Exocytosis, Dependent on Calcium Release from Calcium Stores, is Regulated by Calcium Microdomains

The relationship between the cellular Ca2+ signal and exocytic vesicle fusion is a key determinant of the regulation of the kinetics and magnitude of the secretory response. Here, we have investigated secretion in epithelial cells where the exocytic response is controlled by Ca2+ release from intracellular Ca2+ stores (Ueda & Petersen 1977). Using live-cell 2-photon microscopy, identifying each individual exocytic fusion event (Thorn et al., 2004) and recording cytosolic Ca2+ signals with Fura-2 (Nemoto et al., 2001), we show no evidence for spatial clustering of exocytosis; indeed exocytosis is actually specifically excluded from sites of Ca2+ release hot-spots. Consistent with this data, loading the cells with EGTA potently blocks exocytosis. These results indicate that the control of exocytosis, triggered by Ca2+ release from stores, is through the regulation of cytosolic Ca2+ concentrations within large volume microdomains.Nemoto, T., Kimura, R., Ito, K., Tachikawa, A., Miyashita, Y., Iino, M. & Kasai, H. (2001) Nature Cell Biol.3: 253-258.Thorn, P., Fogarty, K.E. & Parker, I. (2004) Proceedings National Academy Science 101:6774-6779Ueda N. & Petersen OH (1997) Pflugers Archiv370: 179-183