EFFECTS OF AGE ON AORTIC PRESSURE-DIAMETER AND ELASTIC STIFFNESS-STRESS RELATIONSHIPS IN UNANESTHETIZED SHEEP

The effects of aging and smooth muscle activation on the elastic stiffness of the aortic wall were assessed in nine unanesthetized adult sheep, seven newborn (<1 week) lambs, and five near term fetal lambs in utero, previously instrumented with pressure gauges and ultrasonic dimension crystals for measurements of internal pressure and external diameter in the proximal third of the descending thoracic aorta. Angiotensin and nitroglycerin were administered as intravenous (iv) boluses to increase or decrease pressure and diameter. The midwall stress (sigma)-radius data were fitted to an exponential curve (r>0.95) for each animal. The curves obtained in this way were similar to those obtained by inflating and deflating an implanted hydraulic occluder. Incremental elastic modulus (E(inc)) was derived as a linear function of stress. Compared at similar σ levels, E(inc) was significantly (P<0.05) lower in the adults than in either the newborn or the fetuses. For example, at σ = 3.30 x 105 dynes/cm2, E(inc) was 1.41 ± 0.10 x 106 dynes/cm2 in the adults, compared with 2.41 ± 0.35 in the lambs and 2.31 ± 0.15 in the fetuses. However, when E(inc) was calculated at the higher stress value corresponding to baseline mean arterial pressure, (σ = 6.90 ± 0.59 x 105 dynes/cm2 in the adult, 3.36 ± 0.27 in the newborn, 3.62 ± 0.56 in the fetus), E(inc) was only slightly higher in the adults (3.67 ± 0.50 x 106 dynes/cm2) than in the newborns (2.42 ± 0.38) or the fetuses (2.58 ± 0.58).α-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, whereas 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. However, the responsiveness of aortic smooth muscle to α-adrenergic stimulation increases with age.