Two relaxation-mediating P2-purinoceptors in guinea-pig taenia caeci

2-Methylthio ATP, noradrenaline, adenosine 5'-O-(2-thiodiphosphate) (ADP beta S), alpha, beta-methylene ATP, ATP and adenosine caused relaxation of guinea-pig isolated taenia caeci precontracted by carbachol, with potency decreasing in the order indicated. 4,4'-Diisothiocyanatostilbene-2,2'-disulphonate (DIDS) 10, 32 and 100 microM shifted the concentration-response curve of alpha, beta-methylene ATP increasingly to the right, by the factor 141 at DIDS 100 microM. Concentration-response curves of the other agonists were not shifted to the right by DIDS 100 microM, except for the curve of ADP beta S which was shifted by the factor 2.4. The relaxation produced by 2-methylthio ATP faded rapidly. When the fade was complete, further addition of 2-methylthio ATP or ATP did not elicit relaxation, whereas the relaxant effect of alpha, beta-methylene ATP was unchanged. The results indicate that there are at least two relaxation-mediating P2-purinoceptors in guinea-pig taenia caeci, the P2Y-purinoceptor which is relatively insensitive to DIDS and a distinct receptor for alpha, beta-methylene ATP which is very sensitive to DIDS.