| Mechanical ventilation is a life-saving intervention in the critical care of patients who cannot breathe naturally, but it also causes pulmonary damage and dysfunction known collectively as ventilator-induced or ventilator associated lung injury (VILI or VALI). One of the most apparent and most dangerous symptoms of VILI is alveolar edema, or fluid in the airspace. As shown by previous studies, when diseased or injured lungs are ventilated, inhomogeneous material properties lead to disparate regional ventilation volume, including overinflation in some alveoli. In such cases, high alveolar epithelial strain causes increased permeability of proteins and fluids into the alveolar airspace. Vectorial Na+ transport and an actively maintained transepithelial Na+ gradient, driven by Na+/K+-ATPase can help to solve this problem by osmotically clearing edema. This thesis has focused on the function of Na+/K+-ATPase when the alveolar epithelium is stretched.; The overall goal of this research was to determine the effect of stretch on Na+/K+-ATPase activity. Our primary hypothesis was that cyclic stretch increases Na+/K+-ATPase activity by stimulating stretch-activated ion channels and thus triggers traffic of additional Na+/K+-ATPase from intracellular stores to the cells plasma membrane. Although we confirmed these hypotheses, we also found the tonic stretch had no effect on Na+/K +-ATPase activity. To explain this contrast between cyclic and tonic stretch, we hypothesized further that alveolar epithelial cells add additional lipid to their plasma membranes during tonic stretch, resulting in membrane relaxation and an abrogation of stretch-activated channel signaling. Using a patch-clamp technique for measuring whole cell capacitance, a measure of plasma membrane surface area, we found that tonically stretched cells not only expand in apparent surface area, which could be the result of plasma membrane unfolding or lipid insertion, but also increase their capacitance, confirming the latter. Based on our findings and data from the literature relating plasma membrane stretch, tension and lipid insertion, we also developed a model capable of predicting plasma membrane tension and Na+/K +-ATPase stimulation in response to any given cellular deformation pattern. Finally, we used the model to evaluate standard and novel ventilation procedures in terms of their capacity to stimulate edema-clearing Na +/K+-ATPase activity. We purpose that this research lay a foundation upon which medical scientists and clinicians can develop safer and more effective lung ventilation strategies. |
