Autonomic control of heart rate and its variability during normoxia and hypoxia in emu (Dromaius novaehollandiae) hatchlings

Heart rate variability is a common feature of the vertebrate cardiovascular system and is a consequence of variable input from the sympathetic and parasympathetic branches of the autonomic nervous system. The aim of this study was to examine the role of the autonomic nervous system in regulating heart rate and heart rate variability in 1-d-old emu hatchlings in normoxia and during exposure to 10% O₂. The role of the autonomic nervous system in controlling emu heart rate and its variability was examined by blocking the action of the cholinergic and adrenergic pathways by administration of atropine and propranolol. Heart rate of 1-d-old hatchlings exhibited a significant cholinergic tone of 60 ± 22 beats per minute (bpm) and β-adrenergic tone of 28 ± 17 bpm. Cholinergic tone was unchanged during hypoxic exposure (63.5 ± 17.7 bpm), but adrenergic tone doubled to 68 ± 15 bpm. Initially, the majority of hatchlings exhibited high frequency oscillations with a spectral peak at 0.22 ± 0.02 Hz, associated with respiratory sinus arrhythmia. β-Adrenergic blockade had no effect on respiratory sinus arrhythmia or spectral power in high frequency (HF; 0.1 to 0.7 Hz), low frequency (LF; 0.01 to 0.1 Hz), or total frequency (TF) ranges. After cholinergic blockade, spectral power in HF, LF, and TF ranges and respiratory sinus arrhythmia were all abolished. Hypoxia did not initially alter spectral power in the HF, LF, or TF ranges. β-Adrenergic blockade along with hypoxia produced a significant increase in LF oscillations. A distinct LF oscillation appeared in most birds exposed to hypoxia that was abolished by cholinergic blockade. We conclude that although both the sympathetic and parasympathetic system exert a tonic influence on heart rate, the majority of HF and TF heart rate variability is mediated by the parasympathetic system in the emu hatchling. The sympathetic system contributes to LF heart rate oscillations by suppressing the influence of the parasympathetic system on LF oscillations.