Lipoprotein lipase during continuous heparin infusion: Tissue stores become partially depleted

Lipoprotein lipase (LPL) and hepatic lipase (HL) are located at vascular su rfaces in extrahepatic tissues and in the liver, respectively. Heparin disp laces the enzymes into the circulating blood. Animal studies have shown tha t the liver takes up and degrades LPL. To explore whether heparin leads to a depletion of tissue stores, we followed the lipase activities in plasma d uring an 8-hour primed infusion of heparin in 10 healthy subjects. After an initial peak, the HL activity decreased slowly after a time curve similar to that for activated partial thromboplastin time. The time curve for LPL w as different. After the initial peak, the activity dropped by almost 80%, f rom 30 to 120 minutes, and then leveled off to a plateau that corresponded to about 15% of the peak level. A second bolus of heparin was given to 4 su bjects after 4 hours. The plasma LPL activity increased, but only to about 35% of the original peak level. We conclude that when heparin releases LPL into plasma, the lipase becomes liable to be taken up and degraded by the l iver. After less than 1 hour, the stores of LPL have been exhausted, and re cruitment of lipase into plasma depends on a slow but stable delivery of ne wly synthesized molecules.