21 April 2004
ATP has a long-lasting vasodilatory effect, possibly due to its capability to induce a prolonged increase in the intracellular Ca<sup>2+</sup> concentration ([Ca<sup>2+</sup>]<sub>i</sub>) in endothelial cells (EC) and activate constitutive nitric oxide synthase. However, contradictory data have been reported regarding the time course of ATP-evoked Ca<sup>2+</sup> signals in in situ EC. In particular, short-duration Ca<sup>2+</sup> 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 Ca<sup>2+</sup> signals. 20 µ M ATP evoked a short-lasting Ca<sup>2+</sup> signal. However, medium stirring, high agonist concentrations, inhibition of ectonucleotidases and application of a poorly hydrolyzable agonist evoked long-lasting Ca<sup>2+</sup> signals (up to 20 min at 37°C). These studies suggest that ATP is able to sustain a prolonged [Ca<sup>2+</sup>]<sub>i</sub> increase, unless ectonucleotidase activity reduces the agonist concentration near the EC surface to subthreshold values, quickly cutting the Ca<sup>2+</sup> signal. Furthermore, the amplitude of the long-lasting phase of the Ca<sup>2+</sup> signal depended on the balance between agonist degradation by ectonucleotidases and agonist transport, by diffusion and convection, from bulk solution to the EC surface.