J Appl Physiol 102: 972–977, 2007;
doi:10.1152/japplphysiol.00956.2006.
Binge alcohol exposure in the second trimester attenuates fetal cerebral blood
flow response to hypoxia
Dennis E. Mayock,1 Dana Ness,2 Robin L. Mondares,1 and Christine A. Gleason1
1
Division of Neonatology, Department of Pediatrics, and 2Department of Comparative
Medicine, University of Washington School of Medicine, Seattle, Washington
Submitted 29 August 2006; accepted in final form 17 November 2006
of alcohol on the developing fetus
have been perceived since biblical times (15). Today, it is
estimated that the combined incidence of fetal alcohol-related
neurodevelopmental disorders and fetal alcohol syndrome
(FAS) is ⬃1 in every 100 live births (46). Central nervous
system features of FAS include mental retardation, neuronal
migration defects (heterotopia), behavioral problems, and poor
brain growth. Although considerable research has been focused
on understanding alcohol’s detrimental effects on the developing brain, the mechanisms remain largely unknown. Much of
this research effort has focused on alcohol’s direct neurotoxic
effects (52, 53).
Studies in adult humans and animals suggest that, in addition
to neurotoxic effects, chronic alcohol exposure impairs vascular function. Adult alcoholic patients are at high risk of stroke
and cerebral ischemic injury (2, 4, 54). The neuropathy seen in
adult alcoholics includes periventricular vessel sclerosis with
perivascular gliosis (49). These changes suggest that vascular
damage might lead to cerebral injury. In adult rats, chronic
alcohol exposure is associated with impaired cerebral vasodilation, while vasoconstriction is preserved (34). Impaired
endothelium-dependent vasodilation of cerebral arterioles has
also been described in alcohol-fed rats due, in part, to impaired
synthesis and/or release of nitric oxide (22, 35). Although a
great deal of research effort has focused on the long-term
vascular effects of alcohol exposure in the adult, little information is available regarding alcohol’s effect on the fetal
vasculature. We previously reported that early-gestation fetal
alcohol exposure resulted in significant blunting of the cerebral
vasodilatory response to hypoxia in newborn sheep (21). We
speculated that abnormal cerebral vascular function may be
one mechanism for alcohol’s damaging effects.
Different times of alcohol exposure appear to yield different
effects on the fetus. Third-trimester exposure to high doses of
alcohol tends to result in microcephaly (reduced brain weight-tobody weight ratio) (45), while second- and third-trimester exposure is associated with low birth weight (24, 36). Brain
regions affected by neuronal loss from first-trimester exposure
are different from those affected by late-term exposure (44). In
developing rats exposed to alcohol during the brain growth
spurt, capillary diameter in the cerebellum and hippocampus
was decreased (32). Although it is generally accepted that
heavy drinking throughout pregnancy results in the FAS phenotype, the effects of alcohol exposure throughout pregnancy
cannot be easily predicted from the effects of shorter periods of
exposure (50).
We wondered whether chronic prenatal alcohol exposure
would similarly attenuate fetal hypoxic cerebral vasodilation and
whether this attenuated response could limit fetal cerebral oxygen
delivery. We chose this second-trimester binge alcohol model for
the following reasons. 1) Because the pattern of drinking in
pregnant women is more commonly a binge pattern, it is more
clinically relevant. 2) The second trimester is considered to be a
particularly vulnerable time, since alcohol-induced neuronal loss
in rats is reportedly greatest with prenatal alcohol exposure during
the second trimester (38). In sheep, the brain growth spurt occurs
in the second trimester (18). Taken together, the most vulnerable
period with respect to growth of the developing fetal sheep brain
is likely to be the second trimester.
