Effect of vascular endothelium on alpha-adrenoceptor-mediated response in the isolated aorta of the rat

The aim of this work was to study the role played by the endothelial cells in the contractile responses of vascular smooth muscle to α-adrenoceptor agents and to develop elements that can contribute to a better understanding of the mechanism by which endothelial cells modulate the action of these vasoactive drugs. 1. This study has enabled us to demonstrate that endothelium exhibits an inhibitory effect in the contractile responses of rat isolated aorta to different α-adrenoceptor agonists, used in this study, particularly to partial agonists that had a weak effect in intact preparations. The results also indicate that endothelium could affect both Ca2+ entry and Ca2+ release evoked by α-adrenoceptor agonists. We could show that vascular endothelium modifies the mode of antagonism of two selective α1-adrenoceptor antagonists, namely prazosin and doxazosin, which were found to act as non-competitive antagonists in the absence of endothelium. In contrast, the antagonism of two other α-adrenoceptor antagonists, phentolamine and yohimbine, was not affected by the endothelial factor, therefore suggesting that this modulatory effect is related to a factor other than the selective for α-adrenoceptors, which could be related to their common quinazoline structure rendering them non competitive. 3. Experiments with methylene blue and 8-bromo cyclic GMP have demonstrated that both modulatory effects by endothelium of agonistic and antagonistic action are mediated through cGMP which indicates that the endothelial factor responsible for this modulatory effect is the same or similar to the endothelial factor described by Furchgott and Burstyn which mediates vascular smooth muscle relaxation. 4. The results of this work indicate that EDRF could act as physiological or functional antagonist. This hypothesis is based in the finding that EDRF decreases agonist efficacy and receptor reserve. This action of EDRF could account for its inhibitory effect on α-adrenoceptor agonist-induced responses and for its modulatory effect on prazosin mode of antagonism. 5. In view pof the present finding that endothelium constitutes a factor interfering with drug-receptor interaction we have decided to reassess the characteristics of post-synaptic α-adrenoceptors of the rat isolated aorta taking into consideration the endothelial factor. Different parameters determined regarding α-adrenoceptors agonist receptor interaction indicated that the postsynaptic α-adrenoceptor population in the rat aorta is of the α1- subtype. the work reported in this study clearly shows the ability of vascular endothelial cells to modulate vascular smooth muscle response to a group of vasoactive agents of high physiological and pharmacological importance under experimental conditions. This phenomenon was originally observed with acetylcholine and it has become celar that the endothelium releases a diffusible factor which mediates the relaxation to acetylcholine through stimulation of guanylate cyclase. As this factor was found to be the mediator for the relaxation response to other endothelium dependent vasodilators, it is demonstrated here that it is also responsible for the endothelial modulatory effects on α-adrenoceptor mediated responses. The fincind that EDRF could act as a functional antagonist reducing agonist efficacy and receptor reserve could contribute to a better understanding of the different sensitivity of different vascular beds to certain vasoactive agents depending on their receptor density and the relative function of endothelial cells layer. In contrast, the absence or dysfunction of endothelial cells may play a role in pathological vascular events associated with atherosclerosis, thrombogenesis, vasospasm and diabetic vascular pathogenesisThèse de doctorat en sciences médicales -- UCL, 198