Analysis of the influence of nucleotidases on the apparent activity of exogenous ATP and ADP at P2Y 1 receptors

1 ADP is a potent agonist of rat and human P2Y 1 purinoceptors. ATP is a weak competitive antagonist. This study analyses the situation in which P2Y 1 receptors are exposed to ATP in the presence of exogenous ecto‐nucleotidases (apyrases) that have high or low ATPase/ADPase activity ratio. 2 Rat brain capillary endothelial cells of the B10 clone express P2Y 1 receptors that couple to intracellular Ca 2+ mobilization. They have low endogenous ecto‐ATPase and ecto‐ADPase activities. 3 ATP did not raise intracellular Ca 2+ in B10 cells. Addition of apyrases III or VII (1 u ml −1 ) to ATP treated cells induced large intracellular Ca 2+ transients. Apyrases had no action in the absence of ATP. 4 A 1 u ml −1 apyrase III solution generated 20 μ M ADP from 0.1 m M ATP within 15 s. This concentration of ADP was sufficient to produce maximal activation of P2Y 1 receptors. 5 ATP was a full agonist of P2Y 1 receptors in the presence of 1 u ml −1 apyrase III. Dose response curves for the apparent actions of ATP were bell shaped in the presence of 0.1 u ml −1 apyrase III. Apyrase III did not alter ADP dose response curves when coincubated with ADP for 15 s. 6 Apyrase VII (1 u ml −1 ) shifted dose response curves for the actions of ADP to larger concentrations. It induced a bell shaped ATP dose response curve. 7 Results suggest that ATPDases prevent P2Y 1 receptor activation by degrading ADP but may contribute to P2Y 1 receptor activation by generating ADP from ATP.British Journal of Pharmacology (1998) 125 , 675–680; doi: 10.1038/sj.bjp.0702115