The Hydrolysis of Extracellular Adenine Nucleotides by Cultured Vascular Cells and Cardiac Myocytes

Abstract

We have studied the extracellular reaction sequence ATP → ADP → AMP → adenosine, from the point of view that it is important in the regulation of the time course of cell and tissue response during crisis or signaling events. We began with the working hypothesis that the rate of appearance of adenosine from ATP would be governed by feed forward inhibition by ADP and perhaps ATP. It was known that 5′-nucleotidase is inhibited by ATP and ADP1–5; indeed, inhibition by the nonhydrolyzable analog of ADP, α,β-methylene-ADP, has come to be used as a marker to distinguish this enzyme from other phosphatases. None of the previous studies of ectonucleotidase activity had gathered the kind of total progress of reaction data necessary to test this hypothesis. We previously noted that when cultured endothelial cells or smooth muscle cells from pig aorta are incubated with 1 μM ATP very little ADP or AMP accumulate, and there is immediate and linear production of adenosine6. We have now investigated the time course of hydrolysis of ATP by three vascular cell types and cardiac myocytes, using starting concentrations of 100–1000 μ.M, concentrations that could easily be achieved, for example, at the site of platelet degranulation.