Heparin-independent release of lipoprotein lipase activity from perfused rat hearts

Heparin-independent release of lipoprotein lipase activity from isolated perfused rat hearts was measured and related to the rapid turnover of the enzyme. Hearts consistently released lipoprotein lipase activity (2.1 +/- 0.2 U/g released per min) during 60 min of nonrecirculating perfusion without heparin. This rate of release did not significantly differ from that measured in heparin-perfused hearts after the first 10 min of perfusion (2.2 +/- 0.2 U/g released per min). The fractional release rate of lipoprotein lipase activity during nonheparin perfusion was 1.3% per min, which was higher than that calculated for alkaline phosphatase (0.002%) and creatine kinase (0.03%) activities. The lipase activity released was activated 4-fold by serum and inhibited 94 and 88% by 0.5 M NaCl and 3 mg/ml protamine sulfate, respectively. Lipoprotein lipase activity in the 1-min heparin-releasable (extracellular) and residual (intracellular) compartment remained stable during the last 40 min of nonheparin perfusion. During this period total heart, intracellular and extracellular enzyme t1/2 were calculated to be 52, 42 and 10 min, respectively. The results are consistent with the postulate that continuous release of lipoprotein lipase into the vascular compartment may be an important determinant of its rapid turnover in the heart, and possibly other tissues.