Effects of induced hypothermia on renal sympathetic nerve activity and baroreceptor reflex in urethane-anesthetized rabbits

Objective: To evaluate the role of the autonomic nervous system in hemodyna mic changes during induced hypothermia. Design: Prospective, randomized animal study. Setting: An animal research laboratory in a medical university. Subjects: A total of 29 anesthetized rabbits. Interventions: Animals were anesthetized by intraperitoneal urethane. After tracheostomy and administration of gallamine, respiration was maintained b y mechanical ventilatory support. The animals were divided into five groups (one control and the four experimental groups); animals were treated with an intact neuraxis and normothermia (control group), animals with an intact neuraxis (intact group), cervical vagotomized animals (vagotomy group), th e carotid sinus and aortic nerves denervated animals (SAD group), and anima ls with SAD plus vagotomy (SADV group). The left renal sympathetic nerves w ere exposed by a retroperitoneal approach. Measurements and Main Results:We examined the effects of surface cooling on HR, mean arterial pressure, central venous pressure, and renal sympathetic nerve activity (RSNA) in the animals. Changes of baroreflex sensitivity an d plasma catecholamines were also measured simultaneously. Surface cooling caused progressive and profound decreases in DC in all experimental groups. In all groups, RSNAs increased at the early phase, which were followed by return to the precooling level. Conclusions: Hemodynamics and RSNA during induced hypothermia are regulated by mechanisms other than the baroreceptor reflex system, possibly the derm al cold receptors. Suppression of the baroreflex occurred on HR but not on RSNA during hypothermia, which may indicate direct effects of hypothermia o n the heart. RSNA responses may be activated earlier than systemic catechol amine responses during induced hypothermia.