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Abstract
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 degrees C with sulfo-N-hydroxysuccinimidyl-biotin. Incorporated biotin was then labeled with avidin-horseradish peroxidase complex for light microscopy, with avidin-lucifer yellow conjugate for fluorescence microscopy, and quantitative fluorometry, and with avidin-ferritin conjugate for electron microscopy. RESULTS: At 4 degrees 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 degrees 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 degrees C before chilling and labeling with avidin-lucifer yellow decreased the cells' capacity to bind avidin-lucifer yellow by 95%, with t0.5 < 0.5 min. If cells were then incubated with avidin-lucifer yellow at 37 degrees C, they took up the marker with a time course that indicated that 60% of the initial biotin either recycled back to the plasma membrane or remained in intracellular compartments that could be reached by endocytosed extracellular fluid. Internalized biotin communicated with extracellular avidin-lucifer yellow with a t0.5 of 2 min, and this process was accelerated by carbachol at concentrations of 10 mumol/l and 1 mmol/l. CONCLUSIONS: Acinar cell plasma membrane constituents participate in an ongoing, secretagogue-modulated recycling traffic between small surface-expressed pools and 10- to 20-fold larger intracellular pools.