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-Adrenergic contribution to the cardiovascular response to
acute hypoxemia in the chick embryo
1 Department of Pediatrics and Research Institute GROW, Maastricht University, 6202 AZ Maastricht, The Netherlands; and 2 Department of Physiology, University of Cambridge, Cambridge CB2 3EG, United Kingdom
Fetal responses to
acute hypoxemia include bradycardia, increase in blood pressure, and
peripheral vasoconstriction. Peripheral vasoconstriction contributes to
the redistribution of the cardiac output away from ancillary vascular
beds toward myocardial, cerebral, and adrenal circulations. We
investigated the effect of 
-adrenergic receptor blockade on this
fetal response. Fluorescent microspheres were used to measure cardiac
output distribution during basal and hypoxemic conditions with and
without phentolamine treatment. Phentolamine altered basal cardiac
output distribution, indicating a basal -adrenergic tone, but this
was mainly noted at the earlier stages of incubation. During hypoxemia,
phentolamine prevented vasoconstriction in the carcass. At day
19 of incubation, the percent cardiac output distributed to the
carcass increased by 20% compared with a decrease in the control group
by 17%. Phentolamine markedly attenuated the subsequent redistribution
of the cardiac output toward the brain (from +102% in the control
group to 
25% in the phentolamine-treated group) and the heart (from
+196% in the control group to +69% in the phentolamine-treated
group). In the chick embryo, -adrenergic mechanisms contribute to
the maintenance of basal vascular tone and to the redistribution of the
cardiac output away from the peripheral circulations toward the brain
and heart during hypoxemic conditions.
fetus; hypoxia; avian; catecholamines; sympathetic nervous system
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