1 The effect of the P-2-purinoceptor antagonist, suramin, was investigated on contractions of the guinea-pig vas deferens and urinary bladder induced by adenosine 5'-triphosphate (ATP) and by the other naturally occurring nucleoside triphosphates. 2 ATP, guanosine 5'-triphosphate (GTP), cytidine 5'-triphosphate (CTP), inosine 5'-triphosphate (ITP) and uridine 5'-triphosphate (UTP) (0.1-500 mu M) each contracted both the guinea-pig bladder and the guinea-pig vas deferens. In the vas deferens the order of potency of the nucleotides was ATP much greater than CTP > GTP greater than or equal to UTP = ITP, and in the bladder it was ATP much greater than CTP = GTP, UTP = ITP, although maximal responses to these agonists were not achieved in either tissue. 3 Suramin (30 mu M-1 mM) dose-dependently inhibited ATP-induced contractions of the bladder in an apparently non-competitive manner, causing a reduction in the slope of the concentration-response curve to ATP. In contrast, suramin (5 mu M-1 mM) had little inhibitory effect on ATP-induced contractions of the vas deferens, and indeed at concentrations of 100 mu M and above markedly potentiated high concentrations of ATP (100-500 mu M). The contractions induced by CTP, GTP, UTP and ITP (1-500 mu M) were, however, abolished by suramin (1 mM) in each tissue. 4 Desensitization of the P-2X purinoceptors in the guinea-pig vas deferens with adenosine 5'-alpha,beta-methylenetriphosphonate (AMPCPP) (300 mu M) abolished contractions induced by ATP (1 mu M-1 mM) in the absence of suramin. However, the contractions induced in the presence of suramin were unaffected by prior desensitization, indicating that they were not mediated by P-2X-purinoceptors. 5 ATP (100 mu M) was dephosphorylated by both isolated tissue preparations under the conditions of these experiments, breakdown products being detectable after 2 min, with the major breakdown product in the bladder being inosine whereas that in the vas deferens was adenosine. Approximately 35% of the ATP remained intact after incubation for 30 min with the bladder, and approximately 45% remained after incubation for 30 min with the vas deferens. In each tissue this degradation was inhibited by suramin (1 mM), so that after incubation of ATP (100 mu M) in the presence of suramin for 30 min, approximately 50% remained in the case of the bladder and approximately 65% remained in the vas deferens. However, inhibition of the production of the inhibitory agonist, adenosine by suramin did not appear to be responsible for the potentiation observed in the vas deferens, as the P-1-purinoceptor antagonist 8-sulphophenyltheophylline (100 mu M) did not reduce this potentiation. 6 Chelation of divalent cations did not appear to account for the enhancement by suramin of ATP-induced contractions of the vas deferens, as the enhancement was still observed when Mg2+ was omitted from the buffer or when its concentration (normally 1.2 mM) was increased ten fold to 12 mM, or when the concentration of Ca2+ (normally 2.5 mM) was reduced to 0.83 mM. Even in the absence of Mg2+ and with the Ca2+ concentration reduced to 0.83 mM, no inhibition by suramin (1 mM) of ATP-induced contractions was observed. 7 The most likely explanation for the potentiation by suramin of the ATP-induced contractions of the vas deferens is the co-existence of inhibitory P-2Y-purinoceptors. However, no consistent relaxations to ATP (1-100 mu M) Or to the more potent P-2Y-purinoceptor agonist 2-methylthioadenosine 5'-triphosphate (2-MeSATP) (0.01-100 mu M) could be detected in the vas deferens precontracted with KCl (35 mM), even after desensitization of P-2X-purinoceptors with AMPCPP (300 mu M) Similarly, ATP (1-100 mu M) or 2-MeSATP (0.01-100 mu M) added before KCl (35 mM), carbachol (10 mu M) or noradrenaline (10 mu M) did not reduce subsequent contractions to these agents. 8 The differential effect of suramin on the contractions induced by ATP in the bladder and the vas deferens was unexpected, and shows that the receptor populations by which ATP acts in these tissues may not be identical. The failure of suramin to inhibit responses to ATP in the vas deferens suggests that this tissue, in addition to possessing P-2X-purinoceptors may also possess a suramin-insensitive contractile ATP receptor revealed in the presence of suramin.
