Chicken intrapulmonary chemoreceptors: discharge at static levels of intrapulmonary carbon dioxide and their location

We studied 54 intrapulmonary chemoreceptors in the unidirectionally ventilated left lungs of 12 thoracotomized cockerels. We ligated the left pulmonary artery to eliminate CO2 contributed by mixed venous blood. At zero PCO2 many units discharge irregularly, and some cease discharging after several seconds. Discharge frequencies at 13.7 torr PCO2 and above are described by logarithmic regressions. The slopes and intercepts of the logarithmic regressions are correlated so that the average response can be written: frequency = 3.86 -B . 1n (24.5 PCO2-1). Afferent activity above 6.8 torr PCO2 is described by 0.073 + 78.6 exp (-0.11 PCO2) -63.3 exp (-0.15 PCO2). For each unit, receptive site PCO2 in a perfused lung was assumed to be the PCO2 in the unperfused lung which gave the same discharge frequency. Location of the receptor was determined as the fraction of ventilation-perfusion region which had the same PCO2 as receptive site PCO2. Two major concentrations of receptors accounted for 85% of the total, one near the entering gas and one near the middle of the gas-exchange region. Sensitivity of individual receptors did not vary systematically with location.