Purpose. The purpose of this study was to examine internalization and recycling of plasma membrane constituents in lacrimal gland acinar cells. Methods. Acinar cells were isolated from rabbit lacrimal glands. Surface-expressed reactive groups were biotinylated at 4°C with sulfo-N-hydroxysuccinimidyl-biotin. Incorporated biotin was then labeled with avidin-horseradish peroxidase complex for light microscopy, with avidinlucifer yellow conjugate for fluorescence microscopy, and quantitative fluorometry, and with avidin-ferritin conjugate for electron microscopy. Results. At 4°C labels remained at the surfaces of intact cells. Surface avidin-lucifer yellow decreased markedly, giving way to punctate cytoplasmic labeling, on warming to 37°C. Electron microscopy of cells warmed after labeling with avidin-ferritin revealed ferritin in smooth vesicles underlying the plasma membranes, in vesicles adjacent to Golgi membranes, and in multivesicular bodies. Incubation at 37°C before chilling and labeling with avidin-lucifer yellow decreased the cells ' capacity to bind avidin-lucifer yellow by 95%, with t05 < 0.5 min. If cells were then incubated with avidin-lucifer yellow at 37°C, they took up the marker with a time course that indicated that 60 % of the initial biotin either recycled back to the plasm
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