Role of physical forces in compensatory growth of the lung

In many species, partial resection of the lung leads to rapid compensatory growth of the remaining tissue to restore normal lung mass and function. The response to partial pneumonectomy is closely controlled; both its rate and nature are subject to hormonal modulation. Physical factors, particularly distortion of the lung by altered inflation, are likely involved in regulation of the response, although the details of the regulatory mechanisms are not understood. In a number of tissues including the lung, application of external physical force leads to both acute and long-term changes in metabolism. In some cases these include cell growth and division, along with increased production of extracellular matrix components. Similar responses have been described after application of stress to isolated cells in culture. Independent lines of investigation have defined dramatic influences of cell shape on growth, differentiation, and metabolism, but stress-strain relationships at the cellular or subcellular levels are poorly defined. The mechanisms by which changes in cell shape are transduced to intracellular signals likely depend on receptor-mediated interactions with the cytoskeleton, but strain-associated transduction pathways may involve stretch-sensitive ion channels, G protein-dependent reactions, the action of locally produced autocrine or paracrine factors, or a combination of these factors. These observations suggest a general model of the response to pneumonectomy that may be used to formulate specific hypotheses as a basis for future investigations. This approach will provide insight into the mechanisms by which physical forces influence growth and metabolism in the lung and other tissues.