Regulation of arterial PCO2 during inhalation of CO2 in chickens

Since CO2-sensitive intrapulmonary chemoreceptors have been shown to exist in the avian lung, we conducted a series of experiments to determine if birds could regulate PaCO2 when confronted with an inspired CO2 load. Chickens were anesthetized with either pentobarbital (30 mg.kg-1) or phenobarbital (160 mg.kg-1). The PICO2 was either increased or decreased in successive 7 torr steps between 0 and 35 torr. At each level of PICO2, steady-state measurements of PaCO2, PaO2 and [H+]a were made. In three of ten experiments, tidal volume and respiratory frequency were determined and minute ventilation calculated. Our results indicate that as PICO2 is varied between 0 and 21 torr, minute ventilation increases and arterial homeostasis of PCO2 and [H+] is maintained; as the PICO2 is increased above 21 torr, PaCO2 increases. We conclude that the isocapnic hyperpnea associated with inhalation of CO2 in chickens is a CO2-coupled phenomenon and that it is mediated by CO2-sensitive intrapulmonary chemoreceptors.