Effects of diadenosine polyphosphates on the intracellular Ca2+ concentration in endothelial cells

Diadenosine polyphosphates have differential hemodynamic effects. The role of the endothelium in the vascular effects of these agonists is still unclear. Primary cultures of rat aortal endothelial cells and Ea.hy 926 cells (a continuous endothelial cell line) were used to investigate the effects of Ap3A-Ap6A, adenosine triphosphate (ATP), and for comparison, arginine vasopressin (AVP) and angiotensin II (A II) on the intracellular Ca2+ concentration, [Ca2+]i. Fura-2 was used as Ca2+ indicator. In rat aortal endothelial cells, ATP and Ap4A concentration dependently increased [Ca2+]i with an initial peak followed by an elevated plateau. The half-maximal effects were reached at approximately 7 micromol/l for ATP and at approximately 10 micromol/l for Ap4A. The maximal peak effects at 100 micromol/l were 1,035 +/- 413 nmol/l (n = 3) and 437 +/- 271 nmol/l (n = 8) for ATP and Ap4A, respectively. At 100 micromol/l Ap3A and Ap6A slightly increased [Ca2+]i, while Ap5A had no significant effect. The known endothelial agonists AVP (100 nmol/l) and A II (10 nmol/l) increased [Ca2+]i initially by 1,549 +/- 913 nmol/l (n = 7) and 209 +/- 45 nmol/l (n = 9), respectively. In Ea.hy 926 cells an increase in [Ca2+]i was obtained only with ATP (10 micromol/l) and with Ap4A (100 micromol/l). Ap3A, Ap5A, and Ap6A (each 100 micromol/l) and also AVP (100 nmol/l) and A II (10 nmol/l) had no significant effects in these cells. These results show that a considerable increase in [Ca2+]i in endothelial cells can only be induced by Ap4A among the diadenosine polyphosphates, indicating that the vasoactive effects of only this polyphosphate could at least partly be mediated via Ca2+-dependent mechanisms in endothelial cells, comparable to the known effects of AVP, A II, and ATP. The fact that A II and AVP did not influence [Ca2+]i in Ea.hy 926 cells is probably due to the loss of the respective receptors in this cell line.