SKELETAL AND CARDIAC-MUSCLE PROTEIN-TURNOVER DURING SHORT-TERM COLD-EXPOSURE AND REWARMING IN YOUNG-RATS

Young animals exposed to cold environmental temperatures typically hav e decreased skeletal muscle accretion but increased heart masses. To e xplore these phenomena, we measured protein synthesis and degradation in vivo in cardiac and skeletal muscle in weanling rats during short-t erm cold exposure and rewarming. Control rats were housed at 25 degree s C throughout the experiment. Ad libitum-fed and pair-fed (to the int ake of controls) rats were housed at 5 degrees C (cold) for 5 days and then at 25 degrees C (rewarmed) for another 5 days. Cold exposure dec reased rates of protein accretion and synthesis in skeletal muscle, wh ereas degradation did not differ. The effects of cold exposure on skel etal muscle were similar in both pair-fed and ad libitum-fed rats, exc ept growth was lower in pair-fed rats. In cardiac muscle, cold exposur e increased rates of protein synthesis and degradation and resulted in increased cardiac mass. Results in pair-fed animals generally fell be tween those of control and ad libitum-fed cold rats. During rewarming, growth rates were not higher in skeletal muscle in ad libitum-fed rew armed rats, although protein turnover returned toward control values; in pair-fed rats, it remained lower. In heart, growth rates of ad libi tum-fed and pair-fed rewarmed rats decreased due to lower protein synt hesis rates. These alterations appear to be consistent with a strategy designed to improve survival in cold environments.