normoxia. Western blot analysis demonstrated a correlation between HO-1 protein levels and attenuated vasoconstrictor response in CH and posthypoxic rats. These data suggest that the onset and offset of physiologically relevant vascular HO expression occur within 2--3 days.

Renal vasodilatory influence of endogenous carbon monoxide in chronically hypoxic rats

Chronic hypoxia (CH) attenuates systemic vasoconstriction to a variety of agonists in conscious rats. Recent evidence suggests that similarly diminished responses to vasoconstrictors in aortic rings from CH rats may be due to increased endothelial heme oxygenase (HO) activity and enhanced production of the vasodilator carbon monoxide (CO). Thus we hypothesized that a hypoxia-induced increase in HO activity is responsible for decreased vasoconstrictor responsiveness observed in conscious CH rats. CH (4 wk at 0.5 atm) and control rats were renal denervated and instrumented for the measurement of renal blood flow (RBF) and blood pressure. First, renal vasoconstrictor responses to graded intravenous infusion of phenylephrine (PE) were assessed in conscious rats. CH rats demonstrated significantly diminished renal vasoconstrictor responses to PE compared with control responses that persisted even with acute restoration of normoxia. In additional experiments, CH rats exhibited increased renal vascular resistance and decreased RBF in response to the HO inhibitor zinc protoporphyrin IX (11 micromol/kg iv), whereas renal hemodynamics were unaffected by the inhibitor in control animals. Furthermore, we demonstrated greater HO enzyme activity in renal tissue from CH rats compared with controls. These data suggest that enhanced HO activity contributes a tonic vasodilatory influence in the renal vasculature of CH rats that may be responsible for the diminished sensitivity to vasoconstrictor agonists observed under these conditions.

Pulmonary angiotensin-converting enzyme (ACE) binding and inhibition in humans. A positron emission tomography study

Angiotensin-converting enzyme (ACE) inhibition attenuates pulmonary hypertension and delays the development of pulmonary vascular remodeling in animal models. Thus, ACE inhibition might be a useful treatment for primary pulmonary hypertension (PPH). To determine the dose of ACE inhibitor required to specifically block pulmonary ACE in humans, we measured the combined forward rate constant (CFRC) for [(18)F]-fluorocaptopril, which is proportional to the mass of ACE in the lung, using positron emission tomography (PET). In five normal subjects, CFRC was measured twice, 1 wk apart, to assess measurement reproducibility. The CFRC was 0.151 +/- 0.067 for the first measurement and 0.140 +/- 0.060 for the second measurement (p = not significant [NS]). In five normals, CFRC decreased on average 84%, from 0.177 +/- 0.053/s to 0.028 +/- 0.017/s (p < 0.05), after 1 wk ingestion of 5 mg enalapril orally once a day (the scans were performed 24 h after the last medication). Similarly, in five patients with PPH, CFRC decreased on average 76%, from 0.052 +/- 0. 020/s to 0.012 +/- 0.003 (p < 0.01), after 1 wk enalapril, despite much lower baseline values. We conclude that the total mass of pulmonary ACE appears to be significantly reduced in PPH and that only low doses of ACE inhibitors may be needed to block the effects of ACE on vascular remodeling in PPH.

Angiotensin II receptor expression and inhibition in the chronically hypoxic rat lung

