SELECTIVE ANTAGONISM BY PPADS AT P-2X-PURINOCEPTORS IN RABBIT ISOLATED BLOOD-VESSELS

1 Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a P-2-purinoceptor antagonist, was investigated for its ability to antagonize: (1) P-2X-purinoceptor-mediated contractions of the rabbit central ear artery and saphenous artery evoked by either alpha,beta-methylene ATP (alpha,beta-MeATP) or electrical field stimulation (EFS); (2) P-2Y-purinoceptor-mediated relaxations of the rabbit mesenteric artery; (3) endothelium-dependent and endothelium-independent, P-2Y-purinoceptor-mediated relaxations of the rabbit aorta. 2 alpha,beta-MeATP (0.1-100 mu M) caused concentration-dependent contractions of the rabbit ear and saphenous arteries. The negative log[alpha,beta-MeATP] that produced a contraction equivalent to the EC(25) for noradrenaline (ear artery) or histamine (saphenous artery) in the absence of PPADS was 6.60 +/- 0.18 (9) and 6.18 +/- 0.17 (9) in the ear artery and saphenous artery, respectively. These effects of exogenous alpha,beta-MeATP were concentration-dependently inhibited by PPADS (1-30 mu M). In the ear artery, the negative log[alpha,beta-MeATP] producing a contractile response equivalent to the EC(25) of noradrenaline, in the presence of PPADS at 1, 3 and 10 mu M was 6.16 +/- 0.18 (8), 5.90 +/- 0.18 (8) and 4.72 +/- 0.36 (8), respectively (P < 0.01). In the saphenous artery, the negative log[alpha,beta-MeATP] values equivalent to the EC(25) for histamine in the presence of PPADS at concentrations of 1, 3, 10 and 30 mu M were 5.90 +/- 0.19 (8), 5.73 +/- 0.16 (8), 4.99 +/- 0.14 (8) and 4.51 +/- 0.13 (8), respectively (P < 0.01). 3 PPADS at a concentration of 1 mu M had no effect on contractions of the ear artery evoked by EFS (4-64 Hz; 1 mu M phentolamine present). At higher concentrations (3-30 mu M) it caused concentration-dependent inhibition of neurogenic contractions. In the saphenous artery, PPADS (1-30 mu M) concentration-dependently inhibited contractions evoked by EFS at frequencies of 4, 8 and 16 Hz. Contractions evoked by EFS at frequencies of 32 and 64 Hz were significantly inhibited by PPADS only at concentrations of 10 and 30 mu M. 4 PPADS (30 mu M) had no effect on relaxations to 2-methylthio ATP (3 nM-3 mu M) in rabbit mesenteric artery and to ATP (1 mu M-1 mM) in rabbit aorta (with endothelium intact or removed). In addition, PPADS (30 mu M) had no significant influence on the contractile potency of noradrenaline and histamine in rabbit ear and saphenous artery, respectively. 5 In conclusion, these results support the evidence that PPADS is a selective antagonist of P-2X-purinoceptor-mediated responses.