Na(+)-K(+)-ATPase pumps (Na(+) pumps) in the alveolar epithelium create a transepithelial Na(+) gradient crucial to keeping fluid from the pulmonary air space. We hypothesized that alveolar epithelial stretch stimulates Na(+) pump trafficking to the basolateral membrane (BLM) and, thereby, increases overall Na(+) pump activity. Alveolar type II cells were isolated from Sprague-Dawley rats and seeded onto elastic membranes coated with fibronectin or 5-day-conditioned extracellular matrix. After 2 days in culture, cells were uniformly stretched for 1 h in a custom-made device. Na(+) pump activity was subsequently assessed by ouabain-inhibitable uptake of (86)Rb(+), a K(+) tracer, and BLM Na(+) pump abundance was measured. In support of our hypothesis, cells increased Na(+) pump activity in a "dose-dependent" manner when stretched to 12, 25, or 37% change in surface area (DeltaSA), and cells stretched to 25% DeltaSA more than doubled Na(+) pump abundance in the BLM. Cells on 5-day matrix tolerated higher strain than cells on fibronectin before the onset of Na(+) pump upregulation. Treatment with Gd(3+), a stretch-activated channel blocker, amiloride, a Na(+) channel blocker, or both reduced but did not abolish stretch-induced effects. Sustained tonic stretch, unlike cyclic stretch, elicited no significant Na(+) pump response.

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 h, and stretched between 3 and 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 min 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 min to allow lipid insertion and then returned to an unstretched state, reabsorption of excess lipid occurred within 5 min. Finally, we found that lipid insertion induced by tonic stretch was unaffected by F-actin disassembly, ATP depletion, and calcium deprivation.

Fisher, Jacob L., and Susan S. Margulies. Modeling the effect of stretch and plasma membrane tension on Na ϩ -K ϩ -ATPase activity in alveolar epithelial cells. Am J Physiol Lung Cell Mol Physiol 292: L40-L53, 2007. First published August 4, 2006; doi:10.1152/ajplung.00425.2005.-While a number of whole cell mechanical models have been proposed, few, if any, have focused on the relationship among plasma membrane tension, plasma membrane unfolding, and plasma membrane expansion and relaxation via lipid insertion. The goal of this communication is to develop such a model to better understand how plasma membrane tension, which we propose stimulates Na ϩ -K ϩ -ATPase activity but possibly also causes cell injury, may be generated in alveolar epithelial cells during mechanical ventilation. Assuming basic relationships between plasma membrane unfolding and tension and lipid insertion as the result of tension, we have captured plasma membrane mechanical responses observed in alveolar epithelial cells: fast deformation during fast cyclic stretch, slower, time-dependent deformation via lipid insertion during tonic stretch, and cell recovery after release from stretch. The model estimates plasma membrane tension and predicts Na ϩ -K ϩ -ATPase activation for a specified cell deformation time course. Model parameters were fit to plasma membrane tension, whole cell capacitance, and plasma membrane area data collected from the literature for osmotically swollen and shrunken cells. Predictions of membrane tension and stretch-stimulated Na ϩ -K ϩ -ATPase activity were validated with measurements from previous studies. As a proof of concept, we demonstrate experimentally that tonic stretch and consequent plasma membrane recruitment can be exploited to condition cells against subsequent cyclic stretch and hence mitigate stretchinduced responses, including stretch-induced cell death and stretch-induced modulation of Na ϩ -K ϩ -ATPase activity. Finally, the model was exercised to evaluate plasma membrane tension and potential Na ϩ -K ϩ -ATPase stimulation for an assortment of traditional and novel ventilation techniques.ventilator-induced lung injury; lipid trafficking; edema recovery INVESTIGATORS HAVE FOUND alveolar epithelial stretch magnitude to be related to cell injury in vitro (55, 56) and tidal volume to be related to ventilator-induced lung injury (VILI) in animal models (12-15). Studies have revealed that the frequency of epithelial stretch or ventilation can also be an important factor in cell injury (56, 60). The model developed in this study examines how combinations of amplitude and frequency in ventilation procedures affect Na ϩ -K ϩ -ATPase activity via hypothesized pathways including alveolar epithelial cell plasma membrane tension and stretch-activated channel (SAC) stimulation.To date, numerous models have been proposed to describe cellular mechanical behavior, from early continuum viscoelastic representations for erythrocytes and leukocytes (11,27,65) to more sophisticated variations that modeled cell membrane an...

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