Intrapulmonary CO 2 receptors in the duck:III. Functional localization

Abstract

Single-unit activity in the right vagus nerve was recorded from 67 intrapulmonary CO 2 receptors in anesthetized ducks. The right caudal thoracic air sac was cannulated and the mesobronchus was blocked in the right lung between the origins of ventrobronchi and dorsobronchi. The lung could be unidirectionally ventilated in either direction. Two different tests were applied to each receptor in an attempt to determine its location within the lung. 1. (1) Gross localization of receptors in cranial or caudal parts of the lung was assessed on the basis of differences in discharge frequency when gas flow direction was air sac to trachea vs. trachea to air sac. More than 95% of the receptors were in caudal parts of the lung. 2. (2) For finer localization, pure CO 2 or O 2 was insufflated via catheters into the ventilatory gas stream either proximal or distal to the blocking balloon during gas flow in both directions. In this way, the location of 60 receptors (90%) could further be specified. Of these receptors 56% were found to be in the caudal ends of paleopulmonic parabronchi, 27% in neopulmonic parabronchi, and 12% in dorsobronchi or in parabronchial tissue near the dorsobronchi; only 5% were located in the cranial ends of paleopulmonic parabronchi; none were in the ventrobronchi or extrapulmonary air ways. Thus, CO 2 receptors are located in those parts of the lung that experience marked fluctuations in CO 2 concentration during the respiratory cycle. Differences in location permit satisfactory explanation of differences in discharge pattern of CO 2 receptors observed during spontaneous breathing.