Role of endothelium in the contractile response of rat aorta to alpha-adrenoceptor agonists.

The influence of endothelium on the response of rat isolated aorta to alpha-adrenergic agonists has been studied by comparing the response of intact and denuded preparations before and after treatment with calcium entry blockers flunarizine and nifedipine. Endothelium removal enhanced the response of the preparations, especially to alpha 2-agonists that had a weak effect in intact preparations. In the absence of endothelium, about 80% of the maximum response to clonidine was blocked by calcium entry blockers, whereas only 25% of the maximum response to noradrenaline was sensitive to them. In contrast about 40% of maximum noradrenaline-evoked contractions was sensitive to calcium entry blockers in the presence of endothelium. This may be attributed to an increase in the intracellular exchangeable calcium fraction, likely to be due to a slight depolarization of smooth muscle membrane of denuded preparations, which are highly sensitive to the calcium agonist BAY K 8644. The results indicate that a factor liberated by endothelial cells controls both calcium entry and calcium release evoked by alpha-adrenoceptor agonists in vascular smooth muscle.