Blockade of P2X-purinoceptors by trypan blue in rat vas deferens.

1. The possibility of an antagonist effect of trypan blue at P2X-purinoceptors was studied in rat vas deferens. 2. Trypan blue (3.2-320 microM) shifted the concentration-contraction response curve of alpha,beta-methylene ATP (alpha,beta-MeATP) to the right and simultaneously increased the maximum of the curve by up to 40%. The Schild plot had a slope not significantly different from unity and yielded a pA2 value of 5.3 (KB 4.9 microM). 3. Suramin (32 microM) also shifted the concentration-response curve of alpha,beta-MeATP to the right, KB 2.6 microM, and increased the maximum by 31%. In the presence of suramin (32 microM), trypan blue (32 microM) did not change the concentration-response curve of alpha,beta-MeATP. 4. 1-Amino-8-naphthol-3, 6-disulphonate (H-acid) 10 mM, the sulphonic acid-carrying moiety of trypan blue, shifted the concentration-response curve of alpha,beta-MeATP to the right, KB 1.4 mM, and increased the maximum by 33%. 5. Trypan blue did not change contractions elicited by high K+ and noradrenaline. 6. Trypan blue attenuated the purinergic component of neurogenic contractions, IC50 44.9 microM, but did not change the adrenergic component. 7. It is concluded that trypan blue blocks P2X-purinoceptors in rat vas deferens. The increase of the maximum of the alpha,beta-MeATP concentration-response curve is similar in mechanism to the increase produced by suramin.