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Generation
of reactive oxygen and nitrogen species (RONS) is a necessary consequence
of aerobe metabolism. RONS are natural and physiological modulators
of cellular redox milieu and thereby signaling, controlling factors
of a wide range of known and unknown physiological, patho-physiological
processes. Despite of the multi line antioxidant system, the level
of RONS generation can exceed the capability of defense network,
leading to oxidative stress (Askew, 2002).
It is generally assumed that increases in aerobic metabolism or
hyperoxia easily generates increased level of RONS and cause oxidative
damage to lipids, proteins and DNA. Indeed, physical exercise, especially
a single bout of exercise above a certain intensity or duration
can result oxidative challenge and damage to different organs (Radak
et al., 2001). However, it appears that the increased level of RONS
production is not only due to the mitochondrial respiration, because
anaerobic exercise also could cause oxidative damage (Radak et al.,
1998). Moreover
protection of endothelium by exogenous superoxide dismutase (SOD)
prevented both the oxidative damage to lipids and xanthine oxidase
activity, indicating that exercise-associated RONS production occurs
by variety of sources and mechanism.
Similarly to anaerobic physical exercise exposure to high altitude
often result in oxidative damage to macromolecules. Low oxygen pressure
seems to be favorable to low RONS production, but it appears that
high altitude exposure associated with increased oxidative damage,
which could be the consequence of the increased activity of RONS
generating and decreased activity of antioxidant systems. Moreover,
according to our current understanding it cannot be ruled out the
RONS are involved and maybe even play a causative role in the acute
mountain sickness (AMS), high altitude pulmonary edema (HAPE) and
high altitude cerebral edema (HACE) (Bailey et al., 2001,
Baumgartner et al., 2002,
Chao et al., 1999).
The present review will draw upon the available literature on high
altitude, exercise and high altitude and oxidative stress.
High altitude and oxidative damage
In one of our study, we have used intermittent exposure (12
hr in every day) to an altitude of 4000m to study the muscle fiber
type dependent changes in the activity and content of antioxidant
enzymes and the level of lipid peroxidation (Radak et al., 1994).
Our data revealed that the intermittent exposure to high altitude
resulted in significant increase in lipid peroxidation in both,
slow and fast type of muscle fibers of rats. Interestingly, when
we applied 4 wk of continuous exposure to the same altitude, we
did not measure increase lipid peroxidation, but the level of protein
oxidation, measured by carbonyl derivatives was increased (Radak
et al., 1997).
Kumar et al. (1989)
have found the short exposure (5 days) to an altitude of 7576 m
caused increased lipid peroxidation level in plasma of rats. This
result was confirmed by the same experimental protocol adding vitamin
E supplanted groups (Ilavazhagan et al., 2001).
Moreover, Nakanishi and co-workers (1995)
reported that exposure to 5500m result in increased level of malondialdehyde
in serum, lung, liver, heart and kidney.
Human studies revealed similar results. Moller et al., (2001)
exposed twelve healthy subjects to an altitude of 4559 m, which
caused a significant increase in DNA strand breaks, measured from
urine. The damage was more prominent at the endonuclease-III sites.
When humans were exposed simultaneously to high altitude (2700m)
and cold exposure the level of urinary lipid peroxidation, DNA damage
increased significantly (Schmidt et al., 2002).
At the study of Operation Everest III the level of lipid peroxidation
increased by 23% at 6000m, and by 79% at the altitude of 8848m indicating
that the level of oxidative stress is parallel with the increase
in altitude (Joanny et al., 2001).
Thus, both human and animal studies are relatively consequently
reporting that high altitude associated hypoxia is causing oxidative
damage to lipids, proteins and DNA. This damage can be due to the
increased level of ROS production and/or decreased level of antioxidant
capacity.
The effect of high altitude on antioxidant systems
Aerobic cells developed enzymatic and non-enzymatic antioxidant
system to regulate the effects of RONS. The enzymatic system contains
mitochondrial (Mn-SOD), cytosolic (Cu,Zn-SOD, and extra-cellular
SOD to convert reactive superoxide to less powerful hydrogen peroxide.
Glutathione peroxidase (GPX) and catalase decompose hydrogen peroxide
to water. Other enzymes, like thioredoxin and glutaredoxin systems
are not discussed, since data are not available in relation to high
altitude. The nonenzymatic system is very complex and many non-enzymatic
antioxidants exist in cells. High altitude related studies measured
the content glutathione, vitamin E, and vitamin C among the nonenzymatic
antioxidant, therefore these agents are discussed in the present
review.
