ATP ACTIVATES P2X-CONTRACTING AND P2Y-RELAXING PURINOCEPTORS IN THE SMOOTH-MUSCLE OF MOUSE VAS-DEFERENS

1 The mechanism for the low potency of exogenous ATP in producing contraction at the P2x-purinoceptors in the smooth muscle of the mouse vas deferens (VD) was examined. 2 The measure of the breakdown of ATP in contact with the VD showed that its degradation was limited and did not account for its weak contractile effect. 3 Externally applied, ATP induced a small and transient contraction but a marked and prolonged increase of the cytosolic Ca2+ concentration ([Ca2+]i), which suggests an efficient binding to the P2x-purinoceptors. Such a calcium-force dissociation was not observed with beta,gamma-methylene ATP (beta,gamma-Me-ATP), a structural ATP analogue. 4 The force response of precontracted VD to ATP was biphasic, consisting of a small initial contraction followed by a sustained marked relaxation. In contrast, beta,gamma-Me-ATP elicited a pronounced contraction without ensuing relaxation. 5 ATP was more potent than adenosine in producing relaxation, and the relaxation was not antagonized by 8-phenyltheophylline, suggesting the activation of P2-purinoceptors. 6 For this relaxation, the rank order of potency was 2-methyl-thio-ATP (2-MeSATP) > ATP > beta,gamma-Me-ATP, which is characteristic for the P2y-purinoceptors. 7 Reactive Blue 2, a P2y-purinoceptor antagonist, was found to reduce the relaxation mediated by ATP. 8 These results indicate that ATP acts in VD not only on contracting but also on relaxing P2-purinoceptors, eliciting thereby overlapping opposite effects. In VD, the classical low potency of ATP for contraction is thus not explained by its low bioavailability or its low binding, but rather by its low specificity for the contracting P2x-purinoceptors, leading to the activation of the relaxing P2y-purinoceptors.