Functional localization of avian intrapulmonary CO2 receptors within the parabronchial mantle

To determine the location of avian intrapulmonary CO2 receptors, we changed the CO2 stimulus at different regions within the parabronchial mantle and measured the resulting changes in breathing pattern. Three procedures were used to vary the CO2 stimulus: (1) reverse the direction of pulmonary perfusion; (2) stop pulmonary ventilation while maintaining perfusion; and (3) stop pulmonary perfusion while maintaining ventilation. Right and left lungs of adult, anesthetized White Leghorn type chickens were independently, unidirectionally ventilated. The right lung was used to maintain the bird while the left pulmonary artery and vein were cannulated and connected to an extracorporeal gas exchanger, thereby isolating this lung's perfusion. The innervation to both lungs remained intact. When left pulmonary perfusion was reversed, the bird's breathing pattern remained unchanged. The change in breathing pattern that resulted from stopping left pulmonary ventilation was the same during forward perfusion (pulmonary artery to pulmonary vein) as during backward perfusion (pulmonary vein to pulmonary artery). The change in breathing pattern that resulted from stopping forward perfusion was the same as that resulting from stopping backward perfusion. The results indicate that CO2 receptors are not concentrated on the peripheral side of the parabronchial mantle, where venous blood would influence tham, or on the luminal side of the mantle, where arterialized blood would influence them. The CO2 receptors are either distributed symmetrically between the peripheral and luminal sides of the mantle or located in the epithelial lining of the parabronchial lumen.