Characterisation of postjunctional alpha-adrenoceptors in isolated human femoral veins and arteries.

In order to characterise the pharmacological properties of postjunctional alpha-adrenoceptors, both the contractile effects of alpha-adrenoceptor agonists and the blocking potencies of selective alpha-adrenoceptor antagonists were studied in isolated human femoral veins and arteries. The veins were more sensitive to noradrenaline than the arteries. Guanfacine had a higher intrinsic activity in veins than in arteries, whereas the reverse was true for phenylephrine. The antagonists rauwolscine and yohimbine were more potent against noradrenaline in the veins than in arteries, while corynanthine was equally potent in either tissue. They antagonised the noradrenaline response in a competitive manner. Prazosin proved to be the most potent competitive antagonist in arteries, while in veins it exerted weak and non-competitive antagonism. The results suggest that the alpha-adrenoceptor population at the postjunctional site differs between human femoral veins and arteries. The veins seem to contain more alpha 2- than alpha 1-adrenoceptors postjunctionally, whereas in the arteries the alpha 1-subtype prevails. The results indicate the possibility of influencing selectively adrenergic reactions in the capacitance and resistance vessels.