3-Hydroxy-3-methylglutaryl-CoA reductase activity in liver, brown fat, and small intestine of developing rats

3-Hydroxy-3-methylglutarylCoA reductase activity was found to be low in the microsomal fraction of the small intestine, liver, and brown fat in suckling rats. It was high perinatally. Treatment with phosphatase of the microsomal fraction increased activity considerably in gut, somewhat in liver, and not al all in brown fat of infant rats. Activity in the mitochondrial fraction of the small intestine showed no developmental changes. injections of infant rats with triiodothyronine increased and with cortisone decreased hepatic activity but had no effect on gut or brown fat. It is concluded that even though activity decreases in all three tissues after birth, it is regulated differently in liver than in gut and brown fat.