Metabolism of extracellular ATP by rat parotid cells

ATP hydrolysis and the products of ATP metabolism were measured in intact rat parotid acini. The purpose was to determine the contribution of extracellular enzymes in metabolizing ATP and its metabolites. The total enzyme activity accounting for extracellular ATP breakdown was at least 75% dependent on added divalent cations, consistent with the presence of ectoATPase. Approximately 50% of the added ATP was hydrolysed in 1 h by the cells and this percentage was independent of cell protein concentration from 80 to 296 μg/ml and independent of ATP concentration from 4 to 80 μM. ADP, AMP and adenosine were identified as metabolites. Cell adenosine uptake was not a factor in controlling the levels of extracellular adenosine. Generation of adenosine was limited under conditions of higher rates of ATP hydrolysis. Studies in parotid cell membranes showed that very little feedback inhibition of ectoATPase was observed. 5′ Nucleotidase was present at levels of activity of 0.06–0.19 μmol/mg protein/h in intact acini. The results confirm the presence of ectonucleotidases which can generate ADP, AMP and adenosine. Ectonucleotidase could contribute to reducing the effect of extracellular ATP on the parotid cell.