Functional role of EctoATPase activity in goldfish hepatocytes.

SCHWARZBAUM, P. J., FRISCHMANN, M.E., G. KRUMSCHNABEL, R.C. ROSSI AND W. WIESER

AMERICAN JOURNAL OF PHYSIOLOGY-REGULATORY, INTEGRATIVE AND COMPARATIVE PHYSIOLOGY

AMER PHYSIOLOGICAL SOC

Lugar: New York; Año: 1998 vol. 274 p. 1032 - 1038

0363-6119

Extracellular [gamma-32P]ATP added to a suspension of goldfish hepatocytes canbe hydrolyzed to ADP plus g-32Pi due to the presence of anecto-ATPase located in the plasma membrane. Ecto-ATPaseactivity was a hyperbolic function of ATP concentration([ATP]), with apparent maximal activity of 8.3 6 0.4 nmolPi ·(106 cells)21 ·min21 and substrate concentration at which ahalf-maximal hydrolysis rate is obtained of 667 6 123 μM.Ecto-ATPase activity was inhibited 70% by suramin but wasinsensitive to inhibitors of transport ATPases. Addition of 5μM [a-32P]ATP to the hepatocyte suspension induced theextracellular release of a-32Pi [8.2 pmol·(106 cells)21 ·min21]and adenosine, suggesting the presence of other ectonucleotidase(s). Exposure of cell suspensions to 5 μM [2,8-3H]ATPresulted in uptake of [2,8-3H]adenosine at 7.9 pmol·(106cells)21 ·min21. Addition of low micromolar [ATP] stronglyincreased cytosolic free Ca21 (Cai21). This effect could bepartially mimicked by adenosine 58-O-(3-thiotriphosphate), anonhydrolyzable analog of ATP. The blockage of both glycolysisand oxidative phosphorylation led to a sixfold increase ofCai21 and an 80% decrease of intracellular ATP, but ecto-ATPase activity was insensitive to these metabolic changes.Ecto-ATPase activity represents the first step leading to thecomplete hydrolysis of extracellular ATP, which allows 1)termination of the action ofATP on specific purinoceptors and2) the resulting adenosine to be taken up by the cells.