Metabolism and Muscle Activity of Anesthetized Warm and Cold Acclimated Rats on Exposure to Cold

¹ From the Division of Applied Biology, National Research Laboratories, Ottawa, Canada

Two groups of rats, in two different series, acclimated for 4–6 weeks were anesthetized with barbital to a similar degree as judged by maxillary reflex measurements. The 6°C acclimated rats had, at any moment, a higher O₂ consumption and a higher rectal and muscle temperature than the warm acclimated rats when these variables were measured at 30°C. About 30 minutes after transfer to 6°C, cold acclimated rats showed approximately a twofold increase, and the warm acclimated rats a 30–50% increase, in oxygen consumption. The continuous fall in colonic and leg muscle temperatures during the 1 to 1 hr. exposure to 6°C was less marked in the cold than in the warm acclimated rats. Electromyograms from leg and back muscles of warm acclimated rats showed a marked and continuous rise in electrical output that frequently preceded the rise in oxygen consumption by 15 to 20 minutes. Cold acclimated rats on the average showed no change in the electromyogram after exposure to 6°C. The metabolic response of these rats to cold apparently occurred without physical activity of the muscles. Thus, cold acclimated anesthetized rats, exposed to cold can have a greater metabolic response than warm acclimated rats, possibly by virtue of chemical thermogenesis alone.