Effect of hypoxic, hyperoxic or hypercapnic ventilation on inspiratory termination in the cat

Intercostal nerve stimulation was used to examine the effects of altered concentrations of inspired oxygen or carbon dioxide on the termination of inspiration. Experiments were performed in decerebrate cats which were paralyzed, artificially ventilated and bilaterally vagotomized. The threshold current at which electrical stimulation of the T6 intercostal nerve terminated phrenic neural activity was determined at 10 different delays from the onset of phrenic nerve discharge. Ventilation with either hypercapnic normoxic gas (4% CO2) or hypoxic gas (17% O2) increased the threshold current required for inspiratory termination. Hyperoxic ventilation (45% O2), however, decreased the threshold for inspiratory termination. Bilateral section of the carotid sinus nerve abolished the response to hyperoxic ventilation, but did not alter the response to normoxic hypercapnia. These results demonstrate that an oxygen-related stimulus transduced by the peripheral chemoreceptors can influence the mechanism(s) responsible for inspiratory termination.