Pulmonary surfactant, secreted via exocytosis of lamellar bodies (LB) by alveolar type II (AT II) cells, maintains low alveolar surface tension and is therefore essential for normal lung function. Here we describe real-time monitoring of exocytotic activity in these cells by visualizing and quantifying LB fusion with the plasma membrane (PM). Two approaches were used. First, fluorescence of LysoTracker Green DND-26 (LTG) in LB disappeared when the dye was released after exocytosis. Second, phospholipid staining by FM 1–43 resulted in bright fluorescence when this dye entered the LB through the fusion pore. Both processes were restricted to and colocalized with LB and occurred simultaneously. In AT II cells, FM 1–43 offered the unique advantage to independently define the moment and cellular location of single exocytotic events as well as the amount of material released, and to monitor its extracellular fate. Furthermore, both dyes could be used in combination with fura-2. The results indicate considerable diversity in the dynamics of LB exocytosis. In the majority of cells stimulated with ATP and isoproterenol, the first fusion of LB coincided with the rise of [Ca ²⁺ ] _i , but subsequent response of other LB in the same cell considerably outlasted this signal. In other cells, however, the onset of exocytosis was delayed by several minutes. After LB fusion, release of surfactant from LB into an aqueous solution was slow. In summary, stimulated exocytosis in AT II cells occurs at a much slower rate than in most other secretory cells but is still a more dynamic process than predicted from conventional measurements of surfactant released into cell supernatants.

▪ Abstract Exocytosis is fundamental in biology and requires an orchestra of proteins and other constituents to fuse a vesicle with the plasma membrane. Although the molecular fusion machinery appears to be well conserved in evolution, the process itself varies considerably with regard to the diversity of physico-chemical and structural factors that govern the delay between stimulus and fusion, the expansion of the fusion pore, the release of vesicle content, and, finally, its extracellular dispersion. Exocytosis of surfactant is unique in many of these aspects. This review deals with the secretory pathway of pulmonary surfactant from the type II cell to the air-liquid interface, with focus on the distinct mechanisms and regulation of lamellar body (LB) fusion and release. We also discuss the fate of secreted material until it is rearranged into units that finally function to reduce the surface tension in the lung.

In alveolar type II cells, the release of surfactant is considerably delayed after the formation of exocytotic fusion pores, suggesting that content dispersal may be limited by fusion pore diameter and subject to regulation at a postfusion level. To address this issue, we used confocal FRAP and N-(3-triethylammoniumpropyl)-4-(4-[dibutylamino]styryl) pyridinium dibromide (FM 1-43), a dye yielding intense localized fluorescence of surfactant when entering the vesicle lumen through the fusion pore (Haller, T., J. Ortmayr, F. Friedrich, H. Volkl, and P. Dietl. 1998. Proc. Natl. Acad. Sci. USA. 95:1579–1584). Thus, we have been able to monitor the dynamics of individual fusion pores up to hours in intact cells, and to calculate pore diameters using a diffusion model derived from Fick's law. After formation, fusion pores were arrested in a state impeding the release of vesicle contents, and expanded at irregular times thereafter. The expansion rate of initial pores and the probability of late expansions were increased by elevation of the cytoplasmic Ca²⁺ concentration. Consistently, content release correlated with the occurrence of Ca²⁺ oscillations in ATP-treated cells, and expanded fusion pores were detectable by EM. This study supports a new concept in exocytosis, implicating fusion pores in the regulation of content release for extended periods after initial formation.

Ca ²⁺ is considered a key element in multiple steps during regulated exocytosis. During the postfusion phase, an elevated cytoplasmic Ca ²⁺ concentration ([Ca ²⁺ ]) _c leads to fusion pore dilation. In neurons and neuroendocrine cells, this results from activation of voltage-gated Ca ²⁺ channels in the plasma membrane. However, these channels are activated in the prefusion stage, and little is known about Ca ²⁺ entry mechanisms during the postfusion stage. This may be particularly important for slow and nonexcitable secretory cells. We recently described a “fusion-activated“ Ca ²⁺ entry (FACE) mechanism in alveolar type II (ATII) epithelial cells. FACE follows initial fusion pore opening with a delay of 200–500 ms. The site, molecular mechanisms, and functions of this mechanism remain unknown, however. Here we show that vesicle-associated Ca ²⁺ channels mediate FACE. Using RT-PCR, Western blot analysis, and immunofluorescence, we demonstrate that P2X ₄ receptors are expressed on exocytotic vesicles known as lamellar bodies (LBs). Electrophysiological, pharmacological, and genetic data confirm that FACE is mediated via these vesicular P2X ₄ receptors. Furthermore, analysis of fluorophore diffusion into and out of individual vesicles after exocytotic fusion provides evidence that FACE regulates postfusion events of LB exocytosis via P2X ₄ . Fusion pore dilation was clearly correlated with the amplitude of FACE, and content release from fused LBs was accelerated in fusions followed by FACE. Based on these findings, we propose a model for regulation of the exocytotic postfusion phase in nonexcitable cells in which Ca ²⁺ influx via vesicular Ca ²⁺ channels regulates fusion pore expansion and vesicle content release.