INVOLVEMENT OF ENDOTHELIUM-DERIVED NO IN THE BASAL TONE AND IN THE VASODILATOR RESPONSES TO MUSCARINIC AGONISTS IN THE RAT ISOLATED MESENTERIC ARTERIAL BED

To investigate the involvement of nitric oxyde (NO) derived from endothelial cells in the control of vascular tone in the rat mesenteric vascular bed, the effects of different procedures known to interfere with the NO-cyclic GMP pathway were evaluated both on the basal tone and on the vasodilatory responses to four muscarinic agonists. To this aim, rat isolated mesenteric vascular beds were perfused at constant pressure. Water infusion significantly increased the resting perfusion pressure whereas L-NOARG, L-NAME and methylene blue were devoid of effect. In noradrenaline-preconstricted vascular bed, the perfusion pressure was significantly increased after water or L-NAME infusion. The vasodilator response induced by subsequent addition of acetylcholine in bolus was not significantly modified by pre-treatment with indomethacin but was significantly reduced by water infusion. Reponses to acetylcholine and to three other muscarinic agonists -carbachol, oxotremorine or McNeil A 343- were assessed. Incubation with L-NAME did not modify the initial peak falls of the agonists except for McNeil A 343, whereas it significantly reduced the area under the pressure trace for all the substances. The latter effect was reversed after a subsequent incubation with L-Arginine. Finally, L-NAME strongly and significantly increased the drop in perfusion pressure and the area under the pressure trace following bolus of glyceryl trinitrate. These results suggest that in the mesenteric arterial bed of the rat, which can be considered as a resistant arteries preparation, basal tone appears to be controlled by a factor other than NO. Moreover, the vasodilator responses of muscarinic agonists are affected by L-NAME in their second late sustained phase only, which probably relies on a de novo synthesis of endothelium derived-NO. Finally, endothelium derived-NO exerts inhibitory effects both on the sensitivy of the vascular smooth muscle to glyceryl trinitrate and on the magnitude of its contraction in the presence of noradrenaline, two types of effects which are sensitive to L-NAME.