Influence of sympathetic stimulation on the mechanical properties of the aorta in humans

The mechanical properties of the aorta play a major role in the regulation of blood pressure and cardiac performance. The effect of sympathetic stimulation on the mechanical properties of the human abdominal aorta was studied in 19 healthy volunteers, divided into young (25 +/- 2 years) and elderly individuals (69 +/- 2 years) of both sexes. A non-invasive ultrasonic echo-tracking system for measurement of systolic/diastolic variation of aortic diameter in combination with intra-aortic pressure measurements was used to determine wall mechanics. The pressure-diameter (P-D) relationship and the distensibility indices, stiffness (beta) and pressure strain elastic modulus (Ep) of the abdominal aorta were obtained. Measurements were made at rest and during sympathetic stimulation induced by lower body negative pressure (LBNP). As a sign of sympathetic activation, the peripheral resistance increased by 74-96% (P < 0.001) during LBNP. However, the mechanical properties of the abdominal aorta remained unaltered, as estimated either from the P-D relationship or from the indices Ep and beta, both in the young (rest: Ep = 0.53 +/- 0.18, beta = 4.5 +/- 1.5; LBNP: Ep = 0.51 +/- 0.15, beta = 4.5 +/- 1.2, NS) and in the elderly (rest: Ep = 2.17 +/- 0.70, beta = 17.6 +/- 5.8; LBNP: Ep = 2.11 +/- 0.60, beta = 16.9 +/- 3.9, NS). In conclusion, this investigation shows that LBNP-induced sympathetic activation does not change aortic wall mechanics. Thus, sympathetic modulation of the aortic smooth muscle contractile activity seems to be unimportant in the blood pressure regulation.