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Am J Physiol Heart Circ Physiol 287: H553–H559, 2004;
10.1152/ajpheart.00657.2003.
Hyperoxia causes oxygen free radical-mediated membrane
injury and alters myocardial function and hemodynamics in the newborn
K. S. Bandali,1 M. P. Belanger,2 and C. Wittnich1,2,3
1
Department of Physiology and 2Department of Surgery, University of Toronto,
Toronto, M5G 1L5; and 3The Hospital for Sick Children, Toronto, Ontario, Canada M5G 1X8
Submitted 15 February 2003; accepted in final form 16 March 2004
NEWBORN CHILDREN requiring critical care in such cases as
extracorporeal membrane oxygenation (ECMO) or undergoing
cardiopulmonary bypass (CPB) are often exposed to high
oxygen levels (hyperoxia) during their medical and/or surgical
management (1, 2, 35, 41). Pediatric ECMO patients have one
of the lowest survival rates partly as a result of secondary organ
dysfunction after several days of conventional critical management (36). Some of these children have been shown to develop
severe myocardial dysfunction (18). As well, a certain cohort
of children undergoing CPB for primary cardiac repair suffer
from postoperative cardiac dysfunction and low output syndrome (1, 27).
During the medical and/or surgical management of these
patients, exposure to high levels of oxygen (hyperoxia) for
varying durations can occur (1, 2, 35, 41). For example, during
ECMO or CPB, systemic oxygen levels can reach an arterial
PO2 (PaO2) of up to 500 mmHg, which can last several days
during extracorporeal membrane oxygenation or for 2–5 h
during a cardiac operation (1, 35). The original rationale for the
use of hyperoxia was to ensure adequate oxygen delivery,
assuming that higher levels of oxygen could only benefit the
patient receiving critical care (6, 41). The potentially detrimental effects of hyperoxia itself were rarely considered.
In adults, the hemodynamic effects of hyperoxia has been
extensively explored in animal models and patients with heart
failure, coronary artery disease, and myocardial infarction (17,
25, 30 –32). Most of these adult studies have reported significant decreases in heart rate (HR) after oxygen treatment (11,
14, 26, 31). However, the effects of oxygen on blood pressure
are contradictory, with some studies reporting increases and
others reporting no change or even a decrease (11, 17, 25, 26,
31). When focusing on the effects in the heart, research using
adult dogs showed a reduction in cardiac output and compromised ventricular work rates (28).
In the newborn, however, there are only a few studies that
have examined the hemodynamic effects of hyperoxia, and
those were confounded by the presence of congenital heart
disease in children or prior exposure to low oxygen (hypoxia)
(7, 19). In these studies, hyperoxia was found to increase
hemodynamics (7, 19). Work in the newborn pig heart by
Ihnken and colleagues (19, 33) only addressed the myocardial
effects of hyperoxia after a period of acute hypoxia or ischemia
and identified the development of significant myocardial dysfunction. Interestingly, clinical evidence in children with acyanotic congenital heart disease also shows that hyperoxia decreases cardiac index and stroke index (7). Whether clinically
relevant levels of hyperoxia alone affect hemodynamics and
impairs myocardial function in newborns in the absence of
confounding variables such as low oxygen or preexisting
disease remains unknown.
Hyperoxia has been associated with the release of oxygen
free radicals (22). If these radicals are not fully neutralized by
key naturally occurring antioxidant enzymes [superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase
(CAT)], they can cause membrane injury and may represent a
potential mechanism by which hyperoxia may partly contribute
to an impairment of myocardial function. Whether the newborn
is particularly susceptible to this is unknown.
