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-adrenergic receptor to maintain
fetal heart rate and survival
1 Department of Neurobiology and 2 Department of Pediatrics, Duke University Medical Center, Durham, North Carolina 27710; and 3 Department of Neuroscience, Tufts University School of Medicine, Boston, Massachusetts 02111
Mice lacking catecholamines die before
birth, some with cardiovascular abnormalities. To investigate the role
of catecholamines in development, embryonic day 12.5 (E12.5)
fetuses were cultured and heart rate monitored. Under optimal
oxygenation, wild-type and catecholamine-deficient fetuses had the same
initial heart rate (200-220 beats/min), which decreased by 15% in
wild-type fetuses during 50 min of culture. During the same culture
period, catecholamine-deficient fetuses dropped their heart rate by
35%. Hypoxia reduced heart rate of wild-type fetuses by 35-40%
in culture and by 20% in utero, assessed by echocardiography. However,
catecholamine-deficient fetuses exhibited greater hypoxia-induced
bradycardia, reducing their heart rate by 70-75% in culture.
Isoproterenol, a
-adrenergic receptor (
-AR) agonist, reversed
this extreme bradycardia, restoring the rate of catecholamine-deficient
fetuses to that of nonmutant siblings. Moreover, isoproterenol rescued
100% of catecholamine-deficient pups to birth in a dose-dependent,
stereo-specific manner when administered in the dam's drinking water.
An
-AR agonist was without effect. When wild-type fetuses were
cultured with adrenoreceptor antagonists to create pharmacological
nulls, blockade of
-ARs with 10 µM phentolamine or
-ARs with 10 µM bupranolol alone or in combination did not reduce heart rate under
optimal oxygenation. However, when combined with hypoxia,
-AR
blockade reduced heart rate by 35%. In contrast, the muscarinic
blocker atropine and the
-AR antagonist phentolamine had no effect.
These data suggest that
-ARs mediate survival in vivo and regulate
heart rate in culture. We hypothesize that norepinephrine, acting
through
-ARs, maintains fetal heart rate during periods of transient
hypoxia that occur throughout gestation, and that
catecholamine-deficient fetuses die because they cannot withstand
hypoxia-induced bradycardia.
tyrosine hydroxylase; dopamine
-hydroxylase; norepinephrine; hypoxia; adrenoreceptor
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