Regulation of phosphoenolpyruvate carboxykinase during postnatal development in the rat.

Phosphoenolpyruvate carboxykinase (EC 4.1.1.49) (PEPcK) activity is high in the liver, brown fat, and small intestinal mucosa of suckling rats. Starvation results in a rise in hepatic activity in newborn and older animals, but a fall in brown fat from rats younger than 14 days. At that age, activity rises in all three tissues on fasting. Both triiodothyronine (T3) and dexamethazone depress activity in gut and (as previously reported) in brown fat, but elevate it in liver. Cold exposure also has a similar effect. Both alpha-antagonist and agonist injections result in elevated hepatic activity. T3 given together with these more than doubles the effect of either drug or T3 given alone. In brown fat only antagonists decrease PEPcK activity, whereas in gut both agonists and antagonist have an inhibitory effect. It is concluded that PEPcK activity in the three tissues responds differently to the same stimuli with gut mucosa being more similar to, but not the same as, brown fat than to liver.