Reversal by L-NAME of endothelium-independent vascular relaxation induced by Mg2+

Abstract

Acetylcholine (ACh) is known to relax vascular tissues by inducing the release of endothelium-derived relaxing factor which, in major part, is believed to be nitric oxide (NO). Magnesium-induced relaxation is thought to be due to its antagonism of the contractile effect of calcium. A nitric oxide synthase inhibitor, Nw-nitro L-arginine methyl ester, L-NAME, has been widely used in the investigation of the control and actions of nitric oxide in vascular tissues, and has previously been thought to exert all of its vascular effects via its action on NO synthase. In these experiments, we test whether L-NAME has other actions on the vasculature that are not related to NO. Our experiments were performed on rings of isolated femoral veins of dogs. The rings were suspended in organ baths and maintained at a temperature of 37°C. The force of contraction of the rings was measured by force displacement transducers and was recorded on the data translation software ‘Testpoint’. We studied the effect of L-NAME on both, intact as well as endothelium-denuded rings. The denudation of endothelium was achieved by rubbing the internal surface of the rings with the serrated surface of forceps. The effectivity of this technique for the complete removal of endothelium was checked by immunofluorescent staining of the rings before and after the rubbing. The femoral venous rings were precontracted with phenylephrine (1 ÌM), and addition of Mg2+ (2.4 mM) induced 40–80% relaxation, respectively. Addition of L-NAME (100 ÌM), at the steady state, significantly reversed this relaxation. We have also studied this effect of L-NAME on rat aortic rings and have observed similar effects. Our results suggest that L-NAME has direct action on vascular smooth muscle, affecting its reactivity to Mg2+, unrelated to its inhibition of endothelial nitric oxide formation.