Hypoxia inhibits baroreflex vagal bradycardia via a central action in anaesthetized rats

It is known that arterial baroreflexes are suppressed in stressful conditio ns. The present study was designed to determine whether and how hypoxia aff ects arterial baroreflexes, especially the heart rare component, baroreflex vagal bradycardia. In chloralose-urethane-anaesthetized rats, baroreflex v agal bradycardia was evoked by electrical stimulation of the aortic depress or nerve, and the effect of 15 s inhalation of hypoxic gas (4% O-2) was stu died. Inhalation of hypoxic gas was found to inhibit baroreflex vagal brady cardia. The inhibition persisted after bilateral transection of the carotid sinus nerve. Cervical vagus nerves were cut bilaterally and their peripher al cut ends were stimulated to provoke vagal bradycardia of peripheral orig in so as to determine whether hypoxia could inhibit vagal bradycardia by ac ting on a peripheral site. In contrast to baroreflex vagal bradycardia, the vagus-induced bradycardia was not affected by hypoxic gas inhalation. It i s concluded that baroreflex vagal bradycardia is inhibited by hypoxia and t he inhibition is largely mediated by its direct central action.