Stimulation of mouse peritoneal cells by neutral glycerides. A study by light and electron microscope autoradiography using tritiated glycerides

New approaches to the study of sphingolipid enriched membrane domains: the use of electron microscopic autoradiography to reveal metabolically tritium labeled sphingolipids in cell cultures

This paper is the first report on the use of the electron microscopy autoradiography technique to detect metabolically tritium labeled sphingolipids in intact cells in culture. To label cell sphingolipids, human fibroblasts in culture were fed by a 24 hours pulse, repeated 5 times, of 3 x 10(-7) M [1-(3)H]sphingosine. [1-(3)H]sphingosine was efficently taken up by the cells and very rapidly used for the biosynthesis of complex sphingolipids, including neutral glycolipids, gangliosides, ceramide and sphingomyelin. The treatment with [1-(3)H]sphingosine did not induce any morphological alteration of cell structures, and well preserved cells, plasma membranes, and intracellular organelles could be observed by microscopy. Ultrathin sections from metabolic radiolabeled cells were coated with autoradiographic emulsion. One to four weeks of exposition resulted in pictures where the location of radioactive sphingolipids was evidenced by the characteristic appearance of silver grains as irregular coiled ribbons of metallic silver. Radioactive sphingolipids were found at the level of the plasma membranes, on the endoplasmic reticulum and inside of cytoplasmic vesicles. Thus, electron microscopy autoradiography is a very useful technique to study sphingolipid-enriched membrane domain organization and biosynthesis.

Identification of a potential artifact in the use of electron microscope autoradiography to localize saturated phospholipids in cells

The suitability of electron microscope autoradiography for sutdying the uptake and intracellular localization of lipid vesicles (liposomes) containing radiolabeled saturated phospholipids has been examined. Data are presented showing that preparation of specimens for electron microscope autoradiography by conventional methods is accompanied by significant translocation and intercellular redistribution of radiolabeled saturated lipids, causing spurious labeling patterns. Intercellular redistribution of radiolabeled lipid was demonstrated by mixing glutaraldehyde-fixed mous L1210 cells that had been incubated with sonicated lipid vesicles containing [H] dipalmitoyl phosphatidylcholine with an indicator cell population (fixed avian erythrocytes) which had not been exposed to vesicles and showing that after electron microscope processing radiolabeled grains were present in both cell types. The same redistribution artifact also probably affects the intracellular localization of radiolabeled lipids. This artifact is discussed in relation to previous work in which autoradiographic methods have been used for ultrastructural localization saturated phospholipids in cells and tissues.

Endotoxin clearance after intralipid infusion

Healthy 6-8-wk-old New Zealand white rabbits were injected with chromium-chloride- or sodium-chromate-labeled E. coli endotoxin after rapid infusion (10 ml/kg in 1 hr) or slow and repeated infusions (40 ml/kg daily for 7 consecutive days) of 10% Intralipid. Endotoxin clearance rates and RES organ uptakes were determined and the results were compared with those of the controls treated with correspondingly equal volumes of 5% D/W instead of fat. In the acute experiment, the clearance rates were similar in all animals during the first 15 min following endotoxin injection. After this phase, however, experimental animals had faster endotoxin clearance and eventually higher organ uptakes than the controls. In the chronic experiment, there was no significant difference in endotoxin clearance rates or total and per-gram organ uptakes between experimental and corresponding control animals infused with 5% D/W instead of fat. Experimental animals, particularly those having received multiple infusions of fat emulsion, showed deposition of polarizable brown pigment inside and outside the reticuloendothelial cells in the liver, spleen, and bone marrow. None of the controls had these pigments in their organs.