RESPONSES OF CONSCIOUS RABBITS TO CO2 AT AMBIENT-TEMPERATURES OF 5-DEGREES-C, 20-DEGREES-C, AND 35-DEGREES-C

Conscious rabbits were exposed to atmospheric air or to 6% CO2 in air at ambient temperatures (Ta) of 5, 20 and 35.degree. C. Measurements were made of rectal temperature (Tre), metabolic rate (MR), respiratory frequency (f), tidal volume (VT) and minute volume (.ovrhdot.VE). CO2 exposure did not affect Tre at any Ta and only affected MR at 35.degree. C when it caused an increase. At each Ta hypercapnia caused an increase in VT and a decrease in f. At 5.degree. C .ovrhdot.VE was increased by CO2, at 35.degree. C .ovrhdot.VE decreased, and at 20.degree. C the results were variable. The data were examined in the light of theories relating to the relative contributions of inputs from brainstem and from pulmonary stretch receptors, in response to body temperature and CO2 partial pressure. Hypercapnia apparently stimulates an increase in VT via the brainstem, whereas at the same time removing a hypocapnic drive which, along with central thermal inputs, stimulates f.