Evaluation of P2-purinoceptor antagonists at two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli.

The guinea-pig taenia coli possesses two relaxation-mediating receptors for nucleotides: a prototypic P2Y-purinoceptor, which is activated by adenosine 5'-O-(2-thio-diphosphate) (ADP beta S), and a separate receptor for alpha, beta-methylene ATP (alpha,beta-MeATP). Effects of several as yet incompletely characterized P2-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-naphthalenetrisulp honic acid (XAMR0721; at 1000 microM only), pyridoxalphosphate-6-azophenyl-2',5'-disulphonic acid (iso-PPADS), pyridoxal 5-phosphate, trypan blue and Evans blue (at 320 microM 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 microM). 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 microM) and ATP (100 microM) 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 P2-purinoceptors in guinea-pig taenia coli. Comparison of the apparent affinities of the antagonists for the two taenia coli receptors with affinities for the P2X-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 P2Y-purinoceptor in turkey erythrocytes, was very weak at the P2Y-receptor of the taenia, thus supporting the existence of pharmacologic P2Y-receptor subtypes. Evans blue, with little effect in the taenia coli but a marked effect in the rat vas defrens, is the most selective P2X- (versus P2Y-) purinoceptor antagonists presently known, although its effect on the degradation of nucleotides must be kept in mind.