A new model for the mechanism of stimulus-secretion coupling

By combining ultrastructural techniques with a biochemical approach to study the mechanism of mast cell stimulus-secretion coupling and by using purified secretory granules to confirm those early biochemical events which originate from within the secretory granule, a new model for the mechanism of secretory granule exocytosis has emerged. This model not only provides the mechanism by which an activated granule can achieve fusion with the plasma membrane, but it also provides the rationale for the linking of the various early biochemical events to the process of granule activation and thus to exocytosis. Although we still do not understand how the 'activating signal', which results from the stimulation of cell surface receptors, can be conveyed to the granule to cause its activation, we are certain that this 'signal' must cause an influx of water into the matrix of the target granule. This influx of water is what initiates the granule activation process. The major intragranular events which are triggered by this water influx include: (i) de novo membrane assembly; (ii) protein proteolysis; (iii) release of arachidonic acid from matrix-bound phospholipid by phospholipase A2; (iv) initiation of the arachidonic acid cascade and the synthesis of eicosanoids; (v) rapid phospholipid turnover; and (vi) the discharge of matrix materials into the cytoplasm of the activated cell via the fusion of de novo generated vesicles with the perigranular membrane. The ejection of some matrix contents which may include histamine, Ca2+, calmodulin, protease, the products of the arachidonic acid cascade and the products of phospholipid turnover into the cytosole, may serve to turn on the various metabolic machineries needed to initiate a cellular recovery phase.