Local regulation of [3 H]-noradrenaline release from the isolated guinea-pig right atrium by P2X -receptors located on axon terminals

Abstract

In this study the regulation of cardiac sympathetic outflow by presynaptic P_2X receptor‐gated ion channels was examined. ATP (30 μM–1 mM) and other P2‐receptor agonists elicited [³H]‐noradrenaline ([³H]‐NA) outflow from the isolated guinea‐pig right atrium with the potency order of ATP>2‐methyl‐thioATP>α,β‐methylene‐ATP=ADP, whereas β,γ‐methylene‐L‐ATP was inactive. Ca²⁺‐free conditions abolished both electrical field stimulation (EFS)‐ and ATP‐evoked release of tritium. Unlike from EFS‐induced outflow, ATP‐induced [³H]‐NA outflow was not reduced by ω‐Conotoxin‐GVIA (100 nM), Cd²⁺ (100 μM) and tetrodotoxin (1 μM). The rapid extracellular decomposition of ATP was revealed by HPLC analysis. However, the effect of ATP to promote [³H]‐NA release was not prevented by 8‐cyclopentyl‐1,3‐dipropylxanthine (DPCPX, 250 nM), 3,7‐dimethyl‐1‐propargylxanthine (DMPX, 250 nM), or by reactive blue 2 (RB2, 10 μM), antagonists of A₁‐, A₂‐ and inhibitory P₂ receptors. Zn²⁺ (50 μM), the P_2X‐receptor modulator potentiated, and P_2X receptor antagonists, i.e. suramin (300 μM), pyridoxal‐phosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS, 30 μM) and 2′‐o‐(trinitrophenyl)‐adenosine 5′‐triphosphate (TNP‐ATP, 30 μM) antagonized the ATP (1 mM)‐evoked response. RT–PCR study revealed the expression of P_2X2 and P_2X3 receptor mRNAs in guinea‐pig superior cervical ganglion. PPADS (30 μM) significantly reduced the EFS‐induced [³H]‐NA outflow in the presence DPCPX (250 nM) and RB2 (10 μM). In summary a P_2X‐type purinoceptor regulates noradrenaline release from the isolated right atrium of the guinea‐pig. The pharmacological profile of the receptor resemble to homo‐oligomeric P_2X3 or hetero‐oligomeric P_2X2/P_2X3 complexes, and provide a new target to intervene on sympathetic neuroeffector transmission at the presynaptic site. British Journal of Pharmacology (2000) 131, 1775–1783; doi:10.1038/sj.bjp.0703757