1. Angiotensin II (AII) binding density and the effect of chronic AII receptor blockade were examined in the rat model of hypoxia-induced pulmonary hypertension. 2. [125I]-[Sar1,Ile2]AII binding capacity was increased in lung membranes from rats exposed to hypoxia (10% fractional inspired O2) for 7 days compared to normal rats (Bmax 108 +/- 12 vs 77 +/- 3 fmol mg-1 protein; P < 0.05), with no significant change in dissociation constant. Competition with specific AII receptor subtype antagonists demonstrated that AT1 is the predominant subtype in both normal and hypoxic lung. 3. Rats treated intravenously with the AT1 antagonist, GR138950C, 1 mg kg-1 day-1 rather than saline alone during 7 days of exposure to hypoxia developed less pulmonary hypertension (pulmonary arterial pressure: 21.3 +/- 1.7 vs 28.3 +/- 1.1 mmHg; P < 0.05), right ventricular hypertrophy (right/left ventricle weight ratio: 0.35 +/- 0.01 vs 0.45 +/- 0.01; P < 0.05) and pulmonary artery remodelling (abundance of thick-walled pulmonary vessels: 9.6 +/- 1.4% vs 20.1 +/- 0.9%; P < 0.05). 4. The reduction in cardiac hypertrophy and pulmonary remodelling with the AT1 antagonist was greater than that achieved by a dose of sodium nitroprusside (SNP) that produced a comparable attenuation of the rise in pulmonary arterial pressure during hypoxia. 5. The data suggest that AII, via the AT1 receptor, has a role in the early pathogenesis of hypoxia-induced pulmonary hypertension in the rat.

Inhibition of either angiotensin-converting enzyme or neutral endopeptidase induces both enzymes

Synthesis of angiotensin-converting enzyme is induced during its chronic inhibition. Like angiotensin-converting enzyme, neutral endopeptidase (EC 3.4.24.11) is a plasma membrane peptidase. We studied changes of the two enzymes in lung, kidney and serum in a coronary ligation model of experimental congestive heart failure, and during chronic inhibition of the enzymes. Coronary-ligated rats (n = 19) and sham-operated controls (n = 18) were given SCH 34826 [(S)-N-[N-[1-[[(2,2-dimethyl-1,3-dioxolan-4-yl) methoxy]carbonyl]-2-phenylethyl]-L-phenylalanine]-beta-alanine], a specific neutral endopeptidase inhibitor (n = 13), captopril (n = 12), or vehicle (n = 12) for 4 days, and exsanguinated. Pulmonary angiotensin-converting enzyme was induced both by captopril (52% compared to vehicle) and by SCH 34826 (21%). Serum angiotensin-converting enzyme was induced by captopril (44%). Neutral endopeptidase was induced in lung by captopril (73%), and in kidney by SCH 38426 (32%). Compared to controls, the relative heart weight of rats with heart failure was increased by 29%, and the plasma level of atrial natriuretic peptide elevated by 74%, but enzyme activities were not different. We conclude that, in the rat, separate inhibition of either angiotensin-converting enzyme or neutral endopeptidase induces both enzymes, and that the induction varies in different tissues. Alterations in the substrates of the two enzymes, e.g. in bradykinin, might cause these changes.

Obstructive sleep apnea and blood pressure elevation: what is the relationship? Working Group on OSA and Hypertension

Sleep disordered breathing has increasingly been recognised as a frequent cause of ill-health in the community. Moderate or severe forms of the most common condition, obstructive sleep apnea (OSA), occur in up to 12% of the adult male population. A substantial body of literature has been published on the potential relationship between OSA and cardiovascular disease. In particular, OSA has been associated with cardiac failure, stroke, myocardial infarction and hypertension. Part of this association may be explained by other confounders, mainly obesity, which is common in OSA patients. The present review was prepared following a workshop aimed to critically review available scientific evidence suggesting that hypertension is a direct consequence of OSA. In addition, pathophysiologic mechanisms that may be involved in the relationship between OSA and cardiovascular disease, particularly brief intermittent elevation of blood pressure and sustained systemic hypertension, are discussed.

Feedback regulation of angiotensin converting enzyme activity and mRNA levels by angiotensin II

