Characterization of prejunctional purinoceptors on adrenergic nerves of the rat caudal artery.

The effects of a number of purinoceptor agents on the release of endogenous noradrenaline from the electrically stimulated rat caudal artery were determined. Noradrenaline was quantified by high performance liquid chromatography-electrochemical detection techniques. Both P1-receptor and P2-receptor agonists reduced the release of noradrenaline; the relative order of potency being 2-chloroadenosine greater than beta, gamma methylene ATP greater than ATP greater than or equal to adenosine. The adenosine uptake inhibitor S-p-nitro-benzyl-6-thioguanosine potentiated the effects of adenosine but not those of the adenine nucleotides. This suggests that the nucleotides do not need to be converted to adenosine to produce a prejunctional inhibition of the release of noradrenaline. The P1-receptor antagonist 8-(p-sulfophenyl) theophylline reduced the inhibitory effects of both P1- and P2-receptor agonists as did the photolysis of tissues with an intense light source. The findings indicate that prejunctional purinoceptors that mediate an inhibition of the release of noradrenaline from the adrenergic nerves of the caudal artery may not be adequately defined as either P1- or P2-receptors and thus appear to represent a unique receptor. We suggest referring to these receptors as P3-purinoceptors.