ON THE SUITABILITY OF ADENOSINE 3'-PHOSPHATE 5'-PHOSPHOSULFATE AS A SELECTIVE P2Y RECEPTOR ANTAGONIST IN INTACT TISSUES

Agonist and antagonist effects of the putative P2Y(1) receptor antagon ist adenosine 3'-phosphate 5'-phosphosulphate (PAPS) were studied in i ntact tissues. In the carbachol-precontracted guinea-pig taenia coli, PAPS caused prominent relaxation (EC50 3.3 mu M). The response was att enuated by the P2 receptor antagonists 4,4'-diisothiocyanatostilbene-2 ,2'-disulphonate (DIDS) and reactive red 2 with apparent K-d values (0 .27 and 0.29 mu M) indicating that PAPS acts through the non-P2Y recep tor, which is the site of action of alpha,beta-methylene ATP (alpha,be ta-MeATP) in the taenia coli. Incubation with PAPS (10-100 mu M) atten uated the P2Y receptor-mediated relaxation caused by 5'-O-(2-thiodipho sphate) (ADP beta S); PAPS (100 mu M) also attenuated the relaxation c aused by alpha,beta-MeATP, as well as the alpha(1)-adrenoceptor-mediat ed response to noradrenaline. In the noradrenaline-precontracted rat a orta, PAPS caused minor relaxation (EC50 24.7 mu M), which was reduced by the P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',5'-d isulphonate (iso-PPADS; 1 mu M), indicating that PAPS activates endoth elial P2Y receptors. Incubation with PAPS (10 and 100 mu M) attenuated the P2Y receptor-mediated relaxation caused by ADP beta S; PAPS (100 mu M) also attenuated the P2X receptor-mediated relaxation caused by U TP and the muscarine receptor-mediated response to acetylcholine. In r at vas deferens, PAPS (100 mu M) attenuated the P2X receptor-mediated contraction elicited by alpha,beta-MeATP but did not alter the alpha(1 )-adrenoceptor-mediated response to noradrenaline. The results indicat e that PAPS attenuates P2Y receptor-mediated relaxation in intact tiss ues. However, due to its limited subtype selectivity and non-P2 recept or effects, the nucleotide is not a suitable antagonist for this subty pe. (C) 1998 Elsevier Science B.V. All rights reserved.