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The use of liposomes as encapsulating agents for feeding juvenile Pacific oysters (Crassostrea gigas)



Graduation date: 1981The ingestion, uptake, and metabolism of liposomes by juvenile\ud Pacific oysters (Crassostrea gigas) were studied by several methods in\ud an effort to assess their potential as encapsulating agents. Liposomes\ud composed of egg phosphatidylcholine-cholesterol-stearylamine (7:1:2)\ud formed readily and appeared stable in 20°/oo seawater. Radiotracer\ud studies with liposomes made with ¹⁴C-labeled cholesterol or phosphatidylcholine\ud showed uptake of up to 40% of the dose in 24 hrs, with the\ud majority of uptake occurring in the visceral mass. Only slight amounts\ud of label were observed in adductor muscle or mantle tissue. Absence of\ud label in free fatty acids in oysters fed liposomes made with di[l-¹⁴C]\ud palmitoyl phosphatidylcholine indicated a lack of significant amounts of\ud fatty acid hydrolysis from phospholipid in the stomach or lumen of the\ud digestive diverticula. However, radioactivity was observed in lipid\ud other than phosphatidylcholine, including triglyceride, phosphatidylethanolamine,\ud and an unidentified polar lipid. Radioactivity in these\ud lipids resided exclusively in the fatty acids, indicating breakdown of\ud the ¹⁴C-phosphatidylcholine via acyl transfer.\ud To examine metabolism of liposome-encapsulated substances,\ud [1-¹⁴C]glucose and [U-¹⁴C]amino acids were entrapped and fed to oysters.\ud Label from glucose appeared largely in a choloroform-methanol-insoluble\ud fraction, with little radioactivity recovered in the lipid or soluble\ud aqueous fractions. Most label from amino acids was recovered in trichloroacetic\ud acid-precipitable protein. Control oysters given the same\ud amounts of non-encapsulated [1-¹⁴C] glucose or [U-¹⁴C]amino acids as in\ud liposome trials showed (1) the same uptake of label from free amino\ud acids in comparison with encapsulated glucose, and (2) increased uptake\ud of free, amino acids in comparison with encapsulated amino acids. Label\ud from free glucose or amino acids entered the same fractions as encapsulated\ud label.\ud Evidence for intracellular uptake of liposomes was obtained with\ud fluorescence microscopy after feeding oysters with liposomes containing\ud bovine serum albumin conjugated with fluorescein isothiocyante (FITC).\ud The appearance of small fluorescent inclusions within the apical portions\ud of many of the ducts and tubules of the digestive diverticula suggest\ud phagocytosis of intact liposomes. Uptake was not observed in other\ud parts of the alimentary canal. The feeding of liposomes in which the\ud stearylamine had been conjugated with FITC resulted in generalized\ud fluorescence in most of the digestive diverticula and stomach epithelium,\ud perhaps due to extracellular hydrolysis of FITC and its subsequent\ud diffusion into epithelial cells. No fluorescence occurred in tissues\ud other than those of the digestive tract. Autoradiography studies with\ud liposomes containing di[l-¹⁴C]palmitoyl phosphatidylcholine showed\ud radioactivity dispersed throughout the epithelial cells of the ducts\ud and tubules of the digestive diverticula. Only slight radioactivity\ud was observed in the intertubular connective tissue or the lumen of the\ud tubules or stomach. This distribution of liposomal materials resembled\ud that of fluorescence from feeding trials with FITC-tagged liposomes,\ud and indicated uptake of intact liposomes followed by intracellular\ud breakdown and dispersal of the liposomal components.\ud To investigate the process of particle selection in oysters,\ud polyacrylamide beads (2 [plus or minus] 1μ) with aminoethyl side groups, and beads\ud with FITC-conjugated side groups were fed to oysters. Large quantities\ud of both types of beads were observed in the stomach and intestine, but\ud not in the digestive diverticula, indicating recognition as non-food\ud particles despite their organic nature. The ingestion of such derivitizable\ud particles suggests their use in studies of acceptance-rejection\ud processes in the stomach of bivalves.\ud The ingestion, intracellular uptake, and breakdown of liposomes\ud and their contents indicates a use for these particles in studies of\ud nutrition or pollutant-food web relationships in bivalve molluscs or\ud other filter-feeding organisms

Year: 1980
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