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

• Published in: Acta physiologica Scandinavica 1984-10, Vol.122 (2), p.165-174
• Main Author: Skaerby, T
• Format: Article
• Language: English
• Subjects: 3H-noradrenaline release; Adrenergic alpha-Agonists - pharmacology; Adrenergic alpha-Antagonists - pharmacology; Animals; cat cerebral arteries; Cats; Cerebral Arteries - drug effects; Cerebral Arteries - innervation; Electric Stimulation; Female; isometric contractions; Male; Muscle Contraction - drug effects; Muscle, Smooth, Vascular - drug effects; Neuromuscular Junction - drug effects; Pre- and postjunctional α2-adrenoceptors; Vascular Resistance - drug effects
• ISSN: 0001-6772; 1365-201X
• DOI: 10.1111/j.1748-1716.1984.tb07494.x

In cat middle cerebral arteries (CMCA) preincubated with 3H‐noradrenaline (NA), the outflow of tritium evoked by electrical stimulation was reduced to 32% by α‐adrenoceptor (α‐receptor) stimulation with oxymetazoline, and increased to 487% by α‐receptor blockade with HEAT. The relative order of potency for α‐receptor agonists on prejunctional receptors was: clonidine ≥ oxymetazoline > phenylephrine, and the antagonist rauwolscine was more potent than prazosin. This indicates that the prejunctional α‐receptors are mainly of α2‐type. Rauwolscine was more potent than prazosin in inhibiting the contractions induced by NA, indicating a predominance of α2‐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 α2‐receptors in the CMCA have similar pharmacological characteristics and cannot be influenced separately by the presently used drugs.