Diadenosine polyphosphates as antagonists of the endogenous P2Y1 receptor in rat brain capillary endothelial cells of the B7 and B10 clones

1 Diadenosine polyphosphates (Ap(n)As, n = 2-7) are considered as stress mediators in the cardiovascular system. They act both via identified P2 purinoceptors and via yet to be characterized receptors. This study analyses the actions of Ap(n)As in clones of rat brain capillary endothelial cells that express P2Y(1) receptors (B10 cells) or both P2Y(1) and P2Y(2) receptors (B7 cells). 2 B10 cells responded to Ap(3)A with rises in intracellular Ca2+ concentration ([Ca2+](i)), This response was prevented by adenosine-3'-phosphate-5'-phosphate, an antagonist of P2Y(1) receptors. It was largely suppressed by a treatment with apyrase VII or with creatine phosphokinase/creatine phosphate to degrade contaminating ADP. 3 Ap(n)As inhibited ADP induced increases in [Ca2+](i) mediated by P2Y(1) receptors by shifting ADP concentration-response curves to larger concentrations. Apparent K-i values were estimated to be 6 mu M for Ap(4)A, 10 mu M for Ap(5)A and 47 mu M for Ap(6)A. Ap(2)A and Ap(3)A were much less active. 4 Ap(n)As were neither agonists nor antagonists of the endogenous P2Y(2) receptor in B7 cells. 5 Ap(n)As are neither agonists nor antagonists of the G(i)-coupled, ADP receptor in B10 cells. 6 The results suggest that most actions of Ap(n)As in B7 and B10 cells call be accounted for by endogenous P2Y(1) receptors. Ap(4)A, Ap(5)A and Ap(6)A are specific antagonists of endogenous Ca2+- coupled P2Y(1) receptors.