VENTILATORY, CARDIOVASCULAR AND METABOLIC RESPONSES TO HYPOXIA AND HYPERCAPNIA IN THE ARMADILLO

Armadillos have a low resting metabolic rate and high hemoglobin affin ity for their size, a rigid carapace and a semi-fossorial life style. These characteristics could contribute to unusual respiratory response s to hypoxia and hypercapnia which were investigated in this study. Ve ntilatory and oxygen consumption responses of six adult unanesthetized armadillos to 15, 12, 10 and 8% O-2 and 1.5, 3, 5 and 7% CO2 were mea sured by barometric plethysmography and flow-through respirometry. A s ignificant increase in ventilation occurred in response to 10 and 8% O -2 but a decline in oxygen consumption only occurred at 8% inspired O- 2. The convection requirement response has a threshold at a Pa-O2 of a pproximate to 28 Torr which corresponds to a Hb saturation of similar to 70%. Ventilation increased in response to 3% and higher levels of C O2, with no change in oxygen consumption. The magnitude of the ventila tory response to CO2 was similar to other semi-fossorial mammals and l ess than that of nonburrowing species. However, the pattern of the res ponse was unique in being largely a frequency response with little cha nge in tidal volume, contrary to the tidal volume dominated response t o hypercapnia typical of mammals. This feature, not shared by another Xenarthran, the sloth, who lacks a carapace, is likely attributable to the low respiratory system compliance and increased airway resistance resulting from the rigid carapace and small lungs of armadillos and e mphasizes the importance of respiratory mechanics in determining breat hing pattern. (C) 1998 Elsevier Science B.V. All rights reserved.