Metabolic response to ambient temperature and hypoxia in sinoaortic-denervated rats

We tested the hypothesis that the sinoaortic afferents may contribute to normoxic thermogenesis and to the magnitude of the hypometabolic response to hypoxia. Adult rats were either sinoaortic denervated (SAX; n = 20) or sham operated (Sham; n = 20). A few days after the operation, gaseous metabolism [O2 uptake (VO2) and CO2 production (VCO2)] was measured with an open-flow system at ambient temperatures (Tamb) of 20, 25, 30, and 35 degrees C as the animal was resting awake. At thermoneutrality (Tamb 30 degrees C) or higher Tamb there was no difference in VO2 or VCO2. Below thermoneutrality, metabolic rate was significantly lower in SAX than in Sham animals (-14 and -16% at 20 and 25 degrees C, respectively). Colonic temperature and arterial PO2 were also slightly less, whereas arterial PCO2 and pH, mean arterial pressure, and heart rate did not differ significantly between the two groups. Exposure to acute hypoxia (10% inspired O2, 20-30 min) at Tamb 20 and 25 degrees C significantly reduced VO2 in both groups to a similar value; hence, at either Tamb, the metabolic drop during hypoxia in Sham animals was larger than that in SAX animals. Hypercapnia (5% CO2 breathing) did not change VO2 in either group. We conclude that in the rat at Tamb slightly below thermoneutrality, the sinoaortic afferents 1) provide a small but significant contribution to normoxic thermogenesis and 2) are not required for the manifestation of the drop in metabolism during hypoxia.