Peripheral neural modulation of endotoxin-induced hyperventilation

Objective: To delineate the role of the peripheral neural reflexes involved in modulating hyperventilation during endotoxemia. Design: A prospective, randomized, controlled, multigroup study. Setting: Research animal laboratory. Subjects: Adult Sprague-Dawley rats (n = 43; 354 ±24 g) of either gender. Interventions: Eight rats received a sham operation on their vagus, carotid sinus, and aortic nerves before the administration of a saline vehicle to serve as the time control In the endotoxin group, 11 rats received a sham operation before endotoxin challenge. The remaining 24 rats received bilateral vagotomy (n = 8), perivagal capsaicin treatment (n = 8), or denervation of peripheral chemoreceptors (n = 8) before endotoxin challenge. After the breathing pattern returned to a steady state, endotoxin (L-4130, serotype 0111, B4 lipopolysaccharide; 50 mg/kg) was injected into the vein. The rat's respiration was then monitored continuously for 5 hrs or until the animal died. Measurements and Main Results: The respiratory rate and tidal volume did not change over the 5-hr observation period in the time control group. In the endotoxin group, the respiratory rate increased significantly from baseline (135.4%) 2 hrs after endotoxin challenge and increased persistently until the rats died. The tidal volume increased gradually to ≤132.8% of baseline 4 hrs after endotoxin challenge. Bilateral cervical vagotomy and perineural capsaicin treatment of the vagus nerves eliminated the tachypnea response to endotoxin injection. Denervation of the peripheral chemoreceptor accentuated the hyperventilation response to endotoxin, and resulted in the shortest survival time. Conclusions: Both lung vagal C-fiber afferents and peripheral chemoreceptors are involved in modulating the hyperventilation response after endotoxin challenge in rat models. Stimulation of vagal C-fiber afferents increased the respiratory rate. Conversely, the role of peripheral chemoreceptors was to restrain the hyperventilatory response and these receptors may play a protective role during endotoxemia.