Increased venous PCO₂ enhances dynamic responses of avian intrapulmonary chemoreceptors

We quantified the neural discharge of intrapulmonary chemoreceptors (IPC) innervating the left lungs of anesthetized Pekin ducks. Right and left lungs were separately unidirectionally ventilated. Alternating steps in CO₂ concentration (0-6%, 11-s period) were delivered to the left lung under control conditions [mixed venous PCO₂, (Pv̄(CO₂)) 43 ± 4 Torr] and under venous CO₂ load conditions (Pv̄(CO₂) 79 ± 6 Torr). During venous CO₂ loading the right lung was ventilated with 10-20% CO₂, while the left lung was ventilated with a sufficient flow of gas containing 0% CO₂ to maintain normal expired PCO₂ (indicated by constant IPC discharge rate). Venous loading increased the peak-to-peak amplitude of the oscillation in IPC discharge by 4.3 ± 1.8 s^-1 (n = 11, P < 0.05), left lung ventilation was increased 2.6-fold, and the IPC step response became more prompt. The mean IPC discharge rate during the CO₂ stepping cycle was not significantly affected (11.8 ± 1.4 during control vs. 10.3 ± 1.3 s^-1 during venous loading). Increased IPC discharge oscillations were due to enhancement of the dynamic overshoot in receptor discharge after the 6-0% downstep in inspired CO₂ and to a depression of discharge during 6% inspired CO₂. We propose that the phasic enhancement of IPC discharge oscillations during venous CO₂ loading may cause feedback inhibition of ventilatory drive.