Address for reprint requests and other correspondence: C. Wittnich, Univ. of
Toronto, Medical Sciences Bldg. MSB 7256, 1 King’s College Circle, Toronto,
Ontario, Canada M5S 1A8 (E-mail: [email protected]).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
contractility; relaxation; lipid peroxidation; malondialdehyde
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H553
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Bandali, K. S., M. P. Belanger, and C. Wittnich. Hyperoxia
causes oxygen free radical-mediated membrane injury and alters
myocardial function and hemodynamics in the newborn. Am J Physiol
Heart Circ Physiol 287: H553–H559, 2004; 10.1152/ajpheart.
00657.2003.—Newborn children can be exposed to high oxygen
levels (hyperoxia) for hours to days during their medical and/or
surgical management, and they also can have poor myocardial function and hemodynamics. Whether hyperoxia alone can compromise
myocardial function and hemodynamics in the newborn and whether
this is associated with oxygen free radical release that overwhelms
naturally occurring antioxidant enzymes leading to myocardial membrane injury was the focus of this study. Yorkshire piglets were
anesthetized with pentobarbital sodium (65 mg/kg), intubated, and
ventilated to normoxia. Once normal blood gases were confirmed,
animals were randomly allocated to either 5 h of normoxia [arterial
PO2 (PaO2) ⫽ 83 ⫾ 5 mmHg, n ⫽ 4] or hyperoxia (PaO2 ⫽ 422 ⫾
33 mmHg, n ⫽ 6), and myocardial functional and hemodynamic
assessments were made hourly. Left ventricular (LV) biopsies were
taken for measurements of antioxidant enzyme activities [superoxide
dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT)]
and malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) as an
indicator of oxygen free radical-mediated membrane injury. Hyperoxic piglets suffered significant reductions in contractility (P ⬍ 0.05),
systolic blood pressure (P ⬍ 0.03), and mean arterial blood pressure
(P ⬍ 0.05). Significant increases were seen in heart rate (P ⬍ 0.05),
whereas a significant 11% (P ⬍ 0.05) and 61% (P ⬍ 0.001) reduction
was seen in LV SOD and GPx activities, respectively, after 5 h of
hyperoxia. Finally, MDA and 4-HNE levels were significantly elevated by 45% and 38% (P ⬍ 0.001 and P ⫽ 0.02), respectively, in
piglets exposed to hyperoxia. Thus, in the newborn, hyperoxia triggers
oxygen free radical-mediated membrane injury together with an
inability of the newborn heart to upregulate its antioxidant enzyme
defenses while impairing myocardial function and hemodynamics.
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HYPEROXIA AND MYOCARDIAL FUNCTION IN THE NEWBORN
Therefore, the purpose of this study was to examine whether
hyperoxia alone in the absence of any confounding variables
can compromise cardiac function and hemodynamics in the
newborn pig and to explore whether the underlying mechanism
involves the release of oxygen free radicals that overwhelm
myocardial antioxidant enzymes (SOD, GPx, and CAT) leading to subsequent membrane injury marked by increases in
malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) levels. The findings from this work may significantly contribute to
understanding the effects of hyperoxia in the newborn and its
mechanisms.
MATERIALS AND METHODS
Experimental Protocol
Animals were then randomly allocated to 5 h of normoxia control
(PaO2 ⫽ 83 ⫾ 5 mmHg, n ⫽ 4) or hyperoxia (PaO2 ⫽ 422 ⫾ 33
mmHg, n ⫽ 6). The 5-h normoxia ventilatory experiments were
performed to monitor the stability of the model. To assess hemodynamic performance, systolic blood pressure (SBP), diastolic blood
pressure (DBP), developed pressure, and mean arterial blood pressure
(MAP) as well as HR were recorded hourly. Myocardial performance
was derived hourly by differentiating the LV intraventricular pressure
trace to yield indexes of positive (⫹dP/dtmax) and negative maximum
first derivative of LV pressure (⫺dP/dtmax), which were used to assess
myocardial contractility and relaxation, respectively. To account for
the load-dependent nature of these measurements, all derived values
of ⫹dP/dtmax were also normalized for preload [LV end-diastolic
pressure (LVEDP)], afterload [LV systolic pressure (LVSP)], and HR.
