Last update:
21-Jul-2008

OBJECTIVE The aorta, and especially its thoracic portion, does not constitute a mere conduit for the transfer of blood, but through the complicated mechanical properties of its vessel wall contributes significantly to the homeostasis of the cardiovascular system. The aim of this study was the determination of changes in the mechanical characteristics of the thoracic aortic wall following functional blockade of its �-adrenergic receptors.

METHOD Thirty two healthy male Wistar rats, of the same age, were used, which were divided in two groups. In the animals of the control group (n=16) only water was given with their food, while in the experimental group (n=16), propranolol was administered in their water at a daily dose of 100 mg/kg. The animals �f both groups were sacrificed after 3 months. To assess the sufficiency of �-blockade, the effect on the heart rate of isoproterenol (0.5 �g) administered intravenously was examined, before the terminal study in the animals of the two groups. The descending thoracic aorta was excised and longitudinal strips of fixed dimensions were obtained. The specimens were submitted to study of their mechanical characteristics on a tensile-testing device. The stress-strain, elastic modulus-stress and elastic modulus-strain curves were divided into three parts, referring to low, physiologic and high pressures, each of which was subjected to regression analysis.

RESULTS In all animals of the experimental group, sufficient blockade of �-receptors was noted, given that the heart rate was not raised after the intravenous administration of isoprotenerol. The cumulative elastic modulus-stress curve of the experimental group was displaced upwards towards higher elastic modulus values in comparison to the curve of the control group only at physiologic stress levels. The respective elastic modulus-strain curve was displaced towards higher elastic modulus values at physiologic and high strain levels. Statistical evaluation of the regression parameters for the second and third parts of the curves showed significant differences between the two groups, which indicated that the aortic wall of animals of the experimental group was significantly stiffer than that of the control group at physiologic and high levels of pressure.

CONCLUSIONS The pharmacological blockade of �-receptors in experimental animals leads to a significant increase in wall stiffness of the descending thoracic aorta at physiologic and high levels of pressure. The consequences of this pharmacological sympathectomy on the aortic wall and consequently on the maintenance of hemodynamic homeostasis must be taken into consideration in the clinical use of the �-blockers.

Key words: Aorta, �-receptors, Experimental animals, Mechanical properties, Propranolol.