Luminal Membrane Retrieved After Exocytosis Reaches Most Golgi Cisternae in Secretory Cells

Dextran was used to trace membrane retrieved from the luminal surface after induced exocytosis in secretory cells of rat lacrimal and parotid glands. Two different approaches were used: (a) isolated acini were incubated in vitro with dextran followed by stimulation with carbamylcholine (lacrimal) or isoproterenol (parotid) and (b) rats were injected with isoproterenol followed by dextran infusion into the parotid duct in vivo. The main findings were the same regardless of the gland source or experimental approach. Dextran was taken up initially via coated pits into smooth-surfaced apical vesicles. Shortly thereafter it was found in multiple cell compartments: within the stacked Golgi cisternae, in condensing vacuoles, and in lysosomes. Uptake was more rapid and uniform in vivo; dextran was seen in multiple cisternae of numerous Golgi complexes within 5 min after infusion. In acini incubated in vitro uptake into Golgi cisternae was more delayed and occurred with increasing frequency up to 60 min; also, more dextran was taken up into lysosomes, which were more numerous in vitro than in vivo. The results demonstrate that, after exocytosis, membrane is removed from the cell surface via vesicles that fuse with multiple cell compartments. The two novel findings are: (a) the demonstration that the tracer can reach most of the Golgi cisternae in a given stack and (b) the demonstration of the rapidity with which the process takes place (i.e., within 5 min). The findings imply that at least some membrane retrieved from the cell surface after exocytosis fuses with the stacked Golgi cisternae.