Evaluation of P-2-purinoceptor antagonists at two relaxation-mediating P-2-purinoceptors in guinea-pig taenia coil

The guinea-pig taenia coli possesses two relaxation-mediating receptors for nucleotides: a prototypic P-2Y-purinoceptor, which is activated by adenosine 5'-O-(2-thiodiphosphate) (ADP beta S), and a separate receptor for alpha,beta-methylene ATP (alpha,beta-MeATP). Effects of several as yet incompletely characterized P-2-purinoceptor antagonists at these receptors were examined. The concentration-relaxation curve of ADP beta S was shifted to the right by reactive blue 2, suramin, 8-(3,5-dinitro-phenylenecarbonylimino)-1,3,5-naphthalenetrisulphonic acid (XAMR0721; at 1000 mu M only), pyridoxalphosphate 6-azophenyl-2',5'-disulphonic acid (iso-PPADS), pyridoxal 5-phosphate, trypan blue and Evans blue (at 320 mu M only). Schild plots for the antagonism of reactive blue 2, suramin, iso-PPADS and pyridoxal 5-phosphate against ADP beta S had slopes <1. The concentration-relaxation curve of alpha,beta-MeATP was shifted to the right by reactive blue 2, suramin, XAMR0721, iso-PPADS, pyridoxal 5-phosphate and trypan blue but not by Evans blue (320 mu M). Schild plots for the antagonism of suramin, XAMR0721 and iso-PPADS against alpha,beta-MeATP had slopes >1. Only XAMR0721 differed clearly in potency against the two nucleotides: it was considerably more potent against alpha,beta-MeATP than against ADP beta S. 2-Methylthio ATP (MeSATP; 1 mu M) and ATP (100 mu M) were degraded by pieces of taenia coli. All antagonists except trypan blue attenuated the degradation of either or one of the two nucleotides. The selective effect of XAMR0721 against alpha,beta-MeATP confirms the existence of two relaxation-mediating P-2-purinoceptors in guinea-pig taenia coli. Comparison of the apparent affinities of the antagonists for the two taenia coli receptors with affinities for the P-2X-purinoceptor of the rat vas deferens shows that reactive blue 2, suramin, iso-PPADS, pyridoxal 5-phosphate and trypan blue have little selectivity for any of the three receptors. XAMR0721, which has been shown to possess relatively high affinity for the P-2Y-purinoceptor in turkey erythrocytes, was very weak at the P-2Y-receptor of the taenia, thus supporting the existence of pharmacologic P-2Y-receptor subtypes. Evans blue, with little effect in the taenia coli but a marked effect in the rat vas deferens, is the most selective P-2X-(versus P-2Y-) purinoceptor antagonist presently known, although its effect on the degradation of nucleotides must be kept in mind.