Postnatal development of lipid-clearing enzymes in the suckling animal.

Age-related changes in the enzyme activities in the storage and secretory pools of extrahepatic lipoprotein lipase and hepatic triglyceride lipase were determined during a primed/constant infusion of heparin for 2 hr in puppies between birth and 18 wk of life. Lipoprotein lipase activity was low in the first 4 wk of suckling. Its storage pool increased 6-fold in the next 14 wk, with a less dramatic rise in the secretory pool. Sustained increases in the activity of this enzyme were seen late when (1) the pup was being weaned, and (2) the insulin-dependent glucoregulatory mechanism had matured. Both phases of hepatic triglyceride lipase were well developed at birth reflecting the metabolic maturity of the liver at birth. The ability to clear intravenous lipid during a 2-hr infusion was also measured in the pups. The rate of Intralipid utilization increased from 198 +/- 43 mg/kg/hr in the first week of life to 424 +/- 20 mg/kg/hr at 12 wk. Blood Intralipid homeostasis was causally related to increased activity in the storage and secretory pools of lipoprotein lipase. The progressive increase in lipoprotein lipase activity and the maintenance of the secretory capacity was dependent on the growth of the muscle mass and its capillary bed. Endogenous insulin rather than the fat content of the feed appeared significant in the postnatal development of the heparin-releasable lipid clearing enzymes.