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

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Abstract
In cat middle cerebral arteries (CMCA) preincubated with 3H-noradrenaline [norepinephrine] (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 .gtoreq. oxymetazoline > 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. The ratios of EC50[effective median concentration]-values pre- and postjunctionally of rauwolscine, oxymetazoline and clonidine were all close to unity. Pre- and postjunctional .alpha.2-receptors in the CMCA evidently have similar pharmacological characteristics and cannot be influenced separately by the presently used drugs.