Hepatic microsomal 3-hydroxy-3-methylglutaryl-CoA reductase activity in the lactating rat.

Liver microsomal 3-hydroxy-3-methylglutaryl-CoA (HMGCoA) reductase activity increases after parturition in the female rat. The elevated activity observed within 24- to 36-h postpartum was sustained throughout lactation but declined to control levels by day 30, the natural weaning age of the pups. Premature weaning, induced by removal of the offspring, returned HMGCoA reductase activity to normal within 2-4 days. The elevation in HMGCoA reductase was not due to elimination or inversion of the natural diurnal rhythm of this enzyme, which was similar in phase, but not in amplitude, in lactating and control animals. Elevated feed intake in the lactating animal may have contributed to the increase in HMGCoA reductase since pair feeding with a nonpregnant, nonlactating control rat abolished the elevation in enzyme activity observed during lactation. Elevated food intake did not induce the elevation in HMGCoA reductase in lactating rats, however, since the major change in food intake occurred after the initial increase in enzyme activity.