Pharmacological properties of prejunctional α‐adrenoceptors in isolated feline middle cerebral arteries; comparison with the postjunctional α‐adrenoceptors

In cat middle cerebral arteries (CMCA) preincubated with 3H-noradrenaline (NA), the outflow of tritium evoked by electrical stimulation was reduced to 32% by alpha-adrenoceptor (alpha-receptor) stimulation with oxymetazoline, and increased to 487% by alpha-receptor blockade with HEAT. The relative order of potency for alpha-receptor agonists on prejunctional receptors was: clonidine greater than or equal to oxymetazoline greater than phenylephrine, and the antagonist rauwolscine was more potent than prazosin. This indicates that the prejunctional alpha-receptors are mainly of alpha 2-type. Rauwolscine was more potent than prazosin in inhibiting the contractions induced by NA, indicating a predominance of alpha 2-receptors postjunctionally. Apart from clonidine having higher intrinsic activity pre- than postjunctionally, all drugs examined (oxymetazoline, phenylephrine, rauwolscine, HEAT (BE2254), and prazosin) had similar concentration-effect curves on the pre- and postjunctional receptors. Furthermore, the ratios of EC50-values pre- and postjunctionally of rauwolscine, oxymetazoline, and clonidine were all close to unity. These results indicate that pre- and postjunctional alpha 2-receptors in the CMCA have similar pharmacological characteristics and cannot be influenced separately by the presently used drugs.