Quantification in vivo of the effects of insulin on glucose utilization in individual tissues of warm- and cold-acclimated rats.

Cold-acclimation of rats (3 weeks, 4 degrees C) had no effect on basal rates of glucose production or utilization. Under euglycaemic-clamp conditions, in which the circulating insulin concentration was increased by approx. 50 microunits/ml, cold-acclimated rats showed a greater increase in glucose utilization and a greater suppression of endogenous glucose production. Tissue sites of glucose utilization were investigated by using a tracer dose of 2-deoxy-D-[14C]glucose and a glucose metabolic index determined for each tissue. In 5 h-starved warm-acclimated rats, heart had the highest glucose metabolic index. This was increased further by both cold-acclimation and insulin treatment. The glucose metabolic index of skeletal muscle was 3.5-14-fold lower than that of heart, but, as a result of the large muscle mass, skeletal muscle made the largest contribution to whole-body glucose utilization. White and brown adipose tissue had low glucose metabolic indices in warm-acclimated rats under basal conditions, and the indices were not increased by the insulin treatment. However, cold-acclimation produced a significant increase in the glucose metabolic index of brown adipose tissue, but not of white adipose tissue. In contrast with the warm-acclimated rats, insulin treatment of cold-acclimated rats resulted in a marked increase in the glucose metabolic index of brown adipose tissue. The results provide evidence that cold-acclimation produces a selective alteration in the insulin-sensitivity of brown adipose tissue.