alpha-adrenergic contribution to the cardiovascular response to acute hypoxemia in the chick embryo

Fetal responses to acute hypoxemia include bradycardia, increase in blood p ressure, and peripheral vasoconstriction. Peripheral vasoconstriction contr ibutes to the redistribution of the cardiac output away from ancillary vasc ular beds toward myocardial, cerebral, and adrenal circulations. We investi gated the effect of alpha -adrenergic receptor blockade on this fetal respo nse. Fluorescent microspheres were used to measure cardiac output distribut ion during basal and hypoxemic conditions with and without phentolamine tre atment. Phentolamine altered basal cardiac output distribution, indicating a basal alpha -adrenergic tone, but this was mainly noted at the earlier st ages of incubation. During hypoxemia, phentolamine prevented vasoconstricti on in the carcass. At day 19 of incubation, the percent cardiac output dist ributed to the carcass increased by 20% compared with a decrease in the con trol group by 17%. Phentolamine markedly attenuated the subsequent redistri bution of the cardiac output toward the brain (from +102% in the control gr oup 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, alpha -adrenergic mechanisms contribute to the maintenance of basa l vascular tone and to the redistribution of the cardiac output away from t he peripheral circulations toward the brain and heart during hypoxemic cond itions.