effect of endothelium removal on stimulatory and inhibitory modulation of rat aortic prostacyclin synthesis 1 removal of the endothelium de enhanced the in vitro release of prostacyclin pgi2 by rat aortae in response to adrenaline ad noradrenaline na thromboxane a2 analogue u46619 phorbol dibutyrate pdbu and sodium fluoride naf when assessed at 3 h post de at 6 h post de there were no differences between the doseresponse curves obtained from aortic rings with or without endothelium 2 at 3 h post de the antagonism of ad and nastimulated pgi2 synthesis by yohimbine and prazosin and nastimulated pgi2 synthesis by nifedipine was markedly reduced in aortae without endothelium when compared with controls these effects were reversed by protracted incubation of aortic tissue post de 6 h and 9 h 3 acetylcholine carbachol substance p and nitroprusside were without effect on de novo or nastimulated pgi2 synthesis whether or not the endothelium was present and irrespective of incubation time postde 4 these results indicate that a pgi2 synthesis linked to excitatory receptors alphaadrenoceptors thromboxane a2 and associated systems g proteins protein kinase c in the smooth muscle component of the rat aorta is not influenced by endotheliumderived relaxing factor edrf b the changes of response to stimulators and inhibitors of pgi2 synthesis may be due to an increased reactivity of the vessels caused by the trauma of de and c vasodilators parasympathomimetics substance p and nitroprusside that do not act directly on excitatory receptors do not influence pgi2 synthesis