DIFFERENTIAL-EFFECTS OF SURAMIN ON P-2-PURINOCEPTORS MEDIATING CONTRACTION OF THE GUINEA-PIG VAS-DEFERENS AND URINARY-BLADDER

1 The effect of the P-2-purinoceptor antagonist, suramin, was investig ated on contractions of the guinea-pig vas deferens and urinary bladde r induced by adenosine 5'-triphosphate (ATP) and by the other naturall y 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 def erens the order of potency of the nucleotides was ATP much greater tha n 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 respo nses to these agonists were not achieved in either tissue. 3 Suramin ( 30 mu M-1 mM) dose-dependently inhibited ATP-induced contractions of t he bladder in an apparently non-competitive manner, causing a reductio n 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 co ntractions 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 Desensiti zation of the P-2X purinoceptors in the guinea-pig vas deferens with a denosine 5'-alpha,beta-methylenetriphosphonate (AMPCPP) (300 mu M) abo lished contractions induced by ATP (1 mu M-1 mM) in the absence of sur amin. However, the contractions induced in the presence of suramin wer e unaffected by prior desensitization, indicating that they were not m ediated by P-2X-purinoceptors. 5 ATP (100 mu M) was dephosphorylated b y both isolated tissue preparations under the conditions of these expe riments, breakdown products being detectable after 2 min, with the maj or breakdown product in the bladder being inosine whereas that in the vas deferens was adenosine. Approximately 35% of the ATP remained inta ct after incubation for 30 min with the bladder, and approximately 45% remained after incubation for 30 min with the vas deferens. In each t issue 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, ap proximately 50% remained in the case of the bladder and approximately 65% remained in the vas deferens. However, inhibition of the productio n 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 contr actions of the vas deferens, as the enhancement was still observed whe n 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 inhibitio n by suramin (1 mM) of ATP-induced contractions was observed. 7 The mo st likely explanation for the potentiation by suramin of the ATP-induc ed contractions of the vas deferens is the co-existence of inhibitory P-2Y-purinoceptors. However, no consistent relaxations to ATP (1-100 m u M) Or to the more potent P-2Y-purinoceptor agonist 2-methylthioadeno sine 5'-triphosphate (2-MeSATP) (0.01-100 mu M) could be detected in t he vas deferens precontracted with KCl (35 mM), even after desensitiza tion of P-2X-purinoceptors with AMPCPP (300 mu M) Similarly, ATP (1-10 0 mu M) or 2-MeSATP (0.01-100 mu M) added before KCl (35 mM), carbacho l (10 mu M) or noradrenaline (10 mu M) did not reduce subsequent contr actions to these agents. 8 The differential effect of suramin on the c ontractions induced by ATP in the bladder and the vas deferens was une xpected, 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 add ition to possessing P-2X-purinoceptors may also possess a suramin-inse nsitive contractile ATP receptor revealed in the presence of suramin.