HEPARIN-RELEASABLE LIPOPROTEIN-LIPASE ACTIVITY IS INCREASED IN CARDIOMYOCYTES AFTER CULTURE

The activity of lipoprotein lipase (LPL) in adult rat heart cardiomyoc ytes after overnight culture on laminin-coated plates for 18-22 h was compared with enzyme activity in freshly isolated cardiomyocytes. LPL activity in cellular homogenates from cultured cardiomyocytes and fres hly isolated cells was 240 and 233 nmol oleate released.h(-1).mg(-1) p rotein, respectively. LPL specific activity (mU/ng LPL protein) was 0. 07 in cultured cells compared with 0.42 in freshly isolated cells, ind icating an increased content of inactive LPL mass after overnight cult ure. The heparin-induced release of LPL activity into the medium of cu ltured cardiomyocytes (198 nmol.h(-1).mg(-1)) was much greater than he parin-releasable LPL (HR-LPL) activity (59 nmol.h(-1).mg(-1)) from fre shly isolated cells. HR-LPL activity from cultured cardiomyocytes was dependent on serum (16.3-fold activation) and was inhibited by high io nic strength (1 M NaCl) and by a polyclonal antibody to LPL. Cultured cardiomyocytes also had more immunodetectable LPL on the cell surface compared with freshly isolated cardiomyocytes, consistent with increas ed HR-LPL activity. Therefore, overnight culture may permit cardiomyoc ytes time to recover from the stress of isolation by increasing the co ntent of LPL on the cell surface.