EVANS BLUE BLOCKS P(2X)-PURINOCEPTORS IN RAT VAS-DEFERENS

In rat vas deferens, Evans blue 100 muM increased contractions elicited by high K+ and by noradrenaline but markedly reduced contractions elicited by the P2X-purinoceptor-selective agonist alpha,beta-methylene ATP (3 muM). The concentration-response curve of alpha,beta-methylene ATP was shifted to the right by Evans blue 30 muM and the maximal contraction was increased. In tissues incubated with nifedipine 10 muM, Evans blue 100 muM tended to increase the residual contraction elicited by noradrenaline and abolished the residual response to alpha,beta-methylene ATP (3 muM). The concentration-response curve of alpha,beta-methylene ATP was progressively shifted to the right by increasing concentrations of Evans blue in the presence of nifedipine; maximal contractions were increased by Evans blue 10 and 30 but not 100 muM. From the shifts to the right caused by Evans blue 30 muM, apparent pK(B) values of 5.9 (no nifedipine) and 6.0 (nifedipine present) were calculated. It is concluded that Evans blue blocks P2X-purinoceptors in rat vas deferens and in addition causes a non-receptor-specific enhancement of contractions.