AJP - Regulatory, Integrative and Comparative Physiology, Vol 267, Issue 6 1510-R1515, Copyright © 1994 by American Physiological Society
Ventilatory and metabolic responses to hypoxia in the echidna, Tachyglossus aculeatus
P. B. Frappell, C. E. Franklin and G. C. Grigg
Department of Zoology, La Trobe University, Bundoora, Victoria, Australia.
Oxygen consumption (VO2), CO2 production (VCO2), and minute ventilation
(VE) together with breathing pattern were measured in echidnas during
normoxia and hypoxia. In normoxia, VO2, VCO2, and VE were all found to be
approximately 30% of the allometric prediction for a eutherian. As a
consequence VE/VO2 and VE/VCO2 are as predicted for a mammal. This is in
contrast to previous reports on the echidna in which the VE was shown to be
low and the echidna, subsequently, to be in a state of hypoventilation. It
is possible that the difference between this and previous studies is
related to the resting state of the echidna; echidnas in this study adopted
a curled-up "sleeping" posture, and measurements were made without tactile
disturbance. Breathing pattern was typical of a semifossorial species in
that inspiration time to total breath time was short when compared with the
normal eutherian value. In graded hypoxia VE increased [threshold
fractional concentration of inspired O2 (FIO2) = 0.125], predominantly the
result of changes in frequency achieved through a shortening in expiration
time. In acute hypoxia (FIO2 = 0.10) VE/metabolic rate showed a tendency to
increase, mainly because of the increase in VE. Approximately 50% of the
increase in VE could be attributed to the 25% increase in VO2 and VCO2 that
occurred in acute hypoxia. Given that the general mammalian response to
hypoxia is a drop in metabolic rate, possible reasons as to why the echidna
does not decrease metabolic rate in hypoxia are discussed.
