Effects of age on aortic pressure-diameter and elastic stiffness-stress relationships in unanesthetized sheep.

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Abstract
The effects of aging and smooth muscle activation on the elastic stiffness of the aortic wall were assessed in 9 unanesthetized adult sheep, 7 newborn (< 1 wk) lambs, and 5 near term fetal lambs in utero, previously instumented with pressure gauges and ultrasonic dimension crystals for measurements of internal pressure and external diameter in the proximal 3rd of the descending thoracic aorta. Angiotensin and nitroglycerin were administered as i.v. boluses to increase or decrease pressure and diameter. The midwall stress (.sigma.)-radius data were fitted to an exponential curve (r [correlation coefficient] > 0.95) for each animal. The curves obtained were similar to those obtained by inflating and deflating an implanted hydraulic occluder. Incremental elastic modulus (Einc) was derived as a linear function of stress. Compared at similar .sigma. levels, Einc was significantly (P < 0.05) lower in the adults than in either the newborn or the fetuses, i.e., at .sigma. = 3.30 .times. 105 dyn/cm2, Einc was 1.41 .+-. 0.10 .times. 106 dyn/cm2 in the adults, compared with 2.41 .+-. 0.35 in the lambs and 2.31 .+-. 0.15 in the fetuses. When Einc was calculated at the higher stress value corresponding to baseline mean arterial pressure, (.sigma. = 6.90 .+-. 0.59 .times. 105 dyn/cm2 in the adult, 3.36 .+-. 0.27 in the newborn, 3.62 .+-. 0.56 in the fetus), Einc was only slightly higher in the adults (3.67 .+-. 0.50 .times. 106 dyn/cm2) than in the newborns (2.42 .+-. 0.38) or the fetuses (2.58 .+-. 0.58). .alpha.-Adrenergic activation of aortic smooth muscle, induced by methoxamine infusion, shifted pressure-diameter and stress-radius relationships toward higher pressure or stress for any given radius in the adult but no alteration was observed in unanesthetized fetal or newborn lambs or in adults anesthetized with pentobarbital. Thus the aortic elastic modulus at any given wall stress is lower in the unanesthetized adult sheep than in the newborn or the fetus. The responsiveness of aortic smooth muscle to .alpha.-adrenergic stimulation increases with age.