EFFECTS OF OSMOTIC-STRESS ON MAST-CELL VESICLES OF THE BEIGE MOUSE

The large size of the vesicles of beige mouse peritoneal mast cells (4 -mu-m in diameter) facilitated the direct observation of the individua l osmotic behavior of vesicles. The vesicle diameter increased as much as 73% when intact cells were perfused with a 10 mM pH buffer solutio n; the swelling of the vesicle membranes exceeded that of the insolubl e vesicle gel matrix, which resulted in the formation of a clear space between the optically dense gel matrix and the vesicle membrane. Hype rtonic solutions shrank intact vesicles of lysed cells in a nonideal m anner, suggesting a limit to the compressibility of the gel matrix. Th e nonideality at high osmotic strengths can be adequately explained as the consequence of an excluded volume and/or a three-dimensional gel- matrix spring. The observed osmotic activity of the vesicles implies t hat the great majority of the histamine known to be present is reversi bly bound to the gel matrix. This binding allows vesicles to store a l arge quantity of transmitter without doing osmotic work. The large siz e of the vesicles also facilitated the measurement of the kinetics of release as a collection of individual fusion events. Capacitance measu rements in beige mast cells revealed little difference in the kinetics of release in hypotonic, isotonic, and hypertonic solutions, thus eli minating certain classes of models based on the osmotic theory of exoc ytosis for mast cells.