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

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Abstract
Pulmonary surfactant, a critical determinant of alveolar stability, is secreted by alveolar type II cells by exocytosis of lamellar bodies (LBs). To determine exocytosis mechanisms in situ, we imaged single alveolar cells from the isolated blood-perfused rat lung. We quantified cytosolic Ca2+ concentration ([Ca2+]i) by the fura 2 method and LB exocytosis as the loss of cell 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 oscillations. Fluorescence of the lipidophilic dye FM1-43 indicated multiple exocytosis sites per cell. Intracellular Ca2+ chelation and gap junctional inhibition each blocked [Ca2+]i oscillations and exocytosis in type II cells. We demonstrated the feasibility of real-time quantifications in alveolar cells in situ. We conclude that in lung expansion, type II cell exocytosis is modulated by the frequency of intercellularly communicated [Ca2+]i oscillations that are likely to be initiated in type I cells. Thus during lung inflation, type I cells may act as alveolar mechanotransducers that regulate type II cell secretion.