Role of vagal and sympathetic afferents in reflex respiratory responses to capsaicin in dogs.

The respiratory responses following stimulation of type J (pulmonary C fiber) receptors by right atrial injections of capsaicin were assessed in spontaneously breathing anesthetized dogs. At the reflexly effective threshold dose, the primary respiratory response elicited was tachypnoea. With higher doses of capsaicin, the tachypnoea was replaced by apnoea. Left atrial injections of capsaicin also resulted in apnoea, which was abolished or reduced by injecting Xylocaine into the pericardial sac, and after vagotomy, apnoea was replaced by tachypnoea. The latter findings suggested that the apnoea produced by left atrial injection of capsaicin might be due to stimulation of receptors with vagal afferents coursing through the pericardium. In vagotomized dogs, administration of capsaicin into the abdominal aorta above the origin of the iliac arteries (the iliac arteries were kept occluded) resulted in a hyperpnoeic response. Following the transection of the spinal cord between L4 and L5, capsaicin injection into the abdominal aorta caused apnoea instead of hyperpnoea. The apnoeic response elicited was abolished by transecting the spinal cord between L1 and L2. It is suggested that the respiratory responses observed were due to stimulation of receptors in the splanchnic bed connected to sympathetic afferents.