We chose to study fetal sheep, since a wealth of information
regarding fetal cerebral physiology is available for comparison
Address for reprint requests and other correspondence: D. E. Mayock,
Pediatrics, Box 356320, Univ. of Washington, Seattle, WA 98195-6320
(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.
cerebral oxygen delivery; brain injury; fetal alcohol syndrome
THE DETRIMENTAL EFFECTS
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8750-7587/07 $8.00 Copyright © 2007 the American Physiological Society
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Mayock DE, Ness D, Mondares RL, Gleason CA. Binge alcohol
exposure in the second trimester attenuates fetal cerebral blood flow
response to hypoxia. J Appl Physiol 102: 972–977, 2007;
doi:10.1152/japplphysiol.00956.2006.—Alcohol is detrimental to the
developing brain and remains the leading cause of mental retardation
in developed countries. The mechanism of alcohol brain damage
remains elusive. Studies of neurological problems in adults have
focused on alcohol’s cerebrovascular effects, because alcoholism is a
major risk factor for stroke and cerebrovascular injuries. However,
few studies have examined similar cerebrovascular effects of fetal
alcohol exposure. We examined the effect of chronic binge alcohol
exposure during the second trimester on fetal cerebrovascular and
metabolic responses to hypoxia in near-term sheep and tested the
hypothesis that fetal alcohol exposure would attenuate cerebrovascular dilation to hypoxia. Pregnant ewes were infused with alcohol (1.5
g/kg) or saline intravenously at 60 –90 days of gestation (full term ⫽
150 days). At 125 days of gestation, we measured fetal cerebral blood
flow (CBF) and oxygen metabolism at baseline and during hypoxia.
Maternal blood alcohol averaged 214 ⫾ 5.9 mg/dl immediately after
the 1.5-h infusion, with similar values throughout the month of
infusion. Hypoxia resulted in a robust increase in CBF in salineinfused fetuses. However, the CBF response to hypoxia in fetuses
chronically exposed to alcohol was significantly attenuated. Cerebral
oxygen delivery decreased in both groups of fetuses during hypoxia
but to a greater degree in the alcohol-exposed fetuses. Prenatal alcohol
exposure during the second trimester attenuates cerebrovascular responses to hypoxia in the third trimester. Altered cerebrovascular
reactivity might be one mechanism for alcohol-related brain damage
and might set the stage for further brain injury if a hypoxic insult
occurs.
ALCOHOL AND FETAL CEREBRAL BLOOD FLOW
and we had previously used newborn sheep to study cerebral
vascular effects of fetal alcohol exposure. This model also
avoids the potential confounding effects of anesthesia (21).
MATERIALS AND METHODS
J Appl Physiol • VOL
were confirmed, and the brain was removed for blood flow determination. All supratentorial brain tissue was counted to determine
cerebral blood flow (CBF). Regional blood flow to other brain areas
was determined as previously described (20, 21). Radioactivity in
tissue and blood samples was determined using a multichannel
gamma counter (Minaxi gamma counting system, model 5550,
Packard, Downers Grove, IL). Each sample was corrected for
decay time, background counts, and spillover by a matrix inversion
method (47).
Blood samples for pH, respiratory blood gases, hemoglobin concentration, and oxygen saturation were drawn anaerobically into
heparinized Natelson glass pipettes. Simultaneous fetal arterial samples were drawn from the brachiocephalic trunk, and fetal venous
samples were drawn from the sagittal sinus. Respiratory blood gases
and pH were measured at ewe core body temperature using a Radiometer ABL 5 (Copenhagen, Denmark). Oxygen saturation and hemoglobin concentration were measured using a hemoximeter (model
OSM-3, Radiometer).
Experimental protocol. Measurements at each time point included
fetal arterial and sagittal sinus venous blood pressure, blood gases and
pH, heart rate, and CBF. After two baseline measurements were
obtained, the ewe breathed lowered inspired oxygen concentration
(8 –10%) from a plastic bag placed around her head to induce fetal
hypoxemia. Carbon dioxide gas (⬃4%) was blended into this hypoxic
gas mixture to maintain stable fetal arterial PCO2 levels. Fetal hypoxemia was defined as a 50% decrease from baseline in fetal arterial
oxygen saturation (SaO2). This is predicted to cause an approximate
doubling in CBF in preterm fetal sheep (30). After initiation of
hypoxia, frequent fetal arterial blood samples were analyzed for
hemoglobin oxygen saturation, and the oxygen concentration administered to the ewe was carefully adjusted to maintain the desired 50%
decrease in fetal SaO2 from baseline. Once two fetal serial arterial
samples separated by 2 min were at the desired saturation level, fetal
arterial and sagittal sinus venous samples were obtained for blood gas
analysis and for blood flow calculations. CBF was then assessed with
microspheres. These measurements were repeated in a similar fashion
⬃10 min later.
