Pharmacological characterization of postjunctional α‐adrenoceptors in isolated feline cerebral and peripheral arteries

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Abstract
The vascular α‐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>oxymetazoline>noradrenaline>phenylephrine which indicates that the postjunctional α‐receptor in this vessel is of α2‐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 α1‐receptors in a variety of other tissues, thus suggesting the occurrence of an α1‐receptor in these arteries. However, the pA2‐values for yohimbine and rauwolscine correlated well with an α2‐receptor, suggesting also the presence of α2‐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 α1‐and α2‐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 α‐receptor unable to differentiate between substances that in other tissues act preferentially on α1‐ or α2‐receptors.

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