There are only a few studies which examined the level of antioxidant
enzyme capacity at high altitude. We have reported that 6 month
of intermittent exposure to high altitude (4000m) resulted in decreased
activity and protein content of mitochondrial SOD in skeletal muscle
of rats (Radak et al., 1994).
This was confirmed by Nakanishi et al (1995),
who have found that 5500m simulated altitude increased the level
immunoreactive Mn-SOD in the serum and decreased it in liver and
lung of the animals. The activity of glutathione peroxidase (GPX)
also decreased in liver suggesting that liver might especially sensitive
to high altitude induced oxidative stress (Nakanishi et al., 1995).
In our other study we could not detect significant effect of 4 wk
exposure to 4000m on the activities of antioxidant enzymes (Radak
et al. 1997).
Imai et al. (1995)
compared the activity of GPX in serum of native highlanders (4000m)
and subjects from sea level. They have found that people from high
altitude had lower level of GPX activity. The activity and effectiveness
of GPX is strongly dependent upon state of thiol system. Glutamyl-cysteinyl-glycine,
is one of the main thiol/ antioxidant source of the cell, which
continuously synthesized by glutamyl cycle. High altitude exposure
decreases the level of reduced glutathione (GSH) and increase oxidized
glutathione concentration (Ilvazhagan et al., 2001,
Joanny et al., 2001).
Thus, it appears that the capacity of enzymatic and non-enzymatic
antioxidant systems is somewhat decreasing at high altitude. There
are trials to prevent the high altitude associated oxidative damage
by supplementation of antioxidants. Schmidt et al., (2002)
have applied an antioxidant mixture containing vitamin E, beta-carotene,
ascorbic acid, selenium, alpha-lipoic acid, N-acetyl 1-cysteine,
catechin, lutein, and lycopene to reduce oxidative stress caused
by altitude. This mixture was effective and the level of oxidative
damage was reduced.
Supplementation of vitamin E (40 mg per rat·day-1) orally, 5 days
prior to and during the period of hypoxic exposure of 7,576m to
rats, significantly reduced the high altitude-induced increase in
lipid peroxidation (Ilvazhagan et al., 2001).
On the other hand, the antioxidant supplement mixture containing,
20,000 IU beta-carotene, 400 IU vitamin E, 500 mg vitamin C, 100
micrograms selenium, and 30 mg zinc, (in a daily base) did not prevented
the oxidative damage of macromolecules (Pfeiffer et al., 1999).
A very short exposure to rats to an altitude of 8000 m resulted
in increased melatonin level in the blood (Kaur et al., 2002).
Melatonin besides a wide range of effects can act as an antioxidant.
After the first 4 days following the exposure, the mitochondrial
number and lipid droplets in the pinealocytes appeared to be reduced
compared with those in control rats suggesting another source besides
pinealocytes also produce melatonin.
It appears that exposure to high altitude decrease the activity
and content of some antioxidant enzymes. Moreover, the effectiveness
of thiol system is also reduced by high altitude. There are some
indications that antioxidant supplementation reduces or prevents
the high altitude induced oxidative damage to macromolecules.
RONS
generating systems at high altitude
It is well demonstrated that massive oxygen supply results in increased
formation in mitochondrial ROS production. However, it also appears
that hypoxia can lead to reductive stress, which also results in
increased ROS production by the mitochondrial electron transport
system (Mohanraj et al., 1998).
This is believed that ROS is generated at complex I and complex
III of the electron transport chain. During hypoxia, less O2
is available to be reduced to H2O at cytochrome oxidase,
causing accumulation of reducing equivalents within the mitochondrial
respiratory sequence. This called as reductive stress, which leads
to ROS formation by the auto-oxidation of one or more mitochondrial
complexes such as the ubiquinone-ubiquinol redox couple. Khan and
O Brien (1995)
demonstrated increases in the cellular NADH/NAD+ ratio during hypoxia
associated reductive stress.
The xanthine dehydrogenase/oxidase system is a potent ROS generator
during hypoxia/reperfusion conditions. Intermittent exposure to
high altitude has similar characteristics than ischemia/reperfusion
(Radak et al., 1994).
On the other hand the changing pattern of ROS and nitric oxide (NO)
is different during ischemia/reperfussion and exposure to high altitude.
During ischemia/reperfusion the initial response is accompanied
by a reversible increase in the generation of ROS and is blocked
by antioxidants and by interventions that increase the tissue levels
of NO. In contrast to ischemia/ reperfusion, ROS levels increase
during hypoxia and return towards pre-hypoxic values after return
to normoxia. Acclimatization involves up-regulation of inducible
NO synthase (iNOS), suggesting that hypoxia leads to an alteration
of the ROS/NO balance which is eventually restored during the acclimatization
process (Gonzalez and Wood, 2001).
