Immunocytochemical localization of the functional fraction of lipoprotein lipase in the perfused heart.

The functional (heparin-releasable) fraction of myocardial lipoprotein lipase (LPL) has been located at the lumen surface of capillary endothelium by means of an indirect immunocytochemical perfusion method for electron microscopy. The primary step immunoreactant was an IgG fraction of goat antiserum directed against LPL from rat heart. The second step antibody, conjugated with horseradish peroxidase, was rabbit IgG directed against goat IgG. Peroxidase reaction product, when present, appeared at the surface an in invaginations of the lumenal plasma membrane of capillary endothelium and also on chylomicrons adherent to that membrane. The highest coverage by such product occurred when the highest heparin-releasable heart LPL activity was attained after fat-feeding of rats. Coverage was low when a low level of heparin-releasable heart LPL activity was induced by carbohydrate-feeding. Coverage was very low in the perfused hearts after heparin-release of functional LPL activity. The positive association between these immunocytochemical results and actual levels of functional LPL activities indicates that functional LPL in the isolated rat heart is at the lumen surface of capillary endothelium.