Intrapulmonary CO 2 receptors in the duck: I. Stimulus specificity

Abstract

Single unit vagal recordings were made from intrapulmonary CO 2 receptors in five domestic ducks. Gas, containing various concentrations of CO 2, was unidirectionally passed through the respiratory system either from the trachea to the surgically opened, right caudal thoracic air sac or vice versa. Intrapulmonary gas pressure (Pip) and CO 2 concentration in the ventilatory gas (F i CO 2 were varied independently. The units were identified and studied during unidirectional ventilation and during phasic ventilation with a Starling pump. The static discharge frequency of these receptors increased as Fico, decreased. In most receptors, step changes in F I CO 2 also produced dynamic responses. The static discharge frequency at various levels of F i CO 2 did not significantly diner when Pip was low (about 2 cm H 2O) or high (about 10–20 cm H 2O). Likewise, the peak dynamic discharge frequency, elicited when F I CO 2 was suddenly decreased to 0%, was not affected by these levels of Pip. The receptor discharge frequency was also unaltered by transient changes in Pip provided F i CO 2 and gas flow remained constant. All receptors exhibited cyclic discharge patterns during pump ventilation and peak frequencies occurred either during inspiration, or during expiration or during both phases of the respiratory cycle. Their discharge frequency decreased when CO 2 was added to the inhaled gas. The results indicate that intrapulmonary CO 2 receptors are not stretch-sensitive and therefore are not mechanoreceptors. Their cyclic discharge patterns during pump ventilation were similar to those seen during spontaneous breathing and appear to result from changes in intrapulmonary CO 2 concentration at the receptor site. These receptors may be able to monitor alterations in intrapulmonary CO 2 concentration resulting from changes in the metabolic activity of the bird, and may thereby be involved in the control of breathing.