Relative contribution of ecto‐ATPase and ecto‐ATPDase pathways to the biphasic effect of ATP on acetylcholine release from myenteric motoneurons

Background and purpose:  The relative contribution of distinct ecto‐nucleotidases to the modulation of purinergic signalling may depend on differential tissue distribution and substrate preference.

Experimental approach:  Extracellular ATP catabolism (assessed by high‐performance liquid chromatography) and its influence on [³H]acetylcholine ([³H]ACh) release were investigated in the myenteric plexus of rat ileum in vitro.

Key results:  ATP was primarily metabolized via ecto‐ATPDase (adenosine 5′‐triphosphate diphosphohydrolase) into AMP, which was then dephosphorylated into adenosine by ecto‐5′‐nucleotidase. Alternative conversion of ATP into ADP by ecto‐ATPase (adenosine 5′‐triphosphatase) was more relevant at high ATP concentrations. ATP transiently increased basal [³H]ACh outflow in a 2′,3′‐O‐(2,4,6‐trinitrophenyl)adenosine‐5′‐triphosphate (TNP‐ATP)‐dependent, tetrodotoxin‐independent manner. ATP and ATPγS (adenosine 5′‐[γ‐thio]triphosphate), but not α,β‐methyleneATP, decreased [³H]ACh release induced by electrical stimulation. ADP and ADPβS (adenosine 5′[β‐thio]diphosphate) only decreased evoked [³H]ACh release. Inhibition by ADPβS was prevented by MRS 2179 (2′‐deoxy‐N⁶‐methyl adenosine 3′,5′‐diphosphate diammonium salt, a selective P2Y₁ antagonist); blockade of ADP inhibition required co‐application of MRS 2179 plus adenosine deaminase (which inactivates endogenous adenosine). Blockade of adenosine A₁ receptors with 1,3‐dipropyl‐8‐cyclopentyl xanthine enhanced ADPβS inhibition, indicating that P2Y₁ stimulation is cut short by tonic adenosine A₁ receptor activation. MRS 2179 facilitated evoked [³H]ACh release, an effect reversed by the ecto‐ATPase inhibitor, ARL67156, which delayed ATP conversion into ADP without affecting adenosine levels.

Conclusions and implications:  ATP transiently facilitated [³H]ACh release from non‐stimulated nerve terminals via prejunctional P2X (probably P2X₂) receptors. Hydrolysis of ATP directly into AMP by ecto‐ATPDase and subsequent formation of adenosine by ecto‐5′‐nucleotidase reduced [³H]ACh release via inhibitory adenosine A₁ receptors. Stimulation of inhibitory P2Y₁ receptors by ADP generated alternatively via ecto‐ATPase might be relevant in restraining ACh exocytosis when ATP saturates ecto‐ATPDase activity.

Mandarin translation of abstract

DOI: 10.1111/j.1476-5381.2008.00058.x

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