Receptive fields of intrapulmonary chemoreceptors in the Pekin duck

Reflex experiments indicate a uniform distribution of CO2 chemosensitivity in avian lungs, but neural recording experiments suggest a non-uniform distribution of intrapulmonary chemoreceptor (IPC) endings. To reconcile these observations, blood gases and PECO2 were measured while recording discharge frequencies of 32 IPC innervating the unidirectionally ventilated lungs of 14 Pekin ducks. IPC discharge frequencies, recorded from the left vagus, were determined while ventilating the perfused left lung with caudocranial and craniocaudal flows of 1% CO2 in air, and then while ventilating the unperfused left lung with known levels of CO2 in air. Lung PCO2 profiles were predicted using an eight-compartment computer model of cross-current gas exchange with log-normal ventilation-perfusion inequality and shunt. The PCO2 profiles and IPC discharge frequencies were used to calculate receptor location. At the 99% confidence limit, estimates of IPC location changed significantly in all but 7 IPC when the direction of ventilation was reversed, indicating many IPC have multiple endings. Eighteen of 32 IPC had receptive fields extending at least 50% of the parabronchial length, which may explain the uniform reflex chemosensitivity to intrapulmonary CO2 noted by others.