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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1995;15:972-981.)
© 1995 American Heart Association, Inc.

Articles

Uptake of Radiolabeled and Colloidal Gold–Labeled Chyle Chylomicrons and Chylomicron Remnants by Rat Platelets In Vitro

Ning Xu; Li Zhou; Rolf Odselius; Åke Nilsson

From the Department of Cell Biology 1 (N.X., L.Z., Å.N.), Electron Microscopy Unit (R.O.), and Department of Medicine (Å.N.), University Hospital of Lund, Sweden.

Correspondence to Åke Nilsson, MD, PhD, Department of Internal Medicine, University Hospital of Lund, S-221 85 Lund, Sweden.

Abstract This study examined the uptake of chyle chylomicrons (CMs) and chylomicron remnants (CMRs) by rat platelets in vitro. CMs and CMRs were doubly labeled with [³H]arachidonate ([³H]-20:4) and [¹⁴C]cholesterol and were incubated with platelets for up to 4 hours. A significant uptake (binding and/or internalization) of CMs by the platelets occurred, as indicated by the parallel increase of [³H]20:4 and [¹⁴C]cholesterol in platelets with incubation time. Addition of unlabeled CMs, VLDLs, LDLs, and HDLs decreased the uptake of labeled CMs. The competition experiments suggested that there is both a saturable binding and a nonspecific uptake of CMs. During incubation with CMs, the proportion of [³H]20:4 in phospholipids decreased and that in 1,2-x-diacylglycerol increased. The data indicated that a phospholipase C–mediated degradation of phosphatidylcholine and phosphatidylethanolamine occurred, whereas [³H]20:4 in triglyceride and ¹⁴C in cholesteryl ester did not change. Electron microscopic studies after incubation with colloidal gold–labeled CMs (CM-Au's) demonstrated an accumulation of CM-Au particles in the open canalicular system of the platelets. Some CM-Au particles were localized in cytoplasmic vacuoles that were not stained by ruthenium red. Some CM-Au's or free gold particles were in vacuoles that showed acid phosphatase activity, indicating that some true endocytosis of CM occurred. The uptake of [³H]-20:4– and [¹⁴C]cholesterol-labeled CMRs was low compared with the uptake of CMs. After incubation with colloidal gold–labeled CMRs (CMR-Au's), only a few platelets contained CMR-Au in their open canalicular systems, and no CMR-Au particles were seen in the cytoplasm or in acid phosphatase–positive vacuoles. Rat platelets can thus interact with CMs by a process that leads to a sequestration in the open canalicular system and endocytosis and a net degradation of CM phospholipids. The conversion of CMs to CMRs counteracts this interaction.

Key Words: lipoproteins • metabolism • endocytosis • electron microscopy • cytochemistry

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