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

Abstract

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 µM 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 37°C). 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.