Ca2+ entry is essential for cell strain-induced lamellar body fusion in isolated rat type II pneumocytes

Using a new equibiaxial strain device, we investigated strain-induced Ca2+ signals and their relation to lamellar body (LB) exocytosis in single rat alveolar type II (AT II) cells. The strain device allows observation of single cells while inducing strain to the entire substratum. AT II cells tolerated high strain amplitudes up to 45% increase in cell surface area (Delta CSA) without release of lactate dehydrogenase or ATP. Strain exceeding a threshold of approximately 8% Delta CSA resulted in a transient rise of the cytoplasmic Ca2+ concentration in some cells. Higher strain levels increased the fraction of Ca2+-responding cells. The occurrence of strain-induced Ca2+ signals depended on cell-cell contacts, because lone cells (i.e., cells without cell-cell contacts) did not exhibit Ca2+ signals. Above threshold, the amplitude of the Ca2+ signal as well as the number of stimulated LB fusions correlated well with the amplitude of strain. Furthermore, stimulated LB fusions occurred only in cells exhibiting a Ca2+ signal; 50 microM Gd3+ in the bath affected neither Ca2+ signals nor fusions. Intracellular Ca2+ release was triggered at higher strain amplitudes and inhibited by thapsigargin. Removal of bath Ca2+ completely inhibited Ca2+ signals and fusions. We conclude that strain of AT II cells stimulates a Ca2+ entry pathway that is highly sensitive to strain and a prerequisite for subsequent Ca2+ release. Both mechanisms result in a graded response of fusions to strain. Our data also allow us to introduce the term "effective strain" as the physiologically relevant portion of the strain amplitude.