Pharmacological characterization of postjunctional alpha-adrenoceptors in isolated feline cerebral and peripheral arteries.

The vascular alpha-adrenoceptors in isolated feline cerebral, lingual and mesenteric arteries were characterized and compared. In the middle cerebral artery the relative order of potency for agonists was: clonidine greater than oxymetazoline greater than noradrenaline greater than phenylephrine which indicates that the postjunctional alpha-receptor in this vessel is of alpha 2-type. This view is further supported by the finding that yohimbine, but not prazosin, had a potent, mainly competitive blocking action. In peripheral arteries, clonidine was without effect. In these vessels, the potency difference between phenylephrine and oxymetazoline was more than 40 times less than in cerebral vessels. The pA2-values for prazosin correlated well with pA2-values found for the interaction of this drug with alpha 1-receptors in a variety of other tissues, thus suggesting the occurrence of an alpha 1-receptor in these arteries. However, the pA2-values for yohimbine and rauwolscine correlated well with an alpha 2-receptor, suggesting also the presence of alpha 2-receptors. Schild plots for prazosin and rauwolscine in lingual arteries displayed slopes significantly lower than unity, which also supports the view of a mixture of alpha 1- and alpha 2-receptors in these vessels. However, the Schild plots for the antagonists in mesenteric arteries did not differ significantly from unity, a finding possibly indicating the presence of an alpha-receptor unable to differentiate between substances that in other tissues act preferentially on alpha 1- or alpha 2-receptors.