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 i s 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 act ivity ratio. 2 Rat brain capillary endothelial cells of the B10 clone express P2Y(1) receptors that couple to intracellular Ca2+ mobilizatio n. They have low endogenous ecto-ATPase and ecto-ADPase activities. 3 ATP did not raise intracellular Ca2+ in B10 cells. Addition of apyrase s III or VII (1 u ml(-1)) to ATP treated cells induced large intracell ular Ca2+ transients. Apyrases had no action in the absence of ATP. 4 A 1 u ml(-1) apyrase III solution generated 20 mu M ADP from 0.1mM ATP within 15 s. This concentration of ADP was sufficient to produce maxi mal 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 cu rves 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 respon se curves when coincubated with ADP for 15 a. 6 Apyrase VII (1 u ml(-1 )) shifted dose response curves for the actions of ADP to larger conce ntrations. 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 AD P from ATP.