Pulmonary C-fibers do not play a role in ammonia-induced brief tachypnea in the rabbit.

To determine the role of pulmonary C-fibers in evoking a brief tachypnea induced by ammonia vapor, we examined the responses of diaphragm electromyogram (DIAP EMG) to ammonia inhalation before and after procaine treatment to the contralateral vagus nerve in urethane-anesthetized, spontaneously breathing rabbits with unilateral vagotomy. Procaine treatment that blocked the conduction of vagal afferent C wave did not significantly alter the response of brief tachypnea to ammonia. Furthermore, the stimulation of pulmonary C-fibers by ammonia inhalation did not coincide with the induction of brief tachypnea. In addition, we also examined the responses of rapidly adapting receptors (RARs) and slowly adapting receptors (SARs) to ammonia inhalation in rabbits, particularly in which inhalation of this chemical gas produced a brief tachypnea. The burst activity of RARs evoked by ammonia inhalation coincided with the phase of rapid shallow breathing for a few breaths. The discharge rates of SARs during both inspiration and expiration increased when ammonia inhalation caused a brief tachypnea. From these results, it can be suggested that the ammonia-induced brief tachypnea is probably mediated by transient stimulation of both SAR and RAR activities but does not occur as a result of the pulmonary C-fiber stimulation.