Effect of the amino-oxazoline derivative S-3341 on pre- and postjunctional alpha-adrenoceptors in isolated blood vessels.

In segments of isolated dog saphenous veins, S-3341 (10(-8) to 10(-4) M) induced concentration-dependent contractions which, like those to clonidine, were progressively inhibited by increasing concentrations of yohimbine (10(-8) to 10(-6) M) but not by prazosin (10(-8) to 10(-6) M). In strips of the rabbit main pulmonary artery, previously labelled with [3H]noradrenaline and mounted for superfusion, S-3341 and clonidine both decreased the overflow of [3H] caused by electrical stimulation; these effects of S-3341 and clonidine were inhibited by 10(-6) M of yohimbine but not by 10(-6) M of prazosin. Our results illustrate that both S-3341 and clonidine cause contraction by activation of postjunctional alpha 2-adrenoceptors and decrease noradrenaline-release by stimulating prejunctional alpha 2-adrenoceptors. At the postjunctional alpha 2-adrenoceptors clonidine is 27 times more potent than S-3341, while at the prejunctional alpha 2-adrenoceptors, clonidine is only 15 times more potent than S-3341.