Short communication: Baroreflex function in embryonic emus (Dromiceius novaehollandiae).

The baroreflex involves cardiovascular homeostatic mechanisms that buffer the system against acute deviations in arterial blood pressure. It is comprised of the cardiac limb which involves adjustments in heart rate and the peripheral limb which involves adjustments in vascular resistance. This negative feedback loop mechanism has been investigated in numerous species of adult vertebrates, however our understanding of the maturation and functional importance of the reflex in developing animals remains poorly understood. In egglaying species, our knowledge of this mechanism is limited to the domestic chicken embryo and the embryonic alligator. While each of these species possess a cardiac baroreflex prior to hatching, they differ in the timing when it becomes functional, with the embryonic chicken possessing the reflex at 90% of incubation, while the alligator possesses the reflex at 70% of incubation. In an effort to determine if bird species might share similar patterns of active baroreflex function, we studied embryonic emus (Dromiceius novaehollandiae). However, we hypothesized that emus would possess a pattern of baroreflex function similar to that of the American alligator given the emu embryo possesses functional vagal tone at 70% of incubation, possibly indicating a more mature collection of cardiovascular control mechanism than those found in embryonic chickens. Our findings illustrate that emu embryos possess a hypotensive baroreflex at 90% of incubation. Therefore, our data fail to support our original hypothesis. While only two species of birds have been studied in this context, it could indicate that baroreflex function is not essential for cardiovascular homeostasis in birds for the majority of in ovo development.

Maturation of cardiovascular control mechanisms in the embryonic emu (Dromiceius novaehollandiae).

Our understanding of avian embryonic cardiovascular regulation has been based on studies in chickens. The present study was undertaken to determine if the patterns established in chickens are generally applicable to the emu, a ratite bird species. We studied cardiovascular physiology over the interval from 60% to 90% of the emu's 50-day incubation period. During this period, embryonic emus exhibit a slight fall in resting heart rate (from 171 beats min(-1) to 154 beats min(-1)) and a doubling of mean arterial pressure (from 1.2 kPa to 2.6 kPa). Exposures to 15% or 10% O(2) initially decreased heart rate during the first period of emu incubation studied [60% of incubation (60%I)] but increased heart rate in the 90%I group. Arterial pressure responded to hypoxia with an initial depression (-1.6 kPa) at 60%I and 70%I but showed no response during the later periods of incubation (80%I and 90%I). In addition, tonic stimulation of both cholinergic and adrenergic (alpha and beta) receptors was present on heart rate at 70%I, with the cholinergic and beta-adrenergic tone increasing in strength by 90%I. Arterial pressure was dependent on a constant beta-adrenergic and constant alpha-adrenergic tone from 60%I to 90%I. A comparison with embryonic white leghorn chickens over a similar window of incubation revealed that emus and white leghorn chickens both possess an adrenergic tone on heart rate and pressure but that only emus possess a cholinergic tone on heart rate. Collectively, these data indicate that the maturation of cardiovascular control systems differs between white leghorn chickens and emus, inviting investigation of additional avian species to determine other patterns.