[Ca2+](i) oscillations regulate type II cell exocytosis in the pulmonary alveolus

Pulmonary surfactant, a critical determinant of alveolar stability, is secr eted by alveolar type II cells by exocytosis of lamellar bodies (LBs). To d etermine exocytosis mechanisms in situ, we imaged single alveolar cells fro m the isolated blood-perfused rat lung. We quantified cytosolic Ca2+ concen tration ([Ca2+](i)) by the fura 2 method and LB exocytosis as the loss of c ell fluorescence of LysoTracker Green. We identified alveolar cell type by immunofluorescence in situ. A 15-s lung expansion induced synchronous [Ca2](i) oscillations in all alveolar cells and LB exocytosis in type II cells. The exocytosis rate correlated with the frequency of [Ca2+](i) oscillation s. Fluorescence of the lipidophilic dye FM1-43 indicated multiple exocytosi s sites per cell. Intracellular Ca2+ chelation and gap junctional inhibitio n each blocked [Ca2+](i) oscillations and exocytosis in type II cells. We d emonstrated the feasibility of real-time quantifications in alveolar cells in situ. We conclude that in lung expansion, type II cell exocytosis is mod ulated by the frequency of intercellularly communicated [Ca2+](i) oscillati ons that are likely to be initiated in type I cells. Thus during lung infla tion, type I cells may act as alveolar mechanotransducers that regulate typ e II cell secretion.