Glucocorticoids and regulation of phosphoenolpyruvate carboxykinase activity in rat brown adipose tissue.

Studied were performed to examine the factors that might regulate phosphoenolpyruvate carboxykinase (PEPCK) activity in rat brown adipose tissue (BAT) and to determine the role played by glucocorticoids in regulating this enzyme. Comparison was made to white adipose tissue (WAT) where PEPCK activity is known to be glucocorticoid regulated. PEPCK activity in BAT did not respond to adrenalectomy or dexamethasone, whereas WAT activity was increased and decreased, respectively, by these maneuvers. Three conditions were found in which BAT PEPCK activity was stimulated: 1) fasting, 2) feeding a high-fat/low-carbohydrate diet, and 3) during the neonatal period. In each case glucocorticoid treatment prevented the stimulation in PEPCK activity and restored the enzyme to base-line levels. In conditions 1 and 2, enzyme activity was also stimulated in WAT, but in contradistinction to BAT, glucocorticoid administration reduced activity to low levels significantly below base-line activity. Two conditions were found which suppressed PEPCK activity in BAT: exposure to a cold environment and feeding a high-protein/low-fat diet. WAT PEPCK was unaltered by exposure to cold. Thus, differences in PEPCK regulation between BAT and WAT were demonstrated, and the response to glucocorticoids was unique in BAT.