At the end of the 5-h ventilatory protocol, LV myocardial biopsies
were taken to measure antioxidant enzyme activities (SOD, GPx, and
CAT) as well as for MDA and 4-HNE measurements, which reflect
the extent of lipid peroxidation (29) secondary to oxygen free radicalmediated membrane injury.
All experimental procedures and protocols used in this investigation were reviewed and approved by the University of Toronto
Animal Care and Use Committee and are in accordance with the
National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH Pub. No. 85-23, Revised 1985) and Canadian
Council on Animal Care guidelines.
Assessment of Antioxidant Enzyme Activity
SOD. LV myocardial biopsies were homogenized using cold (0.25
M sucrose) buffer and spun for 10 min at 8,500 g at 4°C. The
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MDA and 4-HNE Measurements
Sample preparation. Heart tissue (⬃100 mg) ground into a powder
in liquid nitrogen was suspended in 0.5 ml deionized water followed
by the addition of EDTA at a final concentration of 400 ␮M and
butylated hydroxytoluene (BHT) and desferal at final concentrations
of 20 ␮M. Samples were then analyzed by a modification of the
previously described method (29) and analyzed by gas chromatography-mass spectrometry (29).
Statistical Analysis
Paired t-tests were used to determine significant differences in
functional and hemodynamic parameters between baseline and the
end of 5 h of hyperoxic ventilation. Independent Student’s t-tests were
used to determine significant differences in MDA, 4-HNE, and antioxidant enzyme levels between normoxic and hyperoxic piglets. All
data are expressed as means ⫾ SE, and statistical significance was
defined as P ⬍ 0.05.
RESULTS
Cardiac Performance and Hemodynamics
Control animals that underwent 5 h of normoxic ventilation
did not show any significant changes in myocardial function or
hemodynamics (Table 1), indicating the stability of the model.
The values shown in Table 1 fall within the normal of physiological range of what is seen in healthy 3-day-old piglets (7,
19). The baseline (normoxic) values of the hyperoxic group fell
within the range of those seen in the normoxic study group,
thereby confirming the appropriate random allocation of the
animals used in this study and their initial normal status.
Five hours of hyperoxia resulted in significant but variable
reductions in myocardial contractility (⫹dP/dtmax), ranging
from 3% to 48% (baseline: 2,023.89 ⫾ 219.08 mmHg/s, 5 h of
hyperoxia: 1,590.11 ⫾ 177.65 mmHg/s; Fig. 1). Four of six
animals also showed varying degrees of impairment in myo-
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Neonatal Yorkshire pigs were chosen as the animal model in which
to study the effects of hyperoxia because the cardiac and pulmonary
systems of newborn humans and pigs have many structural and
functional similarities (12, 15). Yorkshire pigs (3 days old, 1.5–2.5
kg) were anesthetized with an intraperitoneal injection of pentobarbital sodium (65 mg/kg, MTC Pharmaceuticals; Cambridge, Ontario,
Canada), intubated, and mechanically ventilated to normal blood
gases [PaO2 ⫽ 88 ⫾ 6 mmHg, arterial PCO2 (PaCO2) ⫽ 38 ⫾ 5 mmHg]
with medical air. Normothermia (37.9 ⫾ 0.2°C) was maintained in
each piglet. A catheter was inserted into the right carotid artery and
advanced to the aortic arch to monitor arterial blood pressure. After a
sternotomy was performed and the heart was exposed, a second
catheter was placed in the left ventricle (LV) to monitor left intraventricular pressure. Both catheters were connected to pressure transducers (COBE; Lakewood, CO) and a physiological recorder (BIOPAC
Systems; Goleta, CA). The right carotid artery catheter was also used
for sampling of blood gases.