Data analysis/calculations. We utilized SSP as an estimate of
intracranial pressure (ICP). Cerebral perfusion pressure (CPP) was
calculated as the difference between mean arterial pressure (MAP)
and SSP. CBF was calculated as follows: CBF ⫽ CPMbrain/CPMref ⫻
reference withdrawal rate (in ml/min), where CPMbrain and CPMref
represent radioactivity counts per minute in brain and reference
samples, respectively. Cerebral O2 consumption (CMRO2) was calculated as follows: CMRO2 ⫽ [CaO2 ⫺ CvO2] ⫻ CBF, where CaO2 and
CvO2 represent cerebral arterial and sagittal sinus venous oxygen
content, respectively. Cerebral oxygen delivery (OD) was calculated
as follows: OD ⫽ CaO2 ⫻ CBF. Cerebral oxygen extraction was
calculated as follows: (CaO2 ⫺ CvO2)/CaO2. Cerebral vascular resistance (CVR) was calculated as follows: CVR ⫽ (MAP ⫺ SSP)/CBF.
Measurements were calculated and data are reported as means ⫾
SE for all study fetuses. Differences between groups were analyzed by
two-way repeated-measures ANOVA. If the F test was significant,
specific differences were sought with the Student-Newman-Keuls test.
Significance was assumed at P ⬍ 0.05.
RESULTS
Maternal blood alcohol levels were measured on the 2nd day
and the last day of each 5-day infusion period during all 4 wk
of infusion. Blood alcohol levels, before and after infusion,
were similar on days 2 and 5 of infusion over the 4 wk of
infusion and, therefore, were averaged. Maternal blood alcohol
levels averaged 214.2 ⫾ 5.9 mg/dl immediately after the 1.5-h
infusion. Maternal blood glucose and lactate concentrations
were measured before and after infusion in both groups of
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All experimental protocols were approved by Animal Care and Use
Committees at the University of Washington and the University of
Idaho. Mixed-breed ewes were used, and pregnancy was time dated.
Full-term gestation is 150 days in the sheep. We studied 13 near-term
(6 saline-control and 7 alcohol-exposed) fetal sheep at 125 ⫾ 2 days
of gestation.
Maternal infusion protocol. Pregnant ewes were randomized to
receive daily (Monday–Friday) intravenous infusions of alcohol (n ⫽
7) or saline (n ⫽ 6) from 60 to 90 days of gestation. Infusions were
performed at the University of Idaho Animal and Veterinary Science
Department farm (Moscow, ID). Jugular catheters were placed using
sterile technique at 60 days of gestation. Catheter patency was
maintained by heparin flush (1,000 U/ml). Alcohol (1.5 g of pure
ethanol per kilogram of ewe weight, 5.7 ml/kg of 33% solution) or the
equivalent volume of normal saline was infused over 1.5 h. Once the
daily infusion protocol was complete, the jugular catheter was removed.
Maternal blood levels of glucose, lactate, and alcohol were measured before and after an infusion on the second and last day of each
infusion week. Samples were obtained in this manner during all 4 wk
of infusion. Blood glucose, lactate, and alcohol concentrations were
measured enzymatically by glucose oxidase, lactate dehydrogenase, and
alcohol dehydrogenase oxidation utilizing standard assay kits (Sigma
Diagnostics, St. Louis, MO; Diagnostic Chemical, Oxford, CT).
Fetal surgical preparation. For 24 h before fetal surgery, food was
withheld from the ewe, but the animals were allowed free access to
water. Fetal surgery was performed using sterile technique at 120 –123
days of gestation. The ewe was premedicated with atropine (0.2
mg/kg im) and xylazine (0.1 mg/kg im) and then anesthetized with
inhaled isoflurane (0.5–3%). The trachea was intubated, and the ewe
was mechanically ventilated. A 16-gauge venous catheter was placed
percutaneously in a maternal jugular vein for fluid administration
during surgery. After the abdominal skin was prepared, the uterus was
exposed through a midline incision. The top of the fetal head and
limbs were exposed one at a time through small uterine incisions for
placement of catheters into the superior sagittal sinus, the brachiocephalic trunk (via axillary arteries), and the inferior vena cava (IVC)
via pedal veins. A catheter (Tygon tubing) was sewn to a fetal hoof for
measurement of amniotic fluid pressure. Fetal vascular catheters were
filled with heparinized saline (10 U/ml). All incisions were closed
with sutures. The catheters were exteriorized to the ewe’s flank, where
they were secured in a pouch. The ewe received penicillin G benzathine (Bicillin, 1.2 ⫻ 106 U im) on completion of surgery. Ampicillin
(500 mg) was administered into the amniotic fluid via the amniotic
fluid catheter, and the estimated volume of lost amniotic fluid was
replaced with warmed saline. Fetuses were studied 48 h after surgery.
