Published ahead of print on March 11, 2004, doi:10.1165/rcmb.2003-0224OC Am. J. Respir. Cell Mol. Biol., Volume 31, Number 2, August 2004, 200-208 A more recent version of this article appeared on August 1, 2004
Submitted on June 23, 2003 Plasma Membrane Surface Increases with Tonic Stretch of Alveolar Epithelial CellsJacob L Fisher1,1 Department of Bioengineering, University of Pennsylvania, Philadelphia, PA, USA, 2 Department of Pathology, University of Pennsylvania, Philadelphia, PA, USA; Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA, USA, 3 Department of Bioengineering, University of Pennsylvania, Philadelphia, PA, USA; Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA, USA * To whom correspondence should be addressed. E-mail: margulie{at}seas.upenn.edu.
Cyclic stretch stimulates numerous responses in alveolar epithelial cells - some beneficial, some injurious - often through mechanosensitive membrane-associated proteins such as stretch-activated ion channels. Tonic stretch, in contrast, stimulates only some of these responses. In this study, we hypothesized that the plasma membranes of alveolar epithelial cells expand during tonic stretch, not only through cell surface unfolding, but also through recruitment of additional phospholipids. Such plasma membrane expansion would reduce membrane tension and decrease stimulation of mechanosensitive membrane proteins. Primary rat alveolar epithelial cells were isolated, cultured for 48 hours, and stretched between 3-40% change in basal membrane surface area. Gross changes in total cell surface area were obtained from stacks of thin fluorescent confocal micrographs; fine changes in plasma membrane area were measured via whole cell capacitance. A 1:1 correspondence linked changes in basal and total cell surface area, implying that cell surface area change is dominated by stretch of the attached basal surface. We also found that plasma membrane increased proportionally with surface area within 5 minutes of tonic stretch, showing that, given time to occur, plasma membrane expansion via lipid recruitment preponderates the changes in cell surface shape and size demanded by stretching the cell. Similarly, in cells tonically stretched 10 minutes to allow lipid insertion and then returned to an unstretched state, reabsorption of excess lipid occurred within 5 minutes. Finally, we found that lipid insertion induced by tonic stretch was unaffected by F-actin disassembly, ATP depletion, and calcium deprivation.
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