Arterial blood samples were obtained, and appropriate adjustments
were made to ensure normal physiological values for PaO2 and PaCO2
as well as acid-base status [pH and bicarbonate (HCO⫺
3 )] using an
ABL30 Acid-Base Analyzer (Radiometer; Copenhagen, Denmark).
supernatant was then analyzed based on the SOD-mediated increase in
the rate of autoxidation of 5,6,6a,11b-tetrahydro-3,9,10-trihydroxybenzofluorene in aqueous alkaline solution to yield a chromophore
with maximum absorbance at 525 nm. The method determines both
Cu,Zn-SOD and Mn-SOD activity (Bioxytech, Oxis Health Products;
Portland, OR). SOD activity was expressed per milligram of protein.
GPx. LV myocardial biopsies were homogenized using cold buffer
[50 mM Tris 䡠 HCl (pH 7.5), 5 mM EDTA, and 1 mM 2-mercaptoethanol] and spun for 15 min at 8,500 g at 4°C. In this assay, analysis
of the supernatant for GPx activity was based on oxidized glutathione
produced upon reduction of an organic peroxide by GPx, which is
recycled to its reduced state by the enzyme glutathione reductase. The
oxidation of NADPH to NADP⫹ is accompanied by a decrease in
absorbance at 340 nm, which provides the spectrophotometric means
of monitoring GPx activity (Calbiochem; San Diego, CA). GPx
activity was expressed per milligram of protein.
CAT. In the catalase assay, LV tissue samples were first homogenized using cold buffer (100 mM KCl, 50 mM KH2PO4, 2 mM
EDTA, 50 mM Tris 䡠 HCl, and 5 mM MgSO4) and spun for 10 min at
1,500 g at 4°C. Supernatants containing CAT were incubated in the
presence of a known concentration of H2O2. After incubation for
exactly 1 min, the reaction was quenched with sodium azide. The
amount of H2O2 remaining in the reaction mixture was then determined by the oxidative coupling reaction of 4-aminophenazone and
3,5-dichloro-2-hydroxybenzenesulfonic acid in the presence of H2O2
and catalyzed by horseradish peroxidase. The resulting quinoneimine
dye was measured at 520 nm as an indicator of CAT activity
(Calbiochem). CAT activity was expressed per milligram of protein.
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HYPEROXIA AND MYOCARDIAL FUNCTION IN THE NEWBORN
Table 1. Left ventricular performance and hemodynamic
measurements in normoxic piglets
Contractility, mmHg/s
Relaxation, mmHg/s
SBP, mmHg
DBP, mmHg
Developed pressure,
mmHg
MAP, mmHg
HR, beats/min
Table 2. Left ventricular relaxation and diastolic and
developed blood pressure measurements in hyperoxic piglets
Baseline
After 5 h of
Ventilation
P
Value
1,905.94⫾440.87
⫺1,747.91⫾405.53
76.30⫾7.05
38.20⫾5.83
1,979.44⫾360.71
⫺1,650.47⫾170.90
73.60⫾8.53
34.54⫾3.43
0.64
0.80
0.87
0.70
68.51⫾7.30
46.54⫾5.25
201.03⫾8.48
0.66
0.75
0.20
76.13⫾19.93
50.57⫾6.00
180.15⫾18.00
Relaxation, mmHg/s
DBP, mmHg
Developed pressure,
mmHg
Baseline
After 5 h of
Ventilation
P
Value
⫺1,704.53⫾330.79
41.50⫾9.03
⫺1,616.05⫾389.51
31.61⫾8.53
0.68
0.09
79.16⫾18.93
65.70⫾21.57
0.09
Values are means ⫾ SE.
Fig. 1. Myocardial contractility (⫹dP/dtmax; A) and systolic blood pressure (B)
in newborns at baseline and at 5 h of hyperoxia.
Fig. 2. Mean arterial blood pressure (A) and heart rate [in beats/min (bpm); B]
in newborns at baseline and at 5 h of hyperoxia.
