Regulation of fructose 2,6-bisphosphate levels in cold-acclimated brown adipose tissue of rat.

The effects of cold exposure and T4 administration on fructose 2,6-bisphosphate levels, phosphofructokinase-2 and pyruvate kinase activities were examined in rat brown adipose tissue. Cold adaptation (14 days) gave rise to a 2-fold increase in the amount of fructose 2,6-bisphosphate and phosphofructokinase-2 activity, and increased the pyruvate kinase activity 4-fold. If, in addition, the cold-acclimated rats were treated with T4, these parameters were again significantly enhanced. The effect on phosphofructokinase-2 was on the Vmax, without modification of the Km (for both fructose 6-phosphate and ATP) of the enzyme. In the hypothyroid state, however, the activity of pyruvate kinase remains unchanged. These data support previous observations on stimulation of glycolytic flux during cold adaptation in brown adipose tissue, and a permissive role of thyroid hormones in the process.