Postjunctional α1‐ and α2‐adrenoceptors mediating contraction in isolated human groin arteries and veins

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Abstract
By means of selective agonists and antagonists for α1‐ and α2‐receptors, the α‐receptor subtypes in human groin arteries and veins were characterized and compared. In the arteries the α1‐receptor blocker prazosin caused a concentration‐dependent parallel displacement of the noradrenaline (NA) concentration‐response (cr) curve without reduction of maximum (pA2=9.86); the selective α2‐receptor antagonist rauwolscine in the concentration 10‐8M caused a right‐ward shift of the NA cr‐curve without reduction of Emax, but 10‐7M and 10‐6M caused little or no further shift. In the veins, the two antagonists had the opposite effects. Rauwolscine caused a concentration‐dependent right‐ward shift of the NA cr‐curve without depression of maximum (pA2=9.03); prazosin 10‐9M significantly displaced the NA cr‐curve, whereas 10‐8M and 10‐7M caused little or no further shift. The responses to the α2‐receptor agonist clonidine in the arteries were too small to allow calculations of pEC50 values; in the veins contractions were elicited in all vessel segments investigated (pEC50=6.24). Phenylephrine, selective for α1‐receptors, was significantly more potent in arteries than in veins. NA was significantly more potent in veins than in arteries. It is concluded that in human groin vessels, there is a functional predominance of arreceptors in the arteries and of a2‐receptors in the veins.