This phenomenon may have relevance to the microcirculatory alterations
associated with hypoxic exposure, including acute mountain sickness
and high altitude pulmonary and cerebral edema. The findings of
Serrano et al. (2002)
indicates that the involvement of different type of NOS is different
in NO production during high altitude, which can lead to increased
formation of nitrotyrosine level in rat cerebellum after reoxygenation
to sea level. It is well known that the UV radiation is significantly
increasing at high altitude, resulting in enhanced formation of
RONS.
Accordingly to our current understanding it seems that high altitude
associated increase in ROS generation is due to different sources,
including mitochondrial respiratory chain, xanthine oxidase, and
iNOS.
High altitude and exercise
High altitude training is often used by athletes to increase the
number of red blood cells, which is believed to increase endurance
performance. However, the oxidative stress related consequence of
high altitude training is poorly known. It is well accepted that
physical exercise increases the oxygen uptake and flux into the
mitochondria and after a certain intensity and/or duration can lead
to oxidative stress. It was also demonstrated that not only aerobic,
but anaerobic exercise as well can lead to oxidative damage (reviewed
by Radak et al., 2001).
It is suggested that during anaerobic condition XO is one major
source of ROS generation (Radak et al., 1995).
The available data suggests both high altitude exposure and exercise
alone could result in oxidative challenge and shift the redox state
of cells. Therefore it is not surprising that the combined effects
of high altitude and exercise could result in oxidative damage.
We have demonstrated that training at altitude of 4000m resulted
in increased carbonylation of certain muscular proteins, most probably
including actin, which is major contractile protein (Radak et al.,
1997). We have
suggested that exercise escalates effects of altitude on ROS production
and weakens the power of antioxidant system. This hypothesis was
confirmed by human studies as well (Wozniak et al., 2001).
Moller et al. (2001)
concluded that hypoxia undermines the capacity of antioxidant system
and reduce the body capacity to withstand oxidative stress produced
by exhaustive exercise. Joanny et al. (2001)
data further support this suggestion and points out the importance
of antioxidant supplementation for individuals engaged with exercise
at high altitude.
Increased physical activity at high altitude is increases the vulnerability
of body to oxidative stress and can lead to oxidative damage. Therefore,
antioxidant supplementation seems to be an important and natural
tool to reduce the high altitude and exercise induced oxidative
stress.
Acute mountain sickness (AMS) and RONS
Our
current understanding about AMS is still far from being complete.
The most common symptoms of AMS are headache, nausea, anorexia,
insomnia, fatigue/lassitude, vomiting and dizziness. Many physiological
events associated with the pathophysiology of AMS have been documented,
included relative hypoventilation, impaired gas exchange (interstitial
pulmonary edema) fluid retention and redistribution, and increased
sympathetic drive (reviewed Hackett, 1999).
In contrast, increased intracranial pressure and cerebral edema
are documented in moderate to severe AMS, reflecting the continuum
from AMS to HACE. In the development of HACE elevated cerebral capillary
pressure occurs altering the function of brain blood barrier (BBB)
producing brain edema. It appears that free radicals (e.g. oxygen
and hydroxyl radicals), bradykinin, histamine, arachidonic acid
and NO could be involved in the alteration of BBB (Schilling and
Wahl, 1999).
Indeed there are some implications that RONS are involved and even
are the causative factor of AMS (Bailey and Davies, 2001).
HAPE, a potentially fatal clinical condition, represents a serious
complication of AMS. HAPE is an increase in capillary permeability,
which could occur as a result of an inflammatory reaction and/or
free radical-mediated injury to the lung (Figure
1). Upon the findings of their study, Kleger et al. (1996)
suggested that the inflammatory reaction, which was associated with
HAPE, was rather a consequence than a causative factor of high-altitude
pulmonary edema (Kleger et al., 1996).
But, NO inhalation was used with a success to soften or curbed the
symptoms of HAPE (Anand et al., 1998)
and this observation suggests that NO play a causative role. The
beneficial effects of NO inhalation was also nicely demonstrated
on rat model, in which the mortality rate of control rats was 39.5%
and just 6.2% in the NO treated group (Omura et al., 2000).
Therefore it is hypothesized that susceptibility to HAPE may be
related to decreased production of NO, an endogenous modulator of
pulmonary vascular resistance, and that a decrease in exhaled NO
could be detected during hypoxic exposure. Since, an exaggerated
hypoxic pulmonary vasoconstriction is essential for development
of HAPE.
Despite of our limited knowledge about AMS, the available information
suggests that RONS are active players in the process, however it
still not clear whether they are causative or associative agents.
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