Maternal analgesia was maintained with buprenorphine (0.005 mg/kg
im every 12 h) as needed. This medication was used since it has no
effect on the fetus (42).
Physiological measurements. Fetal arterial blood pressure, heart
rate, amniotic fluid pressure, and superior sagittal sinus pressure (SSP)
were continuously monitored. Arterial pressure and SSP were referenced to amniotic fluid pressure. Fetal brain blood flow was determined using the radiolabeled microsphere technique (26). Approximately 1 ⫻ 106 (0.4 ml) microspheres labeled with 57Co, 51Cr, 103Ru,
95
Nb, 113Sn, or 46Sc (DuPont NEN, Boston, MA) were injected into
the fetal IVC. Reference samples were withdrawn from a brachiocephalic artery (1.5 ml/min) beginning 30 s before the microsphere
injection and continuing for 1 min after the injection was completed.
After completion of the study protocols, the ewe and fetus were killed
with an overdose of pentobarbital sodium. Fetal catheter positions
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ALCOHOL AND FETAL CEREBRAL BLOOD FLOW
Table 3. Fetal hypoxia responses
Table 1. Maternal metabolic measurements during
intravenous infusions
Saline
Glucose, mg/dl
Lactate, mg/dl
Alcohol, mg/dl
Saline (n ⫽ 6)
Alcohol
Preinfusion
Postinfusion
Preinfusion
Postinfusion
40.4⫾1.1
14.0⫾1.3
1.1⫾0.3
41.0⫾0.9
8.6⫾0.5*
0.7⫾0.2
43.1⫾1.3
15.9⫾1.7
1.1⫾0.2
35.9⫾1.2*
18.6⫾1.0*
214.2⫾5.9*
Values (means ⫾ SE) are averages from 4 wk of infusion. *P ⬍ 0.05 vs.
preinfusion.
Table 2. Baseline fetal physical and
physiological measurements
Body wt, kg
Brain wt, g
Male, %
Twins or triplets, %
Heart rate, beats/min
Mean blood pressure, mmHg
Arterial pH
PaO2, Torr
PaCO2, Torr
SaO2, %
CBF, ml 䡠 100 g⫺1 䡠 min⫺1
Hemoglobin, g/100 ml
CVR, mmHg 䡠 ml⫺1 䡠 100
g 䡠 min
CaO2, ml/100 ml
Cerebral O2 consumption,
ml 䡠 100 g⫺1 䡠 min⫺1
Cerebral O2 delivery,
ml 䡠 100 g⫺1 䡠 min⫺1
Cerebral O2 extraction
Saline (n ⫽ 6)
Alcohol (n ⫽ 7)
3.09⫾0.21
42.9⫾1.7
100
33
178⫾4
46⫾2
7.37⫾0.01
20⫾1
46⫾1
62⫾4
103.8⫾11.5
10.34⫾0.51
3.38⫾0.21
42.9⫾1.9
71
29
167⫾5
40⫾1
7.37⫾0.01
20⫾1
46⫾1
59⫾3
116.0⫾7.5
10.38⫾0.20
0.43⫾0.03
8.65⫾0.83
0.43⫾0.04
8.37⫾0.41
2.69⫾0.22
3.07⫾0.16
9.32⫾0.94
0.29⫾0.02
9.70⫾0.65
0.30⫾0.02
Values are means ⫾ SE. PaO2 and PaCO2, arterial PO2 and PCO2; SaO2, arterial
O2 saturation; CBF, cerebral blood flow; CVR, cerebral vascular resistance;
CaO2, arterial O2 content. There were no statistical differences between groups
for any parameter.
