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1 Department of Physiology, University of Innsbruck, Innsbruck, Tirol, Austria; Department of Medical Physics, University of Innsbruck, Innsbruck, Tirol, Austria
2 Department of Physiology, University of Innsbruck, Innsbruck, Tirol, Austria
3 Department of Medical Physics, University of Innsbruck, Innsbruck, Tirol, Austria
* To whom correspondence should be addressed. E-mail: paul.dietl{at}uibk.ac.at.
Using a new equi-biaxial 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 (
CSA) without release of lactate dehydrogenase (LDH) or ATP. Strain exceeding a threshold of approximately 8 % CSA resulted in a transient rise of the cytoplasmic calcium concentration ([Ca2+]c) 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, as 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 µM Gd3+ in the bath neither affected 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 which 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 to introduce the term "effective strain" as the physiologically relevant portion of the strain amplitude.
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