Ventilatory responses to CO2 in the chicken: intrapulmonary and systemic chemoreceptors

The independent effects of pulmonary and arterial Pco2 on respiratory amplitude (RA) and respiratory frequency (f) were studied in unidirectionally ventilated chickens anesthetized with phenobarbital (160 mg-kg-1). Pulmonary Pco2 was set by the level of PIco2 ventilating the vascularly isolated right lung (VIL), whereas the systemic arterial Pco2 was set by the level of PIco2 ventilating the denervated left or gas exchange lung (GEL). The following results were obtained: 1) Increasing the PIco2 to the VIL from 0 to 35 torr and maintaining Paco2 constant at 2. torr increased RA from apnea to 76% of the animals' maximal hypercapnic response and decreased f: further increases in PIco2 to VIL had only minimal effects on RA and f. 2) increasing Paco2 from 19 to 61 torr and maintaining pulmonary Pco2 constant increased RA and decreased further increases in Paco2 had only slight effects on RA ulmonary chemoreflex and can dominate the control of RA during hypocapnic conditions, and (2) systemic CO2-sensitive chemoreceptors dominate the control of RA during hypercapnic conditions. It is suggested that the intrapulmonary chemoreceptors may act as a sensory system which plays a pertinent role in the regulation of parabronchial ventilation.