Stimulus-secretion coupling mechanisms in mast cells.

We have investigated several aspects of the complex sequence of events, transmitting the antigen-induced signal of cross-linking immunoglobulin E (IgE) resident in the membrane surface of mast cells into the signals yielding the final process of mediator release. Already the initial phase of this cascade still requires a better understanding. Namely, we are still missing a clear physical description of the effective stimulus-producing antigen-IgE complex in terms of size and spatial requirements. We are investigating this problem on a well-defined cell line (rat basophilic leukemia-RBL-2H3) using synthetic divalent haptens and a monoclonal IgE. A subsequent phase following IgE aggregation is a transient increase in the free calcium concentration in these cells' cytosol. The source of the Ca2+ and the way by which it enters the cytosol are studied predominantly by examining antigen-induced channel activity in the cells' membrane allowing Ca2+ influx from the exterior medium. Finally, we have observed that under certain experimental conditions, antigen-induced degranulation can be achieved even without a rise in cytosolic free calcium. In our search for alternative second messengers, we examine the potential candidacy of the cytosolic Na+/H+ balance. So far, we have found that antigen-stimulated secretion does require extracellular sodium and involves changes in its cytosolic pH. However, further studies are required to clarify its possible role as a coupling element.