Respiratory patterns in anesthetised rats before and after anemic decerebration

Abstract

Experiments were undertaken to test the comparability of changes in respiratory frequency and tidal volume during hypoxia and hypercapnia in rats with and without intact peripheral chemoreceptors and with intact vagi. Neural organisation of respiratory control was perturbed by anemic decerebration, achieved by ligation of the common carotid and basilar arteries. Ischemia of the brain was produced as far candal as the rostral pontine nuclei involved in respiratory control but left the medulla well perfused. The dominant respiratory effect in animals breathing air or oxygen was polypnea with hypocapnia (mean Pa CO 2 when breathing air 24.7 mmHg, when breathing oxygen 29.6 mmHg). After decerebration the increase of ventilation produced by breathing 10% O 2 in N 2 was reduced compared with responses in the intact state but levels of ventilation (V̇ i) in hypoxia were similar to those before decerebration. After decerebration, the increase of ventilation produced by breathing 5% CO 2 was greatly reduced and the level of V̇ in animals breathing CO 2 was significantly less than in the intact state. Intermediate changes were seen in animals breathing 2–3% CO 2 which converted the hypocapnia (Pa CO 2 30.9 mmHg) to eucapnia (Pa CO 2 46.4 mmHg). In the intact state, hypoxia dominantly caused increased frequency (f) and hypercapnia caused increased tidal volume (V t); after decerebration, hypoxia produced reduction of V t while hypercapnia produced reduction of f. Bilateral carotid sinus nerve section in decerebrate animals eliminated the ventilatory response to hypoxia but left the responses to hypercapnia unaltered. The results point to differences in the mechanisms by which hypoxia and hypercapnia influence respiration in both intact and decerebrate animals with carotid sinus and vagus nerves functional. The differences can now be interpreted in terms of specific neural features of respiratory control.