J Appl Physiol • VOL
Baseline
Hypoxia
Baseline
Hypoxia
7.37⫾0.01
46⫾1
20⫾1
8.7⫾0.4
7.33⫾0.01
44⫾1
12⫾1*
4.2⫾0.2*
7.37⫾0.01
46⫾1
20⫾1
8.4⫾0.4
7.30⫾0.02
44⫾1
13⫾1*
4.4⫾0.2*
46⫾2
178⫾4
46⫾3
158⫾7*
40⫾1
167⫾5
54⫾3*
158⫾17
2.69⫾0.22
2.54⫾0.18
3.07⫾0.16
2.69⫾0.23
0.29⫾0.02
0.36⫾0.03
0.30⫾0.02
0.33⫾0.01
Values are means ⫾ SE. Baseline and hypoxia values are averages of 2 time
points. CMRO2, cerebral metabolic rate. *P ⬍ 0.05 vs. baseline.
Despite the increase in CBF, cerebral oxygen delivery decreased during hypoxia in both groups of fetuses (Fig. 3). The
decrease was greater in the alcohol- than in the saline-exposed
fetuses (P ⫽ 0.04). There was no change in oxygen extraction
in either group of fetuses from baseline to hypoxia.
CVR decreased significantly in both groups of fetuses during
hypoxia (Fig. 4). The decrease was greater in the saline- than
in the alcohol-exposed fetuses (P ⫽ 0.039). MAP increased
significantly during hypoxia in the alcohol-exposed, but not in
the saline-control, group (Table 3).
DISCUSSION
The major finding of this study is that binge fetal alcohol
exposure in the second trimester attenuates fetal cerebral vasodilatory responses to hypoxia in the third trimester. This
attenuation of hypoxic vasodilation limits fetal oxygen delivery
to the brain during hypoxic episodes, which could contribute to
fetal alcohol-induced brain injury. Our findings are consistent
with previous studies in newborn lambs exposed to alcohol in
utero (21).
The cerebral vasodilatory response to hypoxia has been
demonstrated in preterm and near-term fetal sheep, newborn
lambs, and adult sheep (30). Similar vascular responses have
been described in most other species, including humans (14).
Cerebral vasodilation occurs as the brain tries to maintain
Fig. 1. Cerebral blood flow as percent change from baseline during hypoxia in
saline-infused (solid line) and alcohol-exposed (dashed line) fetuses. Values
are means ⫾ SE.
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animals (Table 1). No change in blood glucose concentration
was noted in saline-control animals. However, blood glucose
levels decreased significantly after the alcohol infusion. Blood
lactate levels increased after alcohol infusion but decreased in
the saline-control animals.
We examined CBF responses to hypoxia in 13 near-term
fetal sheep [6 saline-control (all male) and 7 alcohol-exposed
(5 male and 2 female) animals] at 125 ⫾ 2 days of gestation.
Two baseline measurements were made in both groups of
fetuses. Since data from both of these assessments were similar
in both groups of fetuses, they were averaged for presentation
(Table 2). No differences were noted between groups for any
of the baseline measurements (Table 2). Because hypoxia
resulted in similar changes from baseline in both groups of
fetuses at both hypoxia points, averaged values are presented
(Table 3). As expected, arterial PO2 and CaO2 decreased during
hypoxia. In response to hypoxia, CBF increased by 90 ⫾ 15%
from baseline in the saline-control fetuses. In the alcoholexposed fetuses, the increase was significantly attenuated,
increasing only 48 ⫾ 10% from baseline (Fig. 1). Regional
brain blood flow increased significantly during hypoxia in all
areas examined. However, flow to various brain regions was
significantly attenuated in alcohol-exposed fetuses (Fig. 2).
Arterial pH
PaCO2, Torr
PaO2, Torr
CaO2, ml/dl
Mean blood pressure,
mmHg
Heart rate, beats/min
CMRO2, ml 䡠 100
g⫺1 䡠 min⫺1
Cerebral O2
extraction
Alcohol (n ⫽ 7)
ALCOHOL AND FETAL CEREBRAL BLOOD FLOW
975
Fig. 4. Cerebral vascular resistance as percent change from baseline during
hypoxia in saline-infused (solid line) and alcohol-exposed (dashed line) fetuses. Values are means ⫾ SE.
Fig. 2. Regional brain blood flow as percent change from baseline during
hypoxia. Values are means ⫾ SE. All changes from baseline in both groups of
fetuses are significant. Regional blood flow was significantly attenuated in
alcohol-exposed compared with saline-control fetuses.
