EFFECTS OF SURAMIN ON CONTRACTIONS OF THE GUINEA-PIG VAS-DEFERENS INDUCED BY ANALOGS OF ADENOSINE 5'-TRIPHOSPHATE

1 Adenosine 5'-triphosphate (ATP) and some of its analogues contract the guinea-pig vas deferens, acting via receptors which have been classified as P-2X-purinoceptors. We have recently shown, however, that the effects of ATP are enhanced, rather than inhibited, by the non-selective P-2 antagonist, suramin, and that this enhancement could not easily be explained in terms of inhibition by suramin of the breakdown of ATP. We therefore investigated the effects of suramin on contractions induced by ATP analogues, to define the structure-activity relationships of the suramin-resistant response. 2 In the absence of suramin, the order of potency for ATP analogues was adenosine 5'-(alpha,beta-methylene)triphosphonate (AMPCPP) = P-1,P-5-diadenosine pentaphosphate (Ap(5)A) = adenosine 5'-tetraphosphate (Ap(4)) > adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) = adenylyl 5'-(beta,gamma-methylene) diphosphonate (AMPPCP)> P-1,P-5-diadenosine tetraphosphate (Ap(4)A)> adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) > 2-methylthioadenosine 5'-triphosphate (MeSATP) greater than or equal to ATP > adenosine 5'-diphosphate (ADP). This is generally in agreement with previously reported structure-activity relationships in this tissue. 3 In the presence of suramin (1 mM), responses to Ap(5)A, Ap(4)A, AMPPCP, ADP beta S and ADP were abolished or greatly reduced, and contractions induced by AMPCPP, Ap(4) and ATP gamma S were inhibited. Contractions induced by MeSATP however, like those induced by ATP itself, were not reduced, but at concentrations above 100 mu M were enhanced. In the presence of suramin (1 mM) the order of potency of analogues was therefore AMPCPP = Ap(4) > ATP = MeSATP > ATP gamma S, with all other analogues tested being essentially inactive at concentrations up to 500 mu M. 4 Contractile responses of the vas deferens to transmural nerve stimulation (1-50 Hz) in the presence of the alpha-adrenoceptor antagonist, phentolamine (10 mu M), were abolished by suramin (1 mM). This is in agreement with previous reports that suramin inhibits the excitatory junction potential, a response thought to be mediated by P-2 purinoceptors. It is however hard to reconcile the evidence implicating ATP as the non-adrenergic transmitter responsible for this response with the failure of suramin to inhibit the contractions induced by ATP itself while abolishing nerve-mediated contractions. 5 In conclusion, these results confirm our previous findings of a suramin-resistant component to the ATP-induced contraction in the guinea-pig vas deferens, and show that the structure-activity relationships of this response are not identical to those of any known P-2-purinoceptor subclass. Although the inhibition by suramin of the breakdown of ATP may contribute to the suramin-resistance of some of the ATP analogues, it does not appear to provide the full explanation.