1. Pig aortic endothelial and smooth-muscle cells in culture rapidly catabolize exogenous ATP, ADP or AMP. 2. In both cell types catabolism is due to Mg2+-stimulated ectoenzymes. 3. Inhibition and substrate-specificity studies suggest that both cell types possess three distinct ectonucleotidases, namely nucleoside triphosphatase (EC, nucleoside diphosphatase (EC and 5′-nucleotidase (EC, as well as nucleoside diphosphate kinase (EC 4. These ectonucleotidase systems could be of importance in the regulation of neurotransmission, blood platelet function and vasodilation.

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