Cardiovascular regulation during hypoxia in embryos of the domestic chicken Gallus gallus

Abstract

Renewed interest in the use of the embryonic chicken as a model of perinatal cardiovascular regulation has inspired new questions about the control mechanisms that respond to acute perturbations, such as hypoxia. The objectives of this study were to determine the cardiovascular responses, the regulatory mechanisms involved in those cardiovascular responses, and whether those mechanisms involved the central nervous system (CNS) of embryonic chickens. Heart rate (fH) and blood pressure were measured in chicken embryos of different incubation ages during exposure to different levels of hypoxia (15, 10, and 5% O2). At all levels of hypoxia and at all developmental ages, a depression of fH and arterial pressure was observed, with the exception of day 20 embryos in 15 and 10% O2. The intensity of the embryonic fH and blood pressure responses were directly related to the level of hypoxia used. Muscarinic and α-adrenergic receptor stimulation limited the hypoxic hypotension on days 15– 19 and 15– 21, respectively, as indicated after blockade with atropine and phentolamine. During the final 3 days of incubation, the intensity of the hypoxic hypotension was magnified due to α-vasodilation caused by β-adrenergic and muscarinic receptor stimulation. In 19- to 21-day-old embryos, the fH response to hypoxia was limited by α-adrenergic receptor stimulation as indicated by the accentuated bradycardia after blockade with phentolamine. Furthermore, on day 21, atropine limited the hypoxic bradycardia, indicating that muscarinic receptors also play a role in the fH response at this age. In addition, the muscarinic actions on the heart and the adrenergic effects on the vasculature appeared to occur through a hypoxic-induced direct release from chromaffin tissue and autonomic nerve terminals. Thus, in embryonic chickens, the only cardiovascular response to hypoxia that involves the CNS was the cholinergic regulation of arterial pressure after day 15 of incubation. Therefore, although embryonic chickens and fetal sheep, the standard models of perinatal cardiovascular physiology, respond to hypoxia with a similar redistribution of cardiac output, the underlying mechanisms differ between these species.