Selective internalization of granule membrane after secretion in mast cells.

[3H]Galactose, covalently bound to cell surface glycoconjugates of rat peritoneal mast cells, was used to study internalization of labeled plasma membrane and granule membrane constituents before or after secretion stimulated by compound 48/80. Internalized label was distinguished quantitatively from label on the cell surface by its inaccessibility to enzymatic removal. Three different situations were compared. (i) With label only on the plasma membrane, and in the absence of secretion, incubation at 37 degrees C (but not at 0 degree C) resulted in a gradual decrease of label on the cell surface until, after approximately equal to 2 hr, a steady state was reached with 93% of all cell-bound label remaining on the cell surface. Recycling of internalized label was demonstrated. (ii) When cells were labeled on the plasma membrane and then stimulated to secrete, subsequent retrieval of (unlabeled) granule membrane did not affect the rate or extent of simultaneous internalization of labeled plasma membrane. (iii) When both plasma membrane and exposed granule membrane were labeled after secretion, subsequent incubation at 37 degrees C (but not at 0 degrees C) resulted in approximately equal to 33% of all cell-bound label becoming internalized during 4 hr, indicating additional internalization of label due to retrieval of labeled granule membrane. In all three cases, loss of label into the medium occurred with a half-life of 8-11 hr, showing that no extensive shedding of granule membrane occurred after secretion. The results suggest either that no mixing of labeled membrane constituents occurred between the plasma membrane and granule membrane or that during retrieval of granule membrane, sorting of membrane was taking place at the cell surface.