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cardial relaxation (⫺dP/dtmax), which did not reach statistical
significance (baseline: ⫺1,704.53 ⫾ 330.79 mmHg/s, 5 h of
hyperoxia: ⫺1,616.05 ⫾ 389.51 mmHg/s; Table 2). When the
effect of hyperoxia on hemodynamics was examined, significant reductions were seen in SBP (Fig. 1) but not in DBP
(Table 2). All animals but one showed marked reductions in
developed pressure by as much as 28%; however, due to the
variability of this response, this did not reach statistical significance (Table 2). MAP was significantly reduced (Fig. 2),
whereas HR was significantly increased (Fig. 2), after the 5-h
hyperoxic exposure. Hyperoxia-mediated reductions in myocardial contractility remained significant even after ⫹dP/dtmax
values were normalized for preload (LVEDP), afterload
(LVSP), and HR (Table 3).
Values are means ⫾ SE. SBP, systolic blood pressure; DBP, diastolic blood
pressure; MAP, mean arterial blood pressure; HR, heart rate.
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HYPEROXIA AND MYOCARDIAL FUNCTION IN THE NEWBORN
Table 3. Measurements of cardiac performance normalized
for preload, afterload, and HR
Normalized for preload
⫹dP/dTmax/LVEDP, s⫺1
Normalized for afterload
⫹dP/dTmax/LVSP, s⫺1
Normalized for HR
⫹dP/dTmax/HR
P ⫽ 0.21). When the relationship between ⫹dP/dtmax values
and antioxidant enzyme activity was examined, a strong and
significant inverse correlation was observed between ⫹dP/
dtmax values and CAT activity (Fig. 6). No correlations were
observed between ⫹dP/dtmax values and SOD activity (r ⫽
0.22, P ⫽ 0.54) or ⫹dP/dtmax values and GPx activity (r ⫽
0.20, P ⫽ 0.61).
After 5 h of
Hyperoxia
P
Value
525.55⫾58.29
427.56⫾58.77
0.01
26.06⫾1.45
22.47⫾1.58
0.05
DISCUSSION
10.60⫾1.05
7.63⫾0.70
0.005
This study for the first time demonstrates that exposure to
high levels of oxygen alone in newborns leads to both a general
impairment of myocardial function and hemodynamics as well
as significant oxygen free radical-mediated membrane injury.
The changes in ventricular contractility seen in the present
study may be due to the direct effect of hyperoxia on myocardial function in the newborn or alternatively may be mediated
by secondary changes in preload, afterload, and/or HR. Previous studies have shown that hyperoxia has strong effects on
vascular smooth muscle, and a number of oxygen-sensing
mechanisms in smooth muscle cells have been proposed and
identified (3, 4, 40). There is also evidence in the literature to
suggest that hyperoxia acts as a systemic vasoconstrictor (11,
30, 31). Interestingly, when corrected for these parameters,
contractility was still significantly reduced after hyperoxic
exposure. It is important to note that there are several methods
to measure ventricular function in a whole animal model, and
each of these methods have been documented to have their
advantages and disadvantages (8). The selection of a whole
animal physiology model for this study was of particular
relevance because it allowed for a systemic reaction to occur in
response to the hyperoxic exposure. In the newborn, the derived indexes of ⫹dP/dtmax and ⫺dP/dtmax are well documented and accepted methods to assess myocardial contractil-
Values are means ⫾ SE. ⫹dP/dtmax, positive maximum first derivative of
left ventricular pressure; LVEDP, left ventricular end-diastolic pressure;
LVSP, left ventricular systolic pressure.
Antioxidant Enzyme Activity and MDA Levels
Those newborns exposed to hyperoxia showed a modest but
significant 11% decrease in SOD activity in the LV compared
with those newborns exposed to normoxia (Fig. 3). In contrast,
LV GPx activity was dramatically decreased by 61%, whereas
no significant changes in CAT activity were seen in newborns
after hyperoxic ventilation (Fig. 3). MDA and 4-HNE levels in
the LV were significantly elevated by 52% and 38%, respectively, in piglets exposed to hyperoxia compared with the
MDA and 4-HNE levels generated through normal cellular
activity in normoxic animals (Fig. 4).
