Reversal of the relation between respiratory drive and airway tone in cats

To examine the relationship between respiratory drive and airway tone in the exercise pressor reflex in the cat, we recorded tension in a tracheal segment and activity in the phrenic nerve, before and after injecting capsaicin in various doses (0.3-20 micrograms/kg) into the femoral artery. Injection of all doses of capsaicin relaxed the tracheal segment. However, high doses of capsaicin evoked neural apnea followed by increased phrenic nerve activity whereas low doses evoked only increased phrenic nerve activity. All responses were abolished by cutting hindleg nerves. We were certain that relaxation was due to decreases of parasympathetic bronchoconstrictor activity and not to increases in activity of other inhibitory pathways, because we found no change in capsaicin-evoked relaxation after beta-adrenergic blockade with propranolol and no capsaicin-evoked relaxation after muscarinic blockade with atropine and restoration of tone with 5-hydroxytryptamine. These results provide good evidence that injecting capsaicin into the femoral artery reverses the direct relation between respiratory drive and airway tone and does so by decreasing parasympathetic bronchoconstrictor activity.