Tissue remodeling of rat pulmonary artery in hypoxic breathing. II. Course of change of mechanical properties.

When cells and the matrix of a tissue remodel, the mechanical properties of the tissue do change. The mechanical properties are expressed by constitutive equations. In this article the remodeling of the constitutive equation of the pulmonary artery is studied. The remodeling was induced in a rat breathing a gas whose oxygen concentration was suddenly decreased as a step function of time and maintained constant (17.2%, 13.6%, or 10%) afterwards. Since the mathematical form of the constitutive equation has been identified in earlier papers, we need to determine only the elastic constants that change in the process of tissue remodeling. We consider arteries subjected to blood pressure and longitudinal stretch, and limit ourselves to two-dimensional problems involving only circumferential and longitudinal stress and strain. In the neighborhood of an in vivo state, the perturbations of stresses and strains are related by linear, anisotropic, tensor equations involving three elastic constants: the incremental Young's modulus in the circumferential direction Ythetaz, that in the longitudinal direction Yzz, and the cross modulus Ythetaz. Over a 24 h period, changes of Ythetatheta between 164 and 187 kN/m2, Yzz between 64 and 92 kN/m2, and Ythetaz between 61 and 88 kN/m2 are statistically insignificant.