Regional blood flow of exercise-trained younger and older cold-exposed rats

O₂ consumption (thermogenesis) and regional blood flows (measured using radioactively labeled microspheres) were evaluated in younger (12 mo) and older (24 mo) sedentary and exercised male Fischer 344 (F-344) rats. These variables were measured at rest and during exposure to 6°C. Exercise-trained rats were run on a motor-driven treadmill 5 days/wk, 1 h/day, at 20 m/min for 6 mo. Resting rates of O₂ consumption did not differ with age or exercise training. However, thermogenesis during cold exposure was significantly greater in the older exercised rats than in the other three groups. This difference did not reflect a greater contribution from brown fat as indicated by the fact that total blood flow to the brown fat depots during cold exposure was not greater in the older exercised vs. the other rat groups. Neither exercise training nor age had a significant effect on specific resting blood flow (expressed as ml·min^-1·g tissue mass^-1) to most of the organs measured, including heart, kidney, brown fat, white fat, and skeletal muscle. The notable exception to this was in the spleen of the older sedentary animals where flow was diminished compared with that in the older exercised animals. We conclude that aging, between 12 and 24 mo of age, and/or exercise training have only a minor effect on regional blood flow of F-344 rats during rest or cold exposure and that the enhanced thermogenesis seen in cold-exposed older exercised vs. sedentary F-344 rats cannot be explained by a greater contribution from brown fat.