Chronic hypoxia (10 kPa O2) stunts overall embryonic growth in alligator embryos (Alligator mississippiensis). At 90 % of a 72 day incubation length (30 °C) hypoxic embryos averaged 25.8 ± 1.7 g (n = 8), which was significantly lower than 38.7 ± 1.3 g (n = 18) of normoxic controls. All data are presented as means ± S.E.M. Wilcoxon and Mann-Whitney U tests were used to test for statistical significance of the data.
However, cardiac growth is maintained, which results in a concurrent increase in the heart-to-body mass index in hypoxic embryos (0.58 ± 0.03 vs. 0.48 ± 0.01 %). Maintenance of cardiac growth during chronic hypoxia while other organ systems are growth restricted has also been observed in other species such as the domestic fowl (Metcalfe et al. 1981).
Chronic hypoxia also had important functional consequences. Measurement of heart rate and arterial blood pressure via catheterization of a tertiary chorioallantoic artery showed that hypoxic embryos at 90 % incubation had significantly lower resting heart rates (69.2 ± 3.6 vs. 86.3 ± 2.4 min-1 in controls) and lower resting blood pressures (1.39 ± 0.14 vs. 2.29 ± 0.18 kPa in controls).
The subsequent sequential injection of antagonists of cholinergic (atropine, 3 mg kg-1), β-adrenergic (propranolol, 3 mg kg-1) and α-adrenergic receptors (phentolamine, 1 mg kg-1) quantified the role of these receptors on the resting cardiovascular status of the embryo. In control embryos (top panels in Fig. 1), atropine and phentolamine induced a significant hypotension and no change in heart rate, while propranolol triggered a significant hypertensive response coupled to a marked bradycardia. This is similar to the response of embryonic chickens (Crossley & Altimiras, 2000; Crossley et al. 2002). Although starting at lower blood pressures and heart rates, chronically hypoxic embryos display the same absolute responses (bottom panels in Fig. 1). All embryos were killed with an overdose of xylocaine.
We conclude that cholinergic and adrenergic responses are not altered under chronic hypoxia despite the lowered heart rates and blood pressures displayed by embryos chronically exposed to hypoxia.
This study was carried out in accordance with USA National guidelines for animal research (IACUC Protocol Number 2000-2180, University of California at Irvine).