P2-purinoceptor Antagonists Discriminate Three Contraction-mediating Receptors for ATP in Rat Vas Deferens

The sites of action at which ATP elicits contraction of the rat vas deferens were studied by means of the P2-purinoceptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (iso-PPADS), suramin and reactive blue 2. Increasing concentrations of iso-PPADS (up to 1 mM), suramin (up to 1 mM) and reactive blue 2 (up to 320 microM) reduced and eventually abolished contractions elicited by the P2x-purinoceptor-selective agonist alpha,beta-methylene ATP 3 microM with IC50 values of 2.1, 10.1 and 27.0 microM, respectively. In contrast, iso-PPADS and suramin caused only a partial inhibition of contractions elicited by ATP 1 mM, maximal reduction by about 40%, IC50 values 1.3 and 5.0 microM, respectively; reactive blue 2 did not change ATP-induced contractions. In tissues exposed to iso-PPADS 320 microM throughout, increasing concentrations of reactive blue 2 or suramin decreased contractions elicited by ATP 1 mM, IC50 values 2.6 and 14.5 microM, respectively. In tissues exposed to suramin 320 microM throughout, increasing concentrations of iso-PPADS decreased contractions elicited by ATP 1 mM, IC50 37.9 microM, whereas reactive blue 2 slightly enhanced these contractions. In tissues exposed to reactive blue 2 100 microM throughout, increasing concentrations of iso-PPADS reduced contractions elicited by ATP 1 mM, IC50 26.6 microM, whereas suramin caused no change. Pre-exposure to alpha,beta-methylene ATP 1 microM to desensitize P2x-purinoceptors reduced the response to ATP 1 mM by 91% in otherwise untreated tissues, but did not reduce the response to ATP 1 mM in tissues exposed throughout to iso-PPADS 320 microM, suramin 320 microM or reactive blue 2 100 microM.(ABSTRACT TRUNCATED AT 250 WORDS)