Correlations
Correlations performed to assess the impact of oxygen free
radical-mediated injury on antioxidant enzyme activity showed
significant inverse correlations between MDA levels and SOD
activity as well as GPx activity (Fig. 5), whereas no correlation
was found between MDA levels and CAT activity (r ⫽ 0.49,
Fig. 3. Antioxidant enzyme [superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT)] levels in normoxic and hyperoxic piglets at the end of 5 h of ventilation. NS,
not significant. †P ⫽ 0.048; ‡P ⬍ 0.001.
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Baseline
(Normoxia)
HYPEROXIA AND MYOCARDIAL FUNCTION IN THE NEWBORN
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ity and relaxation, respectively (38, 39), and, interestingly,
when corrected for load, the reduction in contractility seen in
these newborns after 5 h of hyperoxia remained significant.
This is consistent with work performed by Beekman and
colleagues (7) investigating the effect hyperoxia in children
with acyanotic congenital heart disease in which even after HR
was kept constant by atrial pacing, a significant decrease in
cardiac index and stroke index was seen in children breathing
95% oxygen. These children as well as studies in adult dogs
exposed to hyperoxia show that reductions in cardiac index and
ventricular work rates occurred in the face of increased systemic vascular resistance and hemodynamics, possibly indicating an intracardiac effect (19, 28). The present study for the
first time documents that in newborns, there is a distinct effect
of hyperoxia on myocardial performance and hemodynamics in
a whole animal preparation in the absence of cardiac disease.
Furthermore, our previous work has shown that plasma
epinephrine levels were not significantly elevated in piglets
exposed to hyperoxia (5), thereby suggesting that these animals
did not compensate for cardiac dysfunction through a systemic
adrenergic response. However, this does not exclude the possibility that hyperoxia-mediated reductions in MAP may evoke
a cardiac-specific adrenergic response leading to an increase in
HR through the local release of norepinephrine that would not
manifest in a rise in plasma epinephrine levels.
The magnitude of response to hyperoxia was variable in the
present study, with certain newborns showing a greater degree
of functional and hemodynamic impairment compared with
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Fig. 5. Significant and inverse correlation between cardiac MDA levels and
SOD activity (r ⫽ ⫺0.54, P ⫽ 0.03) as well as GPx activity (r ⫽ ⫺0.79, P ⫽
0.01) in piglets.
others. This variability was inherent to the hyperoxic exposure
because those newborns that underwent normoxic ventilation
were stable over the course of the 5-h study period. It is
therefore apparent that certain newborns have a more exaggerated response to hyperoxia than others and hence are at greater
Fig. 6. Significant and inverse correlation between ⫹dP/dtmax values and CAT
activity (r ⫽ ⫺0.81, P ⫽ 0.02) in piglets.
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Fig. 4. Left ventricular (LV) malondialdehyde (MDA) and 4-hydroxynonenal
(4-HNE) levels in normoxic and hyperoxic piglets at the end of 5 h of
ventilation. ‡P ⬍ 0.001; #P ⫽ 0.02.
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the newborn heart sarcolemma (20). Work in transfected heart
cells as well as in adult rats has shown that oxygen free radicals
not only impair Ca2⫹ channel function but also reduce the
number of Ca2⫹ channels in the cell membrane (10, 16, 23).
Both of these effects contribute to a decrease in calcium influx
into the cardiac cell (10, 16, 23).
If hyperoxia-induced oxygen free radical-mediated injury in
the newborn changes membrane composition, this may not
only lead to an impairment of L-type Ca2⫹ channel function
but also a decrease in channel numbers in the neonatal heart.
