Change in residual strain in rat thoracic aorta due to smooth muscle contraction/relaxation (positional variation along the aortic tree)

The effect of smooth muscle contraction on strain distribution in the arterial wall was investigated. The opening angle of short, ring-shaped specimens obtained at various longitudinal positions of rat thoracic aortas was measured in an aerated Krebs-Ringer solution (37°C) under smooth muscle contraction induced with norepinephrine and under relaxation induced with sodium nitroprusside. The angle was significantly greater under smooth muscle contraction than relaxation at all positions except the branching site in the aortic arch. There was significant variation of the angle along the aortic tree under both conditions, being highest at the aortic root and generally low in the descending aorta. The angle reached a local minimum at the branching site. In spite of the large angle variation, the amount of increase in the residual strain due to smooth muscle contraction was very similar among the sites investigated except for the branching site, where smooth muscle alignment differed from that of the other sites. These results suggest that the effect of smooth muscle contraction on residual strain is significant and quantitatively similar among the sites in the thoracic aorta with ordinal histology.