Although renin and angiotensinogen are known to be subject to feedback regulation, the effects of angiotensin II (Ang II) on the regulation of angiotensin converting enzyme (ACE) gene expression and enzymatic activity have not yet been studied. Therefore, the effects of exogenous Ang II infusion and ACE inhibition on ACE mRNA expression were examined. Ang II was infused intravenously in male Sprague-Dawley rats for 3 days at 100 (low dose), 300 (medium dose), or 1,000 (high dose) ng/kg per minute (n = 8 for each group). Compared with control (vehicle infusion, n = 8), Ang II infusion increased plasma Ang II concentration (62, 101, 126 [p < 0.05], and 187 [p < 0.05] fmol/ml) and mean arterial blood pressure (106, 119 [p < 0.05], 134 [p < 0.05], and 125 mm Hg for control, low, medium, and high doses, respectively). Ang II infusion decreased ACE mRNA levels in the lung (57%, 52%, and 51%; p < 0.05 for each) and testis (49%, 63%, and 53% of control for low, medium, and high doses, respectively; p < 0.05 for each), two major sites of ACE synthesis. There was, albeit less pronounced, a parallel decrease in pulmonary ACE activity (4.38, 3.92, 3.07 [p < 0.05], and 3.48 [p < 0.05] nM/mg per minute for control, medium, and high doses, respectively). In contrast, serum (54, 50, 48, and 38 [p < 0.05] nM/ml per minute) and testicular (2.63, 2.08 [p < 0.05], 2.24, and 2.18 nM/mg per minute for control, low, medium, and high doses, respectively) ACE activities displayed only minimal change in animals infused with Ang II.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin lowers pulmonary artery pressure in hypoxic rats by releasing atrial natriuretic peptide

The authors previously demonstrated that arginine vasopressin (AVP) lowers pulmonary artery pressure in rats with hypoxic pulmonary hypertension by activation of the V1 receptor. The pulmonary depressor effect of AVP in hypoxia-adapted rats is not due to its effect on cardiac output. The current study tested two alternative hypotheses: that AVP lowers pulmonary artery pressure in the hypoxia-adapted lung by (1) dilating pulmonary vasculature directly, or (2) releasing atrial natriuretic peptide (ANP) from the heart. The first hypothesis was tested by injecting AVP into the pulmonary arteries of isolated, buffer perfused lungs and monitoring pulmonary artery pressure, and by exposing preconstricted pulmonary artery rings to graded doses of AVP and monitoring the tension generated. AVP caused minimal vasodilation in perfused lungs and only a small vasodilator effect in pulmonary artery rings. The second hypothesis was tested by injecting AVP (160 ng/kg) or vehicle intravenously in conscious hypoxia-adapted (4 weeks) or air control rats and measuring ANP in arterial blood and atria, and by testing pretreatment with the V1 receptor antagonist d(CH2)5 Tyr(Me)AVP (130 micrograms/kg) on the AVP-induced increase in plasma ANP. AVP produced a 7-fold increase in plasma ANP (209 +/- 33 to 1346 +/- 233 pg/ml; p less than 0.05) in hypoxia-adapted rats and a 5-fold increase in ANP (122 +/- 22 to 573 +/- 174 pg/ml; p less than 0.05) in air controls. ANP release was abolished by pretreatment of both groups with d(CH2)5 Tyr(Me)AVP. The AVP-induced ANP release came mainly from left atrium. These data strongly suggest that the pulmonary depressor effects of AVP in hypoxia-adapted rats is due to augmented V1 receptor-induced release of ANP from left atrium.

Lung angiotensin converting enzyme activity in rats with differing susceptibilities to chronic hypoxia

The decrease in lung angiotensin converting enzyme (ACE) activity occurring in rats during chronic hypoxia might be related to the pulmonary haemodynamic response or to the hypoxia. A study in rats was carried out to investigate this question. Rats from the Hilltop (H) strain are known to develop more severe pulmonary hypertension as a result of chronic hypoxia than rats from the Madison (M) strain despite having virtually identical arterial and mixed venous oxygen tensions. Rats from H and M strains were exposed to hypoxia (0.5 atm) for 3-21 days and lung and serum ACE activities were determined. After three days' hypobaria lung ACE activity was significantly lower and serum ACE significantly higher in H than in M rats. Linear regressions for lung ACE activity and right ventricular:body weight ratios showed significant inverse correlations and were similar in the two strains. The results suggest that pulmonary hypertension and not hypoxia determines the reduction in lung ACE activity, possibly by releasing ACE into the blood stream.