Stimulation of tracheal slowly adapting stretch receptors by hypercapnia and hypoxia

We studied the response of tracheal slowly adapting stretch receptors (SARs), whose activity was recorded from the peripheral cut end of the right vagus nerve, in newborn and adult dogs during hyperoxic hypercapnia and isocapnic hypoxia. Tracheal receptors only were studied since it was possible to maintain the parasympathetic efferent supply partially intact (contralateral vagus nerve and superior laryngeal nerve). The animals were anesthetized and passively ventilated with the chest open. A stimulatory effect was found in 9 out of 20 puppies for hypercapnia and in 3 out of 10 for hypoxia. The maximum amount of stimulation ranged from 114 to 556% of the control discharge for hypercapnia and from 110 to 387% for hypoxia. In the adult dog CO2 stimulated tracheal SARs in 11 out of 16 dogs (103-404% control) while hypoxia stimulated SARs in 9 out of 13 animals (109-268% control). The stimulatory effect was greater during expiration and, for hypercapnia, decreased in intensity with time. This excitatory action was eliminated by cold block of the contralateral vagus nerve or atropine administration, suggesting that the stimulation was mediated by smooth muscle contraction. These findings support the notion that airway stretch receptors are activated not only by passive distension of the airways, but also by an active contraction of the tracheobronchial smooth muscle.