P-2-PURINOCEPTOR ANTAGONISTS .1. BLOCKADE OF P-2-PURINOCEPTOR SUBTYPES AND ECTO-NUCLEOTIDASES BY SMALL AROMATIC ISOTHIOCYANATO-SULFONATES

Effects of eight small aromatic isothiocyanatosulphonates, of the alip hatic 2-isothiocyanatoethene-1-sulphonate (IES), and of the parent ami nes were studied on contractions of the rat vas deferens elicited by a lpha,beta-methylene ATP (alpha,beta-MeATP; mediated by P-2X-purinocept ors), relaxations of the carbachol-precontracted guinea-pig taenia col i elicited by adenosine 5'-O-(2-thiodiphosphate) (ADP beta S; mediated by P-2Y-purinoceptors), and the degradation of ATP by rat vas deferen s tissue. The aromatic isothiocyanato-sulphonates all reduced contract ions of the rat vas deferens elicited by alpha,beta-methylene ATP. The antagonism was non-competitive, with depression of the maximum of the concentration-response curve of alpha,beta-MeATP and incomplete rever sibility. The IC50 values were between 11 and 54 mu M. In the guinea-p ig taenia coli, the aromatic compounds shifted the concentration-respo nse curve of ADP beta S to the right in a surmountable manner (one exc eption), and where three concentrations were tested, the Arunlakshana- Schild regression was linear and its slope did not differ from 1. The apparent K-d values were between 10 and 214 mu M. The removal of ATP f rom the medium by vas deferens tissue was decreased by the aromatic is othiocyanates with IC25% values between 25 and 464 mu M. IES and the p arent amines were inactive or almost inactive (parent amines not teste d on ATP breakdown). The results indicate that the isothiocyanato resi due as well as the aromatic core are essential for P-2-purinoceptor bl ockade. At the P-2X-purinoceptor, potency increases with the size of t he molecules but is independent of the position of the isothiocyanato and sulphonate substituents. No simple structure-activity relationship for the P-2Y-purinoceptor and the ATP-degrading ecto-nucleotidases ca n be derived beyond the apparent lack of a major influence of the posi tion of the substituents. 2-Isothiocyanatonaphthalene-1-sulphonate (be ta-INS) seems to be interesting because of relatively high P-2X-select ivity versus both the P-2Y-purinoceptor and ecto-nucleotidases.