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

Departments of ¹Physiology and ²Medical Physics, University of Innsbruck, A-6020 Innsbruck, Austria

Submitted 15 September 2003 ; accepted in final form 21 September 2003

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 (CSA) without release of lactate dehydrogenase or ATP. Strain exceeding a threshold of 8% 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 µM 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.

surfactant secretion; alveolar type II cells; mechanical strain; stretch

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