Endothelial role in alpha-adrenergic induced vasoconstriction in rat thoracic aorta

ABSTRACT

The effects of endothelial removal on vasoconstrictor responses elicited by phenilephrine (alpha-1 agonist), clonidine (alpha-2 agonist) and norepinephrine (alpha-1 and alpha-2 agomist) were studied on isolated rat thoracic aorta. The removal of vascular endothelium resulted in a significant reduction of the effective concentration 50% (EC 50) for norepinephrine from 9.12 x 10-9 M, without changes in the maximal response, which remained equal to the maximal concentration elicited by a solution of 70 mM K. Similary, the EC 50 for phenilephrine was significantly reduced from 2.19 x 10-8 M to 9.12 x 10-9 M, without changes in the maximal response. On the other hand, the maximal response to clonidine increased significantly in the arteries without andothelium from 5.5% to 35% of the maximal response to K, and the 50 was significantly reduced from 1.82 x 10-7 M to 1.10 x 10-7 . These result suggest that the presence of vascular endothelium increases the vasoconstriction induced by adrenergic, and that its action is predominantly manifested on alpha-2 dependent contractions, probably because these responses are highly dependent on extracellular calcium influx, which is modulated by the activity of the endothelial derived relaxing factor (EDRF)