ATP activates P_2x‐contracting and P_2y‐relaxing purinoceptors in the smooth muscle of mouse vas deferens

Abstract

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+],), which suggests an efficient binding to the P 2X-purinoceptors. Such a calcium-force dissociation was not observed with P,y-methylene ATP (P,y-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, J,T-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>P,TyMe-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.

Post‐receptor pathway of the ATP‐induced relaxation in smooth muscle of the mouse vas deferens

“…We previously showed that ATP can induce a transient contraction followed by a sustained relaxation in depolarized preparations and that the latter effect probably involved P2y-purinoceptors (Boland et al, 1992) Figure 1) does not support this interpretation. ATP can also induce opening of K+-channels and hence hyperpolarization, in guinea-pig taenia coli (Tomita & Watanabe, 1973) and in ileum circular layers (Crist et al, 1992).…”

“…ATP-induced relaxation of depolarized VD was correlated with a reduction of the level of PMLC. Most likely this reflected the inhibition of the myosin light chain kinase (MLCK) when it is phosphorylated by a cyclic AMP-dependent protein kinase A (Adelstein et al, 1978;Kerrick & Hoar, 1981;Nishimura & van Bremen, 1989;Ozaki et al, 1992) On the basis of the rank order of potency of different ATP analogues, of the absence of tachyphylaxis and of the action of selective antagonists, it was proposed that ATP-induced relaxation implies a P2Y-like purinoceptor (Boland et al, 1992). Our results suggest that this effect is linked to the activation of adenylyl cyclase.…”

“…We previously showed that the mechanical counterpart of the P2y activation could be revealed on pre-contracted and depolarized preparations (15 min in 140 mM K+ solution): in these conditions, after a transient P2x-related contraction, ATP produced a sustained and reversible relaxation induced by P2y activation (Boland et al, 1992). The relaxant effect was maximal for a concentration of ATP of 1 mm.…”

“…We essentially followed the procedures previously described (Boland et al, 1992) (McCleskey et al, 1987) or after pharmcological destruction of nerve endings (Wakade, 1979). In the latter case, mice were injected intraperitoneally with 6-hydroxydopamine (6-OHDA), three times at two days interval.…”

“…Stimulation with 100I!M ATP, which induces an obvious calciumforce dissociation (Boland et al, 1992) (Boland et al, 1992) with the presence of isoprenaline (see above). We found that K+-induced contraction was not significantly affected by concomitant addition of 1O jM isoprenaline (97 ± 6.5% of the K+ reference peak, n = 4).…”