Characterization of postsynaptic α‐adrenoceptors in rat aortic strips and portal veins

Abstract

1 Postsynaptic α-adrenoceptors in rat isolated aortic strips and portal veins have been examined using a number of agonist and antagonist drugs which have varying selectivity for α₁- and α₂-adrenoceptors. 2 In both tissues (−)-noradrenaline ((−)-NA), (−)-adrenaline ((−) Adr) (−)-α-methyl noradrenaline ((−)-α-Me-NA) and (−)-phenylephrine ((−)-PE) were full agonists, while clonidine, oxymetazoline and (2-(2,6-dichlorophenyl)-5,6-dihydroimidazo(2,1,b) thiazole (44,549) were partial agonists. Guanfacine was a full agonist in aortic strips but only a partial agonist in portal veins. 3 In aortic strips, pA₂ values for prazosin and yohimbine were not significantly different using (−)-NA, (−)-PE or guanfacine as the agonist, suggesting a single population of α-adrenoceptors. The order of potency of the antagonists, prazosin = 2-(β-(4-hydroxyphenyl)-ethylaminomethyl)-tetralone (BE2254) > phentolamine > yohimbine > rauwolscine, is indicative of an α₁-type of receptor. 4 In portal veins, the order of potency of the antagonists was prazosin > BE2254 > phentolamine > yohimbine > rauwolscine, again indicating an α₁-type of receptor. 5 The mean pA₂ value for yohimbine was not significantly different in either tissue. However, mean pA₂ values for prazosin, BE-2254 and phentolamine were approximately one order of magnitude lower in portal veins than in aortic strips, suggesting that the receptors in the two tissues may not be identical.