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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 μm) shifted the concentration‐contraction response curve of α,β‐methylene ATP (α,β‐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 μm). 3 Suramin (32 μm) also shifted the concentration‐response curve of α,β‐MeATP to the right, KB 2.6 μm, and increased the maximum by 31%. In the presence of suramin (32 μm), trypan blue (32 μm) did not change the concentration‐response curve of α,β‐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 α,β‐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 μm, 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 α,β‐MeATP concentration‐response curve is similar in mechanism to the increase produced by suramin.