Study on membrane fragments released from the sublingual glands of the mouse during secretion in vitro and in vivo.

During in vitro secretion membrane fragments are released by the sublingual glands (SL) of the mouse. After stimulation of saliva with 1 microM carbamylcholine, these membranes have been isolated by centrifugation at 100,000 X g for 1 h. The release of 0.56% of the total tissue content of DNA during a 3 h period denotes that the amount of damaged tissue is very low. After 3 h, based on the determination of sialic acid, 25.4% of the secretory product, the sublingual mucin, has been released. The amount of membrane-bound alkaline phosphatase, released during a period of 3 h is 0.63%. As at least three quarters of this amount is due to broken cells, the amount of membrane-bound alkaline phosphatase released during the secretory process (0.16%) is very low compared to the amount of secretory product (25.4%). The low amount of alkaline phosphatase is in accordance with EM observations, which show that secretory granule membranes lack alkaline phosphatase activity. However, at those locations, where luminal membranes fuse with the granule membranes, alkaline phosphatase has been detected. So, the low alkaline phosphatase activity may be due to the presence of some luminal membranes in the secretory product. The involvement of alkaline phosphatase in the secretory process is also indicated by the complete inhibition of the secretory process by tetramisole (5 mM). The SL membrane preparation, isolated from the incubation fluid, had a relatively simple electrophoretic pattern and some antigenic determinants in common with the granular membranes from the Par and SM glands. The phospholipid composition of the released SL membranes differed strongly from the Par and SM granule membranes, especially in their relatively low amount of sphingomyelin (6.8%) and their high amount of phosphatidylethanolamine (26.4%). The lysophosphatidylcholine was only 3.5%. Among others, the phospholipid composition of the SL membranes may be responsible for the specific properties and behaviour of the sublingual granular membranes during the secretory process.