Contraction and endothelium dependent relaxation via alpha adrenoceptors are variable in various pig arteries

The aim was to examine differences in the vascular response to alpha adrenoceptor stimulation on both smooth muscle and endothelium among large arteries perfusing various organs.

In ring preparations of coronary, carotid, iliac, mesenteric, and renal arteries from pigs (n = 24), the magnitudes of contraction to noradrenaline (with beta blocker), with or without endothelium, and endothelium dependent relaxation to noradrenaline (with alpha 1 and beta blocker) were compared in vitro.

In both endothelium-intact and denuded groups, the contractions to noradrenaline were significantly smaller in coronary and carotid arteries than in iliac, mesenteric, and renal arteries. The presence of endothelium significantly suppressed the contractions in coronary and carotid arteries, but did not affect the contractions in iliac, mesenteric, and renal arteries. The endothelium dependent relaxations to noradrenaline, which were abolished by LNMMA, were more prominent in coronary and carotid arteries than in iliac, mesenteric, and renal arteries.

The contractions via alpha adrenoceptors were small, and endothelium dependent relaxations via alpha 2 adrenoceptors were large, in the coronary and carotid arteries, which perfuse the vital organs (heart and brain), as compared with those in iliac, mesenteric, and renal arteries. These results suggest that in the state of activation of the sympathetic nervous system, arterial tone in response to alpha adrenoceptor stimulation may be regulated not only by alpha adrenoceptors on vascular smooth muscle but also by those on endothelium, through release of endothelium derived relaxing factor (EDRF) via alpha 2 adrenoceptors.