ATP as a Co‐Transmitter with Noradrenaline in Sympathetic Nerves—Function and Fate

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Abstract
ATP and noradrenaline are co‐stored in synaptic vesicles in sympathetic nerves and when co‐released act postjunctionally to evoke contraction of visceral and vascular smooth muscle. In the original purinergic nerve hypothesis it was proposed that ATP would then be sequentially broken down to ADP, AMP and adenosine. Although such breakdown can be measured, it is not clear how the time‐scale of breakdown compares with the time‐course of the postjunctional actions of ATP. We have investigated the role of ectoATPase in modulating purinergic neurotransmission in the guinea‐pig vas deferens using ARL67156 (formerly FPL67516), a recently developed inhibitor of ectoATPase. ARL67156 (1–100 µM) potentiated neurogenic contractions in a concentration‐dependent manner. Onset of potentiation was rapid and the effect reversed rapidly on washout of the drug. The effect was also frequency dependent, being greater at lower frequencies. The purinergic component of the neurogenic contraction was isolated using the α1 antagonist prazosin (100 nM) and ARL67156 caused a similar potentiation. ARL67156 also potentiated contractions evoked by exogenous ATP (100 µM), but had no effect on those of the stable analogue α,β‐methylene ATP (500 nM). In the presence of the P2 purinoceptor antagonist PPADS (100 µM), ARL67156 also had no effect on contractions evoked by noradrenaline (10 µM) or KCl (40 mM). These results are consistent with an inhibitory action of ARL67156 on ectoATPase and suggest that ectoATPase modulates purinergic transmission in the guinea‐pig vas deferens. When released from sympathetic nerves, ATP acts at the P2X purinoceptor, a ligand‐gated cation channel, to evoke depolarization and contraction. In single acutely dissociated smooth muscle cells of the rat tail artery, studied under voltage‐clamp conditions, ATP and its analogues evoke an inward current, with a rank order potency of 2‐methylthioATP = ATP > α,β‐methylene ATP. This is very different from the order of potency for evoking contraction in whole vessel rings, which is α,β‐methylene ATP »2‐methylthioATP≥ATP. This discrepancy can be explained by a previously unrecognized attenuation of the action of ATP and 2‐methylthioATP, but not α,β‐methylene ATP, by ectoATPase in whole tissues.