ACTH secretion and ventilation increase at similar arterial PO2 in conscious rats

To compare the arterial PO2 (PaO2) at which adrenocorticotropic hormone (ACTH) secretion and ventilation are stimulated, conscious rats with chronic femoral arterial catheters were exposed for 50 min to 21, 18, 15, 12, or 9% O2. Decreases in arterial PCO2 (PaCO2) and increases in arterial pH and adrenocortical system activity occurred consistently throughout the exposure period in rats exposed to 9 or 12% O2. In contrast, changes in PaCO2 or pH were only transient or delayed, plasma ACTH did not change, and plasma corticosterone only increased after 20 min in rats exposed to 15 or 18% O2 relative to those breathing 21% O2. Omitting the large blood sample at 20 min for ACTH eliminated the increase in corticosterone in the 15% O2 group. Overall, ACTH increased, and PaCO2 decreased, below PaO2 of approximately 60 Torr. We conclude that ACTH secretion increases at a similar PaO2 as hyperventilation-induced decreases in PaCO2 and thus represents a primary physiological response to acute hypoxia; hemodynamic stimuli may also interact with hypoxia to augment adrenocortical system activity.