Pulmonary C-fibers elicit both apneusis and tachypnea in the rabbit

The effects of phenylbiguanide (PBG) on phrenic nerve and pulmonary C-fibers were studied in anesthetized spontaneously breathing rabbits with unilateral vagotomy. Right atrial injections of PBG at low (10 micrograms/kg) and high (100 micrograms/kg) dose resulted in a shallow tachypnea and apneusis followed by tachypnea, respectively, and these effects were blocked by procaine treatment of the vagus nerve. Also, the injection of PBG (100 micrograms/kg) still evoked the rapid shallow breathing preceded by apneusis in carotid chemoreceptor-denervated animals. Vigorous stimulation of pulmonary C-fibers by PBG (100 micrograms/kg) coincided with apneusis and the response was followed by a more modest increase in activity associated with tachypnea. Administration of PBG (10 micrograms/kg) into the right atrium caused an increase in pulmonary C-fiber activity associated with tachypnea. However, a small dose of PBG injected into the aortic circulation had no effect on the C-fiber activity but did inhibit respiration. These results suggest that the stimulation of pulmonary C-fibers via PBG injection can produce both inspiratory apnea and tachypnea.