Age-related differences in response to acute cold challenge (-10 degrees C) in male F344 rats.

Male F344 rats, 3, 12, and 24 months of age, were acclimated to either 5 degrees C (CA) or 23 degrees C (non-CA). At the end of acclimation, all CA rats exhibited higher basal oxygen uptake than the non-CA when measured at 28 degrees C. When challenged by a 3-hour exposure to -10 degrees C, significant difference in tolerance to cold was observed not only between the CA and non-CA rats but among the different age groups. The 3-month-olds tolerated cold better than 12-month-olds, and the 12-month-olds did better than the 24-month-olds. The ranking order for cold tolerance in -10 degrees C was: 3-month CA greater than 12-month CA greater than 3-month non-CA = 12-month non-CA = 24-month CA greater than 24-month non-CA. Total 3-hour O2 uptake decreased progressively with advancing age. Heat productions were doubled in -10 degrees C in all age groups, and all CA rats had higher heat production than non-CAs. The increase of heat production in -10 degrees C was insufficient to maintain body temperature in all animals. All animals had loss of body heat content, the loss becoming greater with advancing age. Non-CA rats had greater loss of heat content or faster decline of colonic temperature than the CAs. Significant strain-related difference in cold tolerance was noted between the F344 and Sprague-Dawley (S-D) rats, because 24-month-old non-CA male F344 rats still exhibited a better tolerance to -10 degrees C than 3-month-old CA S-D rats.