The duration and amplitude of the plateau phase of ATP- and ADP-evoked Ca2+ signals are modulated by ectonucleotidases in in situ endothelial cells of rat aorta

ATP has a long-lasting vasodilatory effect, possibly due to its capability to induce a prolonged increase in the intracellular Ca2+ concentration ([Ca2+](i)) in endothelial cells (EC) and activate constitutive nitric oxide synthase. However, contradictory data have been reported regarding the time course of ATP-evoked Ca2+ signals in in situ EC. In particular, short-duration Ca2+ signals have been reported, which might be thought to be unable to sustain a prolonged, NO-induced vasodilation. The current experiments were therefore performed in in situ EC of rat aorta in order to more fully define the time course of ATP-evoked Ca2+ signals. 20 muM ATP evoked a short-lasting Ca2+ signal. However, medium stirring, high agonist concentrations, inhibition of ectonucleotidases and application of a poorly hydrolyzable agonist evoked long-lasting Ca2+ signals (up to 20 min at 37degreesC). These studies suggest that ATP is able to sustain a prolonged [Ca2+](i) increase, unless ectonucleotidase activity reduces the agonist concentration near the EC surface to subthreshold values, quickly cutting the Ca2+ signal. Furthermore, the amplitude of the long-lasting phase of the Ca2+ signal depended on the balance between agonist degradation by ectonucleotidases and agonist transport, by diffusion and convection, from bulk solution to the EC surface. Copyright (C) 2004 S. Karger AG, Basel.