Postjunctional alpha-adrenoceptors and influence of angiotensin II in different isolated blood vessels.

The effects of the selective alpha 1-and alpha 2-adrenergic agonists phenylephrine and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine) and the respective antagonists prazosin and yohimbine on smooth muscle activity of isolated rat aorta, rabbit vena cava inferior and rabbit vena ischiadica have been investigated. In addition, the influence of angiotensin II on the effects of these agonists and antagonists was evaluated. Among the two agonists phenylephrine was the most potent in the rat aorta and B-HT 920 in the two venous vessels. Prazosin and yohimbine revealed a competitive antagonism against both agonists in all three vessels. No differentiation between alpha 1- and alpha 2-adrenoceptor mediated responses was seen in rat aorta and rabbit vena ischiadica. In the rabbit vena cava, prazosin was more potent against phenylephrine than against B-HT 920 whilst yohimbine was more potent against B-HT 920 than against phenylephrine, thus pointing to the existence of functional alpha 1- and alpha 2-adrenoceptors. Angiotensin II (5 X 10(-9) mol/l) induced sustained contractions in rat aorta and transient contractions of different relative magnitude in the veins. Angiotensin II pretreatment increased the potency of phenylephrine in all vessels with no influence on maximum contraction. B-HT 920 potency was increased in the vein preparations; maximum contractions were increased in rat aorta, decreased in rabbit vena cava and not influenced in rabbit vena ischiadica.(ABSTRACT TRUNCATED AT 250 WORDS)