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 regula tion of blood pressure and cardiac performance. The effect of sympathe tic stimulation on the mechanical properties of the human abdominal ao rta 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 oi systoiic/d iastolic variation of aortic diameter in combination with intra-aortic pressure measurements was used to determine wail mechanics. The press ure-diameter (P-D) relationship and the distensibility indices. stiffn ess (beta) and pressure strain elastic modulus (E(p)) of the abdominal aorta were obtained. Measurements were made at rest and during sympat hetic stimulation induced by lower body negative pressure (LBNP). As a sign of sympathetic activation, the peripheral resistance increased b y 74-96% (P < 0.001) during LBNP. However. the mechanical properties o f the abdominal aorta remained unaltered. as estimated either from the P-D relationship or from the indices E(p) and beta, both in the young (vest: E(p) = 0.53 +/- 0.18, beta = 4.5 +/- 1.5; LBNP: E(p) = 0.51 +/ - 0.15. beta = 4.5 +/- 1.2, NS) and in the elderly (rest: E(p) = 2.17 +/- 0.70, beta = 17.6 +/- 5.8: LBNP: E(p) = 2.11 +/- 0.60. beta = 16.9 +/- 3.9. NS). In conclusion, this investigation shows that LBNP-induc ed sympathetic activation does not change aortic wall mechanics. Thus, sympathetic modulation of the aortic smooth muscle contractile activi ty seems to be unimportant in the blood pressure regulation.