Correlating Ca²⁺ Responses and Secretion in Individual RBL-2H3 Mucosal Mast Cells*

Abstract

The role of Ca²⁺ in stimulus-response coupling in nonexcitable cells is still not well understood. The Ca²⁺ responses of individual cells are extremely diverse, often displaying marked oscillations, and almost nothing is known about the specific features of these Ca²⁺signals that are important for the functional response of a cell. Using the RBL-2H3 mucosal mast cell as a model, we have studied the temporal relationship between changes in intracellular Ca²⁺ and serotonin secretion at the single-cell level using simultaneous indo-1 photometry and constant potential amperometry. Secretion in response to antigen never occurs until intracellular Ca²⁺ is elevated, nor is it seen during the first few oscillations in Ca²⁺. Exocytotic events tend to be clustered around the peaks of oscillations, but excellent secretion is also seen in cells with sustained elevations in Ca²⁺. Ca²⁺ release from stores in the absence of influx fails to elicit secretion. If refilling and continued release of Ca²⁺ from stores is prevented with thapsigargin, Ca²⁺ influx can still trigger secretion, suggesting that store-associated microdomains of Ca²⁺ are not required for exocytosis. Our findings demonstrate the importance of an amplitude-encoded Ca²⁺ signal and Ca²⁺influx for stimulus-secretion coupling in these nonexcitable cells.