ATP analogues and the guinea-pig taenia coli: a comparison of the structure-activity relationships of ectonucleotidases with those of the P2-purinoceptor

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.

Section: Discussionsupporting

confidence: 91%

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

Boland¹,

Himpens

Vincent

et al. 1992

“…The analogues listed in Table 2, which are mostly stable to enzy- matic and chemical hydrolysis (Welford et al, 1986), gave essentially similar maximal responses. The ADP-analogue AMP-CP and AMP were ineffective at concentrations up to 5mmoll-1 (data not shown).…”

Section: Resultsmentioning

confidence: 91%

P₂‐, but not P₁‐purinoceptors mediate formation of 1,4,5‐inositol trisphosphate and its metabolites via a pertussis toxin‐insensitive pathway in the rat renal cortex

Nanoff

Freissmuth

Tuisl

et al. 1990

“…As ADP is a potent P2y purinoceptor agonist but has little activity at P2u receptors, this could affect the observed potency of ATP-y-S. In addition, ADP is readily dephosphorylated by ectonucleotidases to its corresponding nucleoside adenosine (Welford et al, 1986;, which can then act at PI-purinoceptors (Bailey & Hourani, 1990). In this study, enzymatic degradation of the ADP impurity to inosine, which was inactive in this tissue, slightly shifted both relaxation and contraction concentration-response curves to ATP-y-S, enhancing the contractile responses while depressing the relaxations.…”

Section: Discussionmentioning

confidence: 99%

Contractile effects of uridine 5′‐triphosphate in the rat duodenum

Johnson¹,

Hourani²

1994

1 Previous studies have shown that the rat duodenum relaxes to adenosine and adenosine 5'-triphosphate (ATP) via P1 and P2Y purinoceptors respectively, but in preliminary studies uridine 5'-triphosphate (UTP) was found to contract this tissue. The non-selective P2 antagonist suramin and a number of nucleotides were therefore used to investigate this response further.2 ATP, UTP, adenosine 5'-diphosphate (ADP), adenosine 5'-O-(3-thiotriphosphate) (ATP-7-S), guanosine 5'-triphosphate (GTP) and uridine 5'-diphosphate (UDP) each relaxed the duodenum, with an agonist potency order of ATP = ADP>ATP-7-S>>GTP>>UTP = UDP, consistent with the presence of a P2Y purinoceptor mediating relaxation. 3 ATP-i-S, UTP and UDP each contracted the duodenum with an agonist potency order of ATP-y-S > UTP> UDP, although maximal responses to these agonists were not obtained at a concentration of 267 JAM (ATP-i-S) and 300 pM (UTP and UDP). No contractions were observed with any of the other agonists at concentrations up to 300 JM. 4 Indomethacin (25 JM) did not inhibit the contractions induced by UTP, indicating that they were not mediated via production of prostaglandins. 5 Suramin (100 JAM and mM) inhibited relaxations induced by ATP, shifting the concentration-response curve to the right, with the maximal response to ATP being decreased by the higher concentration of suramin (1 mM). Suramin (1 mM) inhibited relaxations induced by ATP-j-S, shifting the concentration-response curve to the right, and completely abolished contractions induced by ATP-y-S. In contrast, suramin (100 JAM and 1 mM) had no effect on contractions induced by UTP. Contractions induced by UTP were, however, less sustained in the presence of suramin, which also affected the basal tone of some tissues when precontracted with carbachol (0.1IJM). In the presence of suramin (I mM), no contractions to ATP were observed.6 These results confirm that in the rat duodenum there is a P2Y purinoceptor that mediates relaxation in response to a number of purine nucleotides, and at which the pyrimidine nucleotides UTP and UDP are almost inactive. There are also receptors at which UTP and ATP-y-S act to cause contraction. Suramin discriminates between the contractile effects of these two agonists, which may indicate the presence of a suramin-insensitive pyrimidinoceptor as well as a suramin-sensitive receptor for ATP-y-S. An alternative explanation is that the differential effects of suramin are via its actions as an antagonist in addition to its action as an ectonucleotidase inhibitor.