Guo X, Lanir Y, Kassab GS. Effect of osmolarity on the zerostress state and mechanical properties of aorta. Am J Physiol Heart Circ Physiol 293: H2328-H2334, 2007. First published June 15, 2007; doi:10.1152/ajpheart.00402.2007.-Some pathological conditions may affect osmolarity, which can impact cell, tissue, and organ volume. The hypothesis of this study is that changes in osmolarity affect the zero-stress state and mechanical properties of the aorta. To test this hypothesis, a segment of mouse abdominal aorta was cannulated in vivo and mechanically distended by perfusion of physiological salt (NaCl) solutions with graded osmolarities from 145 to 562 mosM. The mechanical (circumferential stress, strain, and elastic modulus) and morphological (wall thickness and wall area) parameters in the loaded state were determined. To determine the osmolarityinduced changes of zero-stress state, the opening angle was observed by immersion of the sectors of mouse, rat, and pig thoracic aorta in NaCl solution with different osmolarities. Wall volume and tissue water content of the rings were also recorded at different osmolarities. Our results show that acute aortic swelling due to low osmolarity leads to an increase in wall thickness and area, a change in the stress-strain relationship, and an increase in the elastic modulus (stiffness) in mouse aorta. The opening angle, wall volume, and water content decreased significantly with increase in osmolarity. These findings suggest that acute aortic swelling and shrinking result in immediate mechanical changes in the aorta. Osmotic pressure-induced changes in the zero-stress state may serve to regulate mechanical homeostasis.swelling; opening angle; elastic modulus; strain THE STRESS AND STRAIN that remain in an organ when all external loads are removed are called residual stress and strain (8). The open sector in the zero-stress state can be characterized by an opening angle revealed by a radial cut of a vessel ring in the no-load state (9). The zero-stress state or opening angle as a measure of residual strain has been shown to significantly change during growth and remodeling (9). It has also been found to change in response to mechanical (pressure or flow) (10, 19) and chemical (smoke exposure or diabetes) perturbations (17, 18).It has become clear that chronic changes in the zero-stress state normalize stress distribution in the vessel wall (10,22). Acute changes in the opening angle observed in hypertension imply that the response to restoration of mechanical homeostasis is almost immediate (10). Fung (9) hypothesized that the opening angle may be viewed as an index of nonuniformity of growth and remodeling; i.e., the intima outgrows the adventitia, and, hence, the opening angle increases (or vise versa).Although this hypothesis accounts for chronic remodeling, it remains unclear how the opening angle can change acutely or immediately.Osmotic pressure plays an important role in controlling the distribution of water across cell membranes, and, thus, the cell volume is closel...