Regional dependency of the vascular smooth muscle cell contribution to the mechanical properties of the pig ascending aortic tissue.

BACKGROUND Dilation and dissection of aneurysmal ascending aortic tissues occur preferentially at the outer curvature of the vessel. In this study we hypothesize that the density and contractile properties of the vascular smooth muscle cells (VSMCs) of the pig ascending aorta (AA) are heterogeneous and could explain the non-uniform remodeling and weakening of the AA during aneurysm formation. METHODS Eleven pig AA rings were collected. Two square samples of 15 x 15 mm were taken from each ring from the inner and outer curvature of the AA. Each sample was subjected to equi-biaxial tensile testing in Krebs-Ringer solution maintained at 37 degrees C. Each test consisted of 8 cycles of preconditioning followed by one experimental run from 0% to 30% strain. Phenylephrine (10(-5) M) was added to contract VSMCs. After biaxial testing, samples were paraffin-embedded and stained with hematoxylin-phloxine-saffron (HPS) to quantify VSMC density. RESULTS Significant differences in cell density, maximum contractile stress resultant magnitude (MCSRM) and orientation (theta(MCSR)) were found between the inner and outer curvature. The inner curvature had the greatest contraction. The outer curvature had the highest VSMC density with the maximum contraction stress resultant oriented towards the axial direction. CONCLUSION VSMC activation with phenylephrine had a significant effect on the stiffness of the pig AA. This effect was independent of location and direction. However, cell orientation, density and contractile properties were dependent on location and suggest variations in the remodeling capabilities, tissue strain and cell phenotype between locations.