P-2-purinoceptor antagonists .1. Blockade of P-2-purinoceptor subtypes and ecto-nucleotidases by small aromatic isothiocyanato-sulphonates

Effects of eight small aromatic isothiocyanatosulphonates, of the aliphatic 2-isothiocyanatoethene-1-sulphonate (IES), and of the parent amines were studied on contractions of the rat vas deferens elicited by alpha,beta-methylene ATP (alpha,beta-MeATP; mediated by P-2X-purinoceptors), relaxations of the carbachol-precontracted guinea-pig taenia coli elicited by adenosine 5'-O-(2-thiodiphosphate) (ADP beta S; mediated by P-2Y-purinoceptors), and the degradation of ATP by rat vas deferens tissue. The aromatic isothiocyanato-sulphonates all reduced contractions 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 reversibility. The IC50 values were between 11 and 54 mu M. In the guinea-pig taenia coli, the aromatic compounds shifted the concentration-response curve of ADP beta S to the right in a surmountable manner (one exception), 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 from the medium by vas deferens tissue was decreased by the aromatic isothiocyanates with IC25% values between 25 and 464 mu M. IES and the parent amines were inactive or almost inactive (parent amines not tested on ATP breakdown). The results indicate that the isothiocyanato residue as well as the aromatic core are essential for P-2-purinoceptor blockade. At the P-2X-purinoceptor, potency increases with the size of the 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 can be derived beyond the apparent lack of a major influence of the position of the substituents. 2-Isothiocyanatonaphthalene-1-sulphonate (beta-INS) seems to be interesting because of relatively high P-2X-selectivity versus both the P-2Y-purinoceptor and ecto-nucleotidases.