Large artery remodeling during aging: Biaxial passive and active stiffness

To examine arterial mechanical changes during aging, pressure-radius and axial force-radius curves were measured in vivo in carotid arteries from 6- and 23-month-old Brown Norway X Fischer 344 rats. Incremental passive circumferential stiffness (measured at 50, 100, and 200 mm Hg) was higher (P<0.01) in the 23- compared with the 6-month-old rats (14.02±1.23 versus 6.58±1.51; 2.68±0.56 versus 0.99±0.34; 1.10±0.24 versus 0.69±0.15 dyne/mm²x10³, respectively). Incremental passive axial stiffness was increased (P<0.01) in the 23-compared with the 6-month-old rats (7.95±0.70 versus 4.24±0.81; 1.91±0.10 versus 0.61±0.16; 0.58±0.09 versus 0.36±0.06 dyne/mm²x10³, respectively). Active incremental circumferential arterial stiffness at 100 and 200 mm Hg was increased (P<0.01) in the older rats. In 6-month-old rats, activation of vascular smooth muscle enhanced (P<0.01) the incremental circumferential and axial stiffness measured at 200 mm Hg. In 23- month-old rats, only active incremental stiffness was increased (P<0.01) at 200 mm Hg. Aging increased (P<0.05) media thickness, collagen content, and the collagen/elastin ratio by 12%, 21%, and 38%, respectively. Elastin density and the number of smooth muscle cell nuclei were decreased by 20% and 31%, respectively, with aging. Thus, structural alterations that occur with aging are associated with changes in both active and passive stiffness. Vascular smooth muscle tone modulates arterial wall anisotropy differently during aging.