BLOCKADE OF P-2X-PURINOCEPTORS BY TRYPAN BLUE IN RAT VAS-DEFERENS

1 The possibility of an antagonist effect of trypan blue at P-2X-purin oceptors was studied in rat vas deferens. 2 Trypan blue (3.2-320 mu M) shifted the concentration-contraction response curve of alpha,beta-me thylene ATP (alpha,beta-MeATP) to the right and simultaneously increas ed the maximum of the curve by up to 40%. The Schild plot had a slope not significantly different from unity and yielded a pA(2) value of 5. 3 (K-B 4.9 mu M). 3 Suramin (32 mu M) also shifted the concentration-r esponse curve of alpha,beta-MeATP to the right, K-B 2.6 mu M, and incr eased the maximum by 31%. In the presence of suramin (32 mu M), trypan blue (32 mu M) did not change the concentration-response curve of alp ha,beta-MeATP. 4 1-Amino-8-naphthol-3, 6-disulphonate (H-acid) 10 mM, the sulphonic acid-carrying moiety of trypan blue, shifted the concent ration-response curve of alpha,beta-MeATP tb the right, K-B 1.4 mM, an d increased the maximum by 33%. 5 Trypan blue did not change contracti ons elicited by high K+ and noradrenaline. 6 Trypan blue attenuated th e purinergic component of neurogenic contractions, IC50 44.9 mu M, but did not change the adrenergic component. 7 It is concluded that trypa n blue blocks P-2X-purinoceptors in rat vas deferens. The increase of the maximum of the alpha,beta-MeATP concentration-response curve is si milar in mechanism to the increase produced by suramin.