The interaction of diadenosine polyphosphates with P-2X-receptors in the guinea-pig isolated vas deferens

1 The site(s) at which diadenosine 5',5'''-P-1,P-4-tetraphosphate (AP(4)A) and diadenosine 5',5'''-P-1,P-5-pentaphosphate (AP(5)A) act to evoke contraction of the guinea-pig isolated vas deferens was studied by use of a series of P-2-receptor antagonists and the ecto-ATPase inhibitor 6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP (ARL 67156). 2 Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (300 nM-30 mu M), suramin (3-100 mu M) and pyridoxal-5'-phosphate (P-5-P) (3-1000 mu M) inhibited contractions evoked by equi-effective concentrations of AP(5)A (3 mu M), AP(4)A (30 mu M) and alpha,beta-methyleneATP (alpha,beta-meATP) (1 mu M), in a concentration-dependent manner and abolished them at the highest concentrations used. 3 PPADS was more potent than suramin, which in turn was more potent than P-5-P. PPADS inhibited AP(5)A, AP(4)A and alpha,beta-meATP with similar IC50 values. No significant difference was found between IC50 values for suramin against alpha,beta-meATP and AP(5)A or alpha,beta-meATP and AP(4)A, but suramin was more than 2.5 times more potent against AP(4)A than AP(5)A. P-5-P showed the same pattern of antagonism. 4 Desensitization of the P-2X1-receptor by alpha,beta-meATP abolished contractions evoked by AP(5)A (3 mu M) and AP(4)A (30 mu M), but had no effect on those elicited by noradrenaline (100 mu M). 5 ARL 67156 (100 mu M) reversibly potentiated contractions evoked by AP(4)A (30 mu M) by 61%, but caused a small, significant decrease in the mean response to AP(5)A (3 mu M). 6 It is concluded that AP(4)A and AP(5)A act at the P-2X1-receptor, or a site similar to the P-2X1-receptor, to evoke contraction of the guinea-pig isolated vas deferens. Furthermore, the potency of AP(4)A, but not AP(5)A, appears to be inhibited by an ecto-enzyme which is sensitive to ARL 67156.