This impairment in channel function and reduction in the
number of L-type Ca2⫹ channels may limit the ability to move
calcium into the cell and potentially lead to a reduction in the
number of actin-myosin cross-bridges that can be formed in the
neonatal heart. This in turn would ultimately have a detrimental impact on myocardial function in the neonate. Oxygen free
radicals have also been implicated in the inhibition of calcium
binding to cardiac troponin in the myocardium (24). This,
coupled with the strong evidence that oxygen free radicals have
a detrimental effect on L-type Ca2⫹ channels, may explain the
significant effect on myocardial contractility in this study.
In conclusion, this study presents novel findings identifying
that in newborns, hyperoxia triggers oxygen free radicalmediated membrane injury together with an inability of the
newborn heart to upregulate its antioxidant enzyme defenses
while impairing myocardial function and hemodynamics.
GRANTS
This study was supported by The Heart and Stroke Foundation of Ontario
Grant T4926. K. S. Bandali was supported by a Doctoral Research Award from
The Heart and Stroke Foundation of Canada.
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risk for hemodynamic and functional impairment. This could
also apply to children in the critical care setting, implying that
every neonate may not mature at exactly the same rate and thus
their susceptibility to stresses including exposure to high oxygen levels may vary.
This varying level of susceptibility in the newborn may in
part be a result of their ability to tolerate oxidative injury.
Although somewhat controversial, the antioxidant defensive
systems are immature in the newborn, making them particularly susceptible to oxygen free radical-mediated injury (13,
34). One mechanism through which hyperoxia could reduce
myocardial function is through membrane injury that is mediated by the release of oxygen free radicals, which could be a
result of insufficient myocardial antioxidant defences in the
newborn heart. The work in the present study, for the first time,
clearly demonstrates that the newborn heart lacks the ability to
upregulate its antioxidant enzyme activity to meet the challenge of an additional oxygen free radical stress that may be
mediated by hyperoxia. Moreover, not only does this work
document the lack of upregulation but further suggests that two
of the three major antioxidant enzymes suffer a reduction in
their activity after a hyperoxic exposure, thereby rendering the
newborn heart at greater risk of oxygen free radical-mediated
membrane injury. This is consistent with the significant increases in MDA levels, strongly suggesting a mechanism
involving oxygen free radical-mediated injury that may partly
account for the impairment in myocardial function and hemodynamics seen in newborns. This was further substantiated by
reductions of both SOD and GPx activity levels being significantly correlated with increases in MDA levels in this study.
GPx activity also showed the greatest reduction in response to
hyperoxia, thereby confirming its role as the main antioxidant
pathway in the heart. The reduction in GPx activity and the
presence of oxygen free radical-mediated membrane injury do
imply the development of some myocardial oxidative stress in
the newborn in response to hyperoxia. Interestingly, however,
GPx activity did not correlate with functional impairment. One
limitation to this is that GPx activity alone does not take into
account the GSH redox status (GSH-to-GSSG ratio) or GSH ⫹
GSSG pool size (21), which is required to fully elucidate the
contribution of the GPx pathway to the hyperoxia-induced
depression of myocardial function in newborns. In contrast to
the GPx findings in response to hyperoxia, CAT activity did
not correlate with MDA levels; however, it did significantly
correlate with reductions in ⫹dP/dtmax values, leading to the
possible speculation that CAT activity may serve as a marker
of functional impairment in the newborn. However, it is also
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Oxygen free radical-mediated injury and an impairment in
myocardial function may also be linked through calcium cycling, which is critical in myocardial function and which can be
susceptible to oxygen free radical-mediated injury. This is
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reliance on sarcolemma calcium influx for myocardial contraction because calcium sequestration in the neonatal heart has
been reported to be immature (9). This greater reliance of the
neonatal heart on extracellular calcium is also consistent with
the greater number and sensitivity of L-type Ca2⫹ channels in
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