Release of lipoprotein lipase and hepatic lipase activities. Effects of heparin and a low molecular weight heparin fragment.

Unfractionated heparin and a low molecular weight heparin fragment (LMWH, mean molecular weight 5000) were compared with respect to the ability to release lipoprotein lipase (LPL) and hepatic lipase (HL) from tissue binding sites. The investigations were carried out in vivo and in vitro in man and in the rat. The in vivo release of LPL activity was greater with heparin in both species. The release of HL activity was equal with both heparins in man, but greater with unfractionated heparin in the rat. In the in vitro studies the LMWH fragment consistently released more LPL activity from the tissues investigated, i.e. fat, skeletal muscle and heart muscle. With isolated adipocytes, however, we could not demonstrate any difference between the two heparins with respect to their lipase-releasing effect. In liver tissue homogenate the two heparins showed the same ability to release HL activity. It is concluded that not only biochemical differences between the two heparins are of importance for the difference in the lipolytic effect. The possibility of different rheological behaviours of the two heparins in the capillary lumen, which might partly explain the greater in vivo lipolytic effects of heparin, is discussed. It is suggested that LMWH enters the tissue preparation more readily in vitro, thereby exerting a greater lipase-releasing effect than heparin.