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

M E Pedersen,M Cohen,M C Schotz

doi:10.1016/s0022-2275(20)37956-6

Abstract

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.

Highlights

The functionalfraction of myocardial lipoprotein lipase (LPL) has been located at the lumen surface of capillary endothelium by means of an indirect immunocytochemicalperfusion method for electron microscopy
We describe here the indirect immuno-electron microscopic localization of functional LPL at capillary lumen surfaces in isolated rat hearts and the positive association between extents of immunocytochemical reaction and levels of heparin-releasable LPL5 resulting from fat- [10] or glucose-feeding [11] or heparin perfusion
With preimmune immunoglobulin G (IgG) substituted for primary step anti-LPL IgG, peroxidase reaction product was extremely low to absent in capillaries of hearts from either glucose- or fat-fed rats (Fig. 4)

Summary

Introduction

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 immunocytochemicalperfusion method for electron microscopy. Lipoprotein lipase (LPL) is generally considered t o be present at the lumen surface of capillary endothelium and to be involved in hydrolysis of adherent chylomicron and very low density lipoprotein triglycerides Evidence for this location as the functional site of LPL includes the rapidity with which passage of heparin through capillary beds releases LPL activity in vivo or in vitro [1] and the subsequent loss of the capacity to hydrolyze chylomicron triglycerides [2]. Positive histochemical reactions involving capillaries have been obtained by electron microscopy with lactating mouse mammary gland after intravenous injection of Intralipid [7] and with chylomicrons perfused through rat parametrial adipose tissue [8] This histochemical approach may reveal free fatty acids in tissues, such an approach would fail to demonstrate LPL-active sites lacking adherent chylomicrons o r very low density lipoproteins. When an antiserum against this enzyme, a pig adipose tissue lipase, was used as primary step antiserum in light microscopic indirect immunohistochemistry of various pig tissues, a positive reaction was Abbreviations: IgG, immunoglobulin G; KRB, Krebs-Ringer bicar-

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