Threshold calcium levels for lamellar body exocytosis in type II pneumocytes

Pulmonary surfactant is secreted via exocytosis of lamellar bodies (LBs) by alveolar type II cells. Here we analyzed the dependence of LB exocytosis o n intracellular Ca2+ concentration ([Ca2+](i)). In fura 2-loaded cells, [Ca 2+](i) was selectively elevated by flash photolysis of a cell-permeant cage d Ca2+ compound (o-nitrophenyl EGTA-AM) or by gradually enhancing cellular Ca2+ influx. Simultaneously, surfactant secretion by single cells was analy zed with the fluorescent dye FM 1-43, enabling detection of exocytotic even ts with a high temporal resolution (T. Haller, J. Ortmayr, F. Friedrich, H. Volkl, and P. Dietl. Proc. Natl. Acad. Sci. USA 95: 1579-1584, 1998). Exoc ytosis was initiated at a threshold concentration near 320 nmoL/1 with both instantaneous or gradual [Ca2+](i) elevations. The exocytotic response to flash photolysis was highest during the first minute after the rise in [Ca2 +](i) and thus almost identical to purinoceptor stimulation by ATP. Corresp ondingly, the effects of ATP on initial secretion could be sufficiently exp lained by its ability to mobilize Ca2+. This was further demonstrated by th e fact that exocytosis is significantly blocked by suppression of the ATP-i nduced Ca2+ signal below similar to 300 nmol/1. Our results suggest a highl y Ca2+-sensitive step in LB exocytosis.