Fig. 3. Cerebral oxygen delivery as percent change from baseline during
hypoxia in saline-infused (solid line) and alcohol-exposed (dashed line) fetuses. Values are means ⫾ SE.
J Appl Physiol • VOL
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adequate cerebral oxygen delivery, despite reduced oxygen
content. In earlier studies using radiolabeled microspheres,
even during hypoxia, the near-term fetal sheep brain maintained its baseline cerebral oxygen delivery by increasing CBF
and, if necessary, increasing cerebral oxygen extraction to meet
tissue oxygen needs (25, 30). In the preterm fetal sheep,
however, similar levels of hypoxia result in significant compromise of cerebral oxygen delivery and limited ability to
maintain cerebral oxygen needs (20). The present study demonstrates that alcohol attenuates the cerebral vasodilatory response to hypoxia in near-term fetuses, and this response is
similar to that noted in preterm fetuses during hypoxia. However, as with most fetal sheep studies, our relatively small
number of animals means that our results may not have been
powered enough to exclude the possibility of type II error. We
could speculate that fetal alcohol exposure delays normal
cerebrovascular maturation.
Recent investigations utilizing a new technique based on
cerebral heat production and laser-Doppler flowmetry have
suggested that increased CBF during moderate hypoxia may
not maintain adequate regional oxygen delivery in the fetal
sheep. In their study of near-term fetal sheep, Hunter and
colleagues (28) found that cerebral metabolic rate fell significantly during moderate hypoxia, as reflected by cerebral tissue
temperature changes. They concluded that the increase in CBF
did not result in maintenance of cerebral oxygen delivery.
However, the increase in fetal CBF in their investigation with
similar levels of hypoxia was less than predicted from other
studies (20). Indeed, Bishai et al. (11) found that the change in
CBF measured by laser-Doppler flowmetry during hypoxia
significantly underestimated that measured by the microsphere
technique. Further studies by these same investigators utilizing
this technique demonstrated that adenosine may mediate the
increase in CBF and the decrease in cerebral metabolic rate
during hypoxia (12). On the basis of this new technique, more
careful studies of fetal regional cerebral responses to various
interventions would be possible and might yield results that are
different from those previously reported.
In our alcohol-exposed near-term fetuses, the attenuated
cerebral vasodilatory response to hypoxia was associated with
a similarly attenuated decrease in CVR (Fig. 3). Although both
groups demonstrated a significant decrease in CVR from baseline normoxic levels, the decrease was significantly less in the
alcohol-exposed than in the saline-control animals. Moreover,
MAP increased in alcohol-exposed fetuses during hypoxemia,
a response that should have resulted in a further increase in
CBF if cerebral autoregulatory capability was compromised.
Despite the increase in CPP and the decrease in CVR, cerebral
oxygen delivery was still compromised. These changes indicate that prenatal alcohol exposure alters fetal vessel reactivity
to hypoxia, although the mechanism is yet to be determined.
The attenuation of fetal hypoxic cerebral vasodilation associated with prior alcohol exposure we observed may be due to
one or more factors. Prenatal alcohol exposure has been demonstrated to cause retardation of central nervous system development (17, 19, 37). If alcohol stunts vascular growth (32), this
may account for the attenuated response to hypoxia, similar to
that seen in more immature fetuses (20). However, because the
cerebrovascular response to hypercapnia is not altered by
alcohol exposure, this explanation is unlikely (21). Alcohol
may indirectly affect developing fetal vessels by causing intermittent fetal hypoxia and/or placental insufficiency. High
concentrations of alcohol produce spasm of umbilical vessels
(3, 41), and chronic alcohol consumption decreases placental
blood flow in rats (31). However, a study in sheep demonstrated that, during late-term gestation, exposure to high
amounts of alcohol in binge-type dosing did not induce acute
hypoxia in the fetal sheep (16).
Alcohol might influence cerebral vasomotor tone by altering
sensitivity to neurotransmitter levels or receptor function.
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ALCOHOL AND FETAL CEREBRAL BLOOD FLOW
J Appl Physiol • VOL
In conclusion, the results of this study demonstrate a significant attenuation of hypoxic cerebral vasodilation in the lategestation fetal sheep chronically exposed to alcohol in the
second trimester. This diminished ability of the alcohol-exposed fetal cerebral circulation to vasodilate when confronted
with hypoxia limits cerebral oxygen delivery. This attenuation
of hypoxic cerebral vasodilation places the fetus at increased
risk of brain injury during such a stress. The inability of the
alcohol-exposed sheep fetus to adequately increase CBF during
hypoxia might explain the similar brain injury pattern in
fetuses exposed to hypoxia or alcohol alone (13). Secondary
insults such as maternal smoking and drug (e.g., cocaine) abuse
may result in brain injury in a fetus previously rendered more
vulnerable to hypoxia or ischemia by alcohol exposure. Cumulative effects of the direct toxic effects of alcohol, in
addition to hypoxic injury, may explain the wide range of
pathological findings in alcohol-related neurodevelopmental
disorder and alcohol-related behavior disorder (29).
GRANTS
This study was supported by National Institute of Alcohol Abuse and
Alcoholism Grant RO1 AA-012403.
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Chronic alcohol exposure leads to potentiated epinephrine
vasoconstriction in rats (1). In the alcohol-exposed fetus, the
increased sympathetic sensitivity during hypoxia could counteract hypoxic vasodilation. Altered vasomotor tone could also
be due to altered neurotransmitter levels. Serotonin neuron
development is altered in alcohol-exposed fetal mice (55).
Additionally, receptor function could be impaired. In young
guinea pigs exposed to alcohol as fetuses, GABA receptors are
altered (10). In preliminary work, we demonstrated that alcohol
exposure early in gestation was associated with fewer vasoactive intestinal peptide-producing neurons in fetal sheep (9).
Decreased expression of vasoactive intestinal peptide, a potent
cerebral vasodilator, might be responsible, in part, for impaired
hypoxic cerebral vasodilation. Ethanol induces lipid peroxidation and nuclear factor-␬B activation in cerebral vascular
smooth muscle, changes that may trigger proinflammatory
events and apoptosis (5, 7, 33). Cerebral vessel alcohol exposure recruits leukocytes with vessel damage and inflammation
(6). Brain alcohol exposure also decreases intracellular magnesium and high-energy metabolites (8). Finally, alcohol infusion led to decreased maternal blood glucose levels and increased lactate levels in our study. These changes certainly
must have a negative effect on the fetus, but the extent of this
compromise is not known. In preliminary work, we have
demonstrated increased levels of glucose transporter protein 1
in fetal brain tissue after binge alcohol exposure. This change
suggests that blood glucose levels were decreased in fetuses
exposed to alcohol in utero (39). These factors, separately or in
combination, could contribute to abnormal vasomotor tone or
vasoreactivity in the alcohol-exposed fetus.
Vasodilatory responses have been shown to be altered in
studies of chronic alcohol exposure in adult animals. Mechanisms for this altered response could include abnormal synthesis of, release of, or response to endothelium-derived vasodilatory substances, such as adenosine or acetylcholine. Hypoxic
cerebral vasodilation is mediated by adenosine in adult (40)
and fetal brain (12, 23, 28) in direct proportion to the severity
of hypoxia. Cerebral vessels from adult rats chronically exposed to alcohol do not vasodilate normally in response to
adenosine, (34, 48) and actually vasoconstrict in response to
acetylcholine (34). Fetal sheep studies implicate adenosine in
the increase in CBF and the decrease in cerebral metabolism
during hypoxia (12). Activation of the adenosine A1 receptor
during asphyxia appears to have a neuroprotective effect in the
fetal sheep by suppressing neural activity (27). Endotheliumdependent vasodilation of cerebral arterioles is impaired in
alcohol-fed rats and is believed to be due in part to impaired
synthesis and/or release of nitric oxide (22, 35).
We chose our study model because of our previous experience and that of others and because of the wealth of cerebral
physiological data available in the sheep (21, 43). Additionally,
we chose a high level of infused alcohol to replicate binge
drinking, since most investigators believe that FAS is caused
by heavy maternal alcohol abuse (50). The blood alcohol levels
achieved in our study (214.2 ⫾ 5.9 mg/dl) greatly exceed the
legal limit for driving while intoxicated in most states. In
humans, significant narcosis occurs at blood alcohol levels of
200 mg/dl; stupor and coma occur at 300 mg/dl. A recent
survey by the Centers for Disease Control and Prevention
found that 10% of pregnant women used alcohol and 2%
engaged in binge drinking (51).
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