Inhibition of rat parotid ecto-ATPase activity

The inhibitory profile of several known and suspected ecto-ATPase inhibitors was compared on ecto-ATPase activity in rat parotid plasma membranes. Those chemicals with high IC50 (above 130 microM) were the nucleotides alpha,beta-methylene ATP, beta,gamma-methylene ATP, 2-methylthio ATP, inosine triphosphate, 5'-p-fluorosulphonylbenzoyladenosine, the sulphonates, 1-amino-2-naphthol-4-sulphonic acid, Coomassie brilliant blue G, and the stilbene disulphonates, DIDS and SITS. Those agents with low IC50 were: Coomassie brilliant blue R (114 microM), ATPgammaS (49 microM), suramin (72 microM) and Reactive blue 2 (28 microM). The last three inhibitors have similar potencies as inhibitors of ATP hydrolysis by whole parotid acinar cells. ARL67156, a selective inhibitor of ecto-ATPase, had an IC50 of approx. 120 microM. Suramin displayed non-competitive inhibition of ecto-ATPase whereas the inhibitory effects of ATPgammaS and Reactive blue 2 were curvilinear on Dixon plots. These results define the effects of various agents on ecto-ATPase in an exocrine tissue that has been